Direct Observation of Interactions between Nanoparticles and

Article pubs.acs.org/accounts

Direct Observation of Interactions between Nanoparticles and Nanoparticle Self-Assembly in Solution Published as part of the Accounts of Chemical Research special issue “Direct Visualization of Chemical and Self-Assembly Processes with Transmission Electron Microscopy”. Shu Fen Tan,†,‡ See Wee Chee,†,‡,§ Guanhua Lin,†,‡,§,∥ and Utkur Mirsaidov*,†,‡,§,∥ †

Department of Physics, National University of Singapore, 117551 Singapore Centre for BioImaging Sciences and Department of Biological Sciences, National University of Singapore, 117557 Singapore § Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 117546 Singapore ∥ NUSNNI-NanoCore, National University of Singapore, 117411 Singapore ‡

CONSPECTUS: Hierarchically organized nanoparticles (NPs) possess unique properties and are relevant to various technological applications. An important “bottom-up” strategy for building such hierarchical nanostructures is to guide the individual NPs into ordered nanoarchitectures using intermolecular interactions and external forces. However, our current understanding of the nanoscale interactions that govern such self-assembly processes usually relies on post-synthesis/assembly or indirect characterization. Theoretical models that can derive these interactions are presently constrained to systems with only a few particles or on short time scales. Hence, except for a number of special cases, a description that captures the detailed mechanisms of NP self-assembly still eludes us. By imaging the assembly of NPs in solution with subnanometer resolution and in real-time, in situ liquid cell transmission electron microscopy (LC-TEM) can identify previously unknown intermediate stages and improve our understanding of such processes. Here, we review recent studies where we explored NP self-assembly at different organization length scales using LC-TEM: (1) we followed the transformation of atoms into crystalline NPs in solution, (2) we highlighted the role of solvation forces on interaction dynamics between NPs, and (3) we described the assembly dynamics of NPs in solution. In the case of nanocrystal nucleation, we identified the existence of three distinct steps that lead to the formation of crystalline nuclei in solution. These steps are spinodal decomposition of the precursor solution into solute-rich and solute-poor liquid phases, nucleation of amorphous clusters within the solute-rich liquid phase, followed by crystallization of these amorphous clusters into crystalline NPs. The next question we ask is how NPs interact in solution once they form. It turns out that the hydration layer surrounding each NP acts as a repulsive barrier that prevents NPs from readily attaching to each other due to attractive vdW forces. Consequently, two interacting NPs form a metastable pair separated by their one water molecule thick hydration shell and they undergo attachment only when this water between them is drained. Next, we explore the self-assembly of many NP systems where the formation of linear chains from spherical NPs or nanorods (NRs) is mediated by linker molecules. At low linker concentration, both spherical NPs and NRs tend to form linear chains because of the need to reduce electrostatic repulsion between NP building blocks. When the concentration of linkers is increased, the attachment of NPs is no longer linear. For example, we find that two NRs undergo side-to-side assembly due to decreased electrostatic repulsion and the anisotropic distribution of linkers on NR surfaces at high linker concentration. Lastly, we look at the formation of NP nanorings directed by ethylenediaminetetraacetic acid (EDTA) nanodroplets in water. Our study shows that continued...

Received: January 31, 2017 Published: May 9, 2017 © 2017 American Chemical Society

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DOI: 10.1021/acs.accounts.7b00063 Acc. Chem. Res. 2017, 50, 1303−1312

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Accounts of Chemical Research

nanoring assemblies form via sequential attachment of NPs to binding sites located along the circumference of the EDTA nanodroplet, followed by rearrangement and reorientation of the attached NPs. Our approach based on real-time visualization of nanoscale processes not only reveals all the intermediate steps of NP assembly, but also provides quantitative description on the interactions between nanoscale objects in solution.

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INTRODUCTION Basic nanomaterials can be hierarchically organized into more complex nanostructures that derive new properties from their organization length scales.1 These nanostructures are promising candidates for various applications, ranging from electronics,2 optics,3 magnetism,4 to catalysis.5 Self-assembly is an important and robust “bottom-up” method for organizing nanoparticle (NP) building blocks into ordered and well-defined nanoarchitectures.5 From the viewpoint of thermodynamics, NPs will organize within a prescribed environment and adopt a structure with minimum free energy.6 The self-assembly of NPs in solution is driven by solution-mediated intermolecular forces between them and the external forces applied on them. Despite recent research efforts to understand the physical and chemical interactions that govern self-assembly, our description of the interactions that lead to the assembly of NPs in solution is still limited.7 The interactions between NPs are often described by classical continuum models such as the Derjaguin−Landau−Verwey− Overbeek (DLVO) theory.8 However, these descriptions break down when the sizes of NPs approach several nanometers.7 At these length scales, the discrete structure of NPs (i.e., the surface structure and nature of the surface−solvent interface) and its

surrounding media (i.e., the finite size of solvent molecules, solvated ions, and surfactants) have to be considered.9 To reveal the key intermediate states of NP assembly, atomistic models based on molecular dynamics (MD) simulation are often used. However, the efficacy of these simulations is limited by the computational power that is currently available7,10 and the dynamics of assembly processes that last more than few seconds cannot be captured. Experimentally, characterization of selfassembly processes is usually performed with the ”quench-andlook” approach where the assembly is stopped at different time points and the intermediate products are examined with scanning electron microscopy (SEM) or transmission electron microscopy (TEM).11 This approach provides high spatial resolution snapshots in time, but lacks the time resolution needed to fully describe assembly pathways. Time evolution of the assembly processes is inferred from indirect spectroscopic methods such as UV−visible spectroscopy,12 which probe the evolution of ensembles, but are not sensitive to individual assembly events. It is clear that neither approach can describe the assembly of NPs in detail. As such, direct experimental techniques that visualize the entire process with nanometer resolution in real-time is needed to develop a better framework for describing intermolecular forces at the nanoscale. Liquid cell

Figure 1. (A) Liquid cell (LC) sandwiches a thin layer (10−100 nm) of solution (light blue) containing either precursor solution and/or nanoparticles (NPs) between two ultrathin (∼10−20 nm) SiNx membrane windows that are separated by a spacer. This liquid cell protects the encapsulated thin solution layer from the vacuum of the transmission electron microscope (TEM). The camera underneath captures the dynamics of the nanoscale process occurring in the solution. (B) Hierarchical organization from atoms to crystalline NPs, interaction of NPs in a solution, and assembly of these NPs into nanostructures can be directly visualized using LC-TEM. 1304


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Figure 2. Three-step pathway for nucleation of gold nanocrystals in solution. (A) Time-series of TEM images show the intermediate steps during nucleation of gold NPs from a supersaturated aqueous Au0 solution. (B) Schematic illustration of the proposed steps in nucleation (gold as orange spheres, with surrounding water as blue bent lines). Adapted with permission from ref 34. Copyright 2017 NPG.

transmission electron microscopy (LC-TEM) provides both spatial and temporal resolution to capture such phenomena.5 In a liquid cell (Figure 1A), the liquid specimen containing metal precursors or NPs is sandwiched between two thin electron transparent membrane windows and the entire system is hermetically sealed to protect the liquid from the vacuum inside the TEM.13,14 The most common membrane material is silicon nitride (SiNx) because it is robust and easy to mass produce using standard microfabrication techniques. These microfabricated liquid cells can integrate multiple capabilities, such as flow, specimen heating and electrochemical biasing.15,16 Depending on membrane material/thickness, and thickness of the liquid layer, the spatial resolution in LC-TEM can reach subnanometer resolution.17 In terms of temporal resolution, events occurring within the liquid cell can be captured at a rate of a few hundred frames per second with state-of-the-art electron detectors.18,19 In this review, we discuss how direct visualization of nanoscale events in liquids can be used to reveal the mechanisms that govern interactions between NPs and their assembly process. First, we will show how NPs nucleate and form in solution. Next, we will describe the interaction between individual NPs in a solution. Finally, we will discuss how tailored interactions between NPs can lead to the formation of simple architectures by emphasizing the assembly pathways (Figure 1B).

This step is the spinodal decomposition of two component solutions into solute-rich and solute-poor liquid phases followed by the formation of nuclei in the solute-rich phase.23,32,33 Other groups have studied postnucleation NP growth at the nanometer-scale13,21 using in situ electron microscopy, and we extend this TEM method to observe each step in the nucleation pathway of gold NPs in water directly. Figure 2A shows that nucleation of gold nanocrystals starts from a homogeneous gold solution (t = 3.0 s). At t = 9.2 s, the solution demixes into gold-rich and gold-poor aqueous phases and forms a two-phase aqueous mixture that is characteristic of spinodal decomposition. This initial phase separation can be explained by the rapid reduction of Au3+ in the aqueous gold solution to Au0 due to solvated electrons being generated as the high-energy electrons interact with the solution. Then, the gold-rich phase condenses into amorphous nanoclusters (t = 11.3 s) that eventually became crystalline solid NPs (t = 15.4 s). Our analysis of the gold concentration in solution reveals that the homogeneous gold concentration reaches 0.2−0.6 M (Figure 3B) just before phase separation of the solution into gold-rich and gold-poor phases, which is 200−600 times more concentrated than the initial 1 mM solution.34 This increase in concentration of AuCl4−, precursor ions, is consistent with the distribution of counterions near the positively charged SiNx surface. The electron dose-dependent measurements of nucleation showed that the formation of initial amorphous nanoclusters is faster for higher electron fluxes (Figure 3A, B), consistent with reported radiolysis-induced silver nucleation in water.21 More importantly, these nucleation rates are in good agreement with previous studies of gold nucleation.35 The formation of gold-rich aqueous phase and subsequent condensation into an amorphous nanocluster can be described by ab initio calculations (Figure 3C) of a hydrated gold-atom pair. This computed ground-state energy of the hydrated gold system shows two energy minima at dAu−Au = 5.45 and 2.60 Å. The metastable state with dAu−Au = 5.45 Å has to overcome an energy barrier of 13.0 kBT at T = 295 K by expelling intervening water

From Atoms to Crystalline Nanoparticles

We begin our discussion with the nucleation of NPs in aqueous metal salt solution since initial phase formation and subsequent growth of nanocrystals can only be captured unambiguously using direct dynamic nanoscale imaging.13,20,21 Nucleation is commonly described by the classical nucleation theory (CNT) where a nucleus that is stable against dissolution emerges from a solution in a single step.22,23 However, CNT fails to account for nucleation processes in more complex systems.24−27 For example, the high nucleation rates during the synthesis of NPs cannot be described by CNT.28 To reconcile these differences, nonclassical nucleation models, such as the introduction of an additional step that precedes nucleation, have been proposed.23−25,29−31 1305


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Figure 3. (A) Details of gold nanocluster formation from spinodal structures at three TEM electron fluxes. The boundaries between gold-poor (dark blue) and gold-rich (light) regions are enhanced in this false color image. Compact boundaries of nascent gold nanoclusters appear after 65, 12.8, and 9.8 s of observation at electron fluxes 830, 1650 and 3300 e Å−2 s−1, respectively. (B) Number of gold atoms in the nanoclusters when imaged with the three electron fluxes, as estimated using bulk gold density. (C) Computed ground-state energy of the hydrated gold system as a function of dAu−Au from ab initio calculations. Adapted with permission from ref 34. Copyright 2017 NPG.

Figure 4. (A) Time-series of TEM images showing two gold NPs undergoing coalescence. (B) Schematic illustration of the interaction between two NPs that eventually leads to their coalescence. (C) Spacing between two gold NPs as a function of time. When the spacing between two interacting NPs is equal to two hydration layers, their approach toward each other ceases and they form a metastable pair. They undergo attachment only after both hydration layers between the NPs are drained. Adapted with permission from ref 43. Copyright 2016 American Chemical Society.

Interaction between Nanoparticles in Solution

molecules to achieve the denser Au−Au packing represented by dAu−Au = 2.60 Å. The energy barrier for solid dissolution is quite high (54.3 kBT at T = 295 K) as shown in Figure 3C, which is consistent with our experimental observations that show most of the amorphous nanoclusters do not redissolve back into the gold-rich aqueous phases. Our experiment allows us to follow the evolution of intermediates present in the multistep nucleation and clearly shows that the gold NPs nucleate from supersaturated aqueous solution of Au0 via a three-step mechanism: spinodal decomposition, followed by solidification into amorphous phase and crystallization.

In a solution, NPs interact through van der Waals (vdW), doublelayer, and solvation forces.7,36 So far, in situ TEM imaging has resolved the self-assembly of spherical37−39 and octapod-shaped NPs40 via vdW forces, and end-to-end attachment of nanorods (NRs) due to electrostatic41 and solvation forces.42 Short-range solvation forces arise due to the presence of solvent molecules. The role of solvation, or hydration forces when the solvent is water, in NP interactions becomes clear when we track the interaction dynamics between NPs separated by a distance comparable to the size of solvent molecules. We captured 1306


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Figure 5. (A) Spacing between 39 pairs of gold NPs during the last 1 s prior to contact (gray lines). Solid black thick line represents the average spacing for all pairs. (B) Histogram of all pairwise separation distances for 39 gold NP pairs. The red curve is a fit to the Boltzmann distribution. (C) Interaction energy between gold NP pairs (black open circles) as a function of separation obtained from the distribution in (B). Green curve represents the pairwise interaction force that is obtained from a fit to the interaction energy (blue curve), F(D) = − 2016 American Chemical Society.

dU (D) . dD

Adapted with permission from ref 43. Copyright

Figure 6. (A) Schematic of gold NP chain assembly. The linker molecules (ethylenediamine) link the surfactant molecules (citrate) of the adjacent gold NPs by forming a hydrogen bond between nitrogen of ethylene and oxygen of citrate. The predicted interparticle distance is ∼1.5 nm. (B) Histogram of distances between two adjacent NPs in the chains where Gaussian fit center at ∼1.5 nm. (C) Time-series of TEM images acquired at 100 frames per second showing that incoming NPs add to the gold NP chains by attachment of single NPs or short chain segments of a few NPs in the presence of 50 μM ethylenediamine. (D) Molecular dynamics simulation of attachment between a NP pair (1,2) and a free NP (3), where NP (3) is placed 6.2 nm away from NP (2); inset shows the magnified view of the attachment region, Water molecules are omitted for clarity. Orange, red, and blue colors represent gold atoms, citrate molecules, and ethylenediammonium linkers, respectively. (E) Time-series of TEM images showing the formation of branched NP network in the presence of high linker concentration (250 μM ethylenediamine). Adapted with permission from ref 64. Copyright 2016 American Chemical Society. 1307


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Figure 7. Schematics and TEM images of (A) end-to-end and (B) side-to-side NR assemblies in the presence of 150 and 500 μM cysteamine, respectively. Time-series of TEM images displaying (C) the formation of end-to-end NR assemblies and the formation of side-to-side NR assemblies through (D) the prealigned attachment and (E) the postattachment alignment. We attribute the gray spots in (E) to CTAB that were displaced from NRs in solution by cysteamine molecules. Adapted with permission from ref 69. Copyright 2017 American Chemical Society.

the detailed interaction dynamics between gold NPs in water43 as shown in Figure 4A, B. We find that as a pair of NPs approach each other, their motion stalls and they form a metastable NP-pair separated by a distance of ∼6 Å, which is comparable to the size of two water molecules (Figure 4C). NPs do not immediately touch once they reach this two water-moleculethick critical separation. Instead, they stay as a transient, sterically stabilized NP dimer for few seconds. We proposed that this short-lived behavior represents the time required to move surface bound molecules out from the two facing solid surfaces. When the hydration layer of NPs keeping them apart is drained of water molecules, they coalesce through an abrupt jump into contact. We used the individual trajectories from 39 coalescence events shown in Figure 5A to obtain the distribution of pairwise distances between NP-pairs (Figure 5B). The interaction energy between NPs can be approximated as

metastable NP-pair (Figure 5C). We obtain from the interaction energies, a decay length of δ = 1.4 Å for the hydration repulsion, which is consistent with previously reported values.48,49 The brief discussion above shows that direct observations of NP interaction dynamics can go beyond revealing the transient stages of pairwise interactions such as the formation of metastable NP-pairs, and allows us to quantify the strength of these interactions between NPs. Self-Assembly of Nanoparticles in Solution: Building Nanoarchitectures

NP assembly is generally driven by interparticle interactions, external forces or both.7,36 The interaction between NPs can be tuned via their surface chemistries. Molecules are often utilized to guide the NP organization by taking advantage of intermolecular forces such as covalent bonding,50,51 vdW forces, electrostatic interactions,52,53 π−π interactions,54 hydrogen bonding,55,56 and hydrophobic interactions.57,58 We present in coming sections the self-assembly of spherical and anisotropic NPs (i.e., NRs) guided by small linker molecules (ethylenediamine and cysteamine). External forces such as adsorption to liquid interfaces,59 electric and magnetic forces,60,61 capillary forces,62 are also known to produce very robust NP assemblies. We provide an example of this case where NPs organize themselves into rings in a mixture of ethylenediaminetetraacetic acid and water. A. Assembly of Nanoparticle Chains using Molecular Linkers. We first explore the simplest of assembly processes: assembly of ligand-capped spherical NP into a 1D chain. A suspension of gold NPs capped with citrate ligands are stabilized via electrostatic repulsion and generally do not aggregate.63 In order to assemble them into stable chains, we need to

U (D) = W exp( −D/δ) − AR /12D

Here, the first term is due to the repulsive hydration force with a decay hydration length scale of δ44−46 and the second term is due to the attractive vdW interaction between two spheres with the radius of R, where A is the Hamaker constant.47 The distribution for pairwise spacing is captured with a Boltzmann distribution: P(D) ∼ exp( −U (D)/kBT )

The approach of NPs toward each other stalls when the repulsive hydration force balances the attractive vdW force (Figure 4B). Therefore, a peak in the distribution of pairwise spacing or minima of pairwise interaction energy (net zero force) corresponds to the 1308


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B. Assembly of Anisotropic Nanoparticles using Molecular Linkers. Anisotropic NPs can form one-, two- and three-dimensional architectures of different shapes because of their geometry and anisotropic affinity for surfactants.51,65 We found that the CTA+-capped gold NRs can be assembled into two modes by tuning the concentration of cysteamine molecules (linker). Cysteamine molecules bind with gold through the Au−S covalent bond.66 Then, the free ends of these molecules connect two adjacent gold NRs via hydrogen bonding (N−H···N) (Figure 7A, B). At low cysteamine concentration (150 μM), NRs attach end-to-end (Figure 7A, C), whereas at high concentration (500 μM) NRs attach side-toside (Figure 7B, D, E). The different attachment modes can be understood in terms where cysteamine molecules bind to the NRs. At low linker concentration, cysteamine molecules displace CTA+ ions bound at the end facets67 because CTA+ ions have less affinity for these facets.68 Hence, only the NR ends bind through hydrogen bonding. At high linker concentration, the cysteamine molecules also displace CTA+ ions at the sides of NRs, leading to side-to-side attachment. Moreover, our real-time observations reveal that at high linker concentration, where both NR sides and ends are functionalized with linkers, side-to-side assemblies form through two different pathways. They can form via sideway contact of two aligned NRs, or via end-to-end attachment followed by rotation into a side-to-side arrangement (Figure 7E).69 This rotation of two NRs into a side-to-side assembly is driven by the continual formation of hydrogen bonds between cysteamine molecules on the sides of two NRs. These intermediate steps in NR attachment

connect them with linker molecules. We connected these NPs by adding ethylenediamine (linker molecules) into the solution (Figure 6A). Here, the underlying concept is that the ethylenediamine molecule, with amine groups (−NH3+) on its two ends, would form hydrogen bonds with carboxylate ends (−COO−) of the citrate ligands that are attached to the NPs. The assembly process shown in Figure 6C reveals that NP chains grow either by attachment of individual NPs or small chain fragments consisting of few NPs. Our analysis of the linear chains formed in situ indicates that the length of the linker molecule determines the separation between neighboring NPs. The spacing between NPs in a chain is equal to the combined length of one linker and two surfactant molecules (Figure 6B), and increasing the linker length correspondingly increases the spacing.64 To understand the preference for linear assembly of NPs, we performed molecular dynamics simulations to see how a free NP would approach an existing pair of linked NPs at two concentrations of ethylenediamine. The simulations revealed that, in the presence of 50 μM ethylenediamine, the effective equilibrium charge of each NP is about +20e− and the incoming NP prefers to approach from the ends of the NP-pair (versus coming in from the side of the pair), where it experiences the least electrostatic repulsion. When the concentration of linker molecules is increased from 50 μM to 250 μM, the effective charge on the NPs drops from +20e− to +4e− due to screening by the divalent counterions of linkers. This screening, in turn, lowers the electrostatic repulsion and increases the efficacy of not only linear attachment (Figure 6D) but also attachment of NPs onto the sides. This was confirmed by direct observation of branched NP network growth at high linker concentration (Figure 6E).

Figure 8. (A) Schematic illustration and (B) time-series of TEM images of platinum NPs binding to the equatorial circumference of a phase separated EDTA nanodroplet in water. NP binding to the nanodroplet occurs via direct attachment (orange arrows) and NP insertion (red arrows) between two closely spaced nanodroplet-bound NPs. (C) The arrangement of the NPs at the circumference of the same nanodroplet reveals the alignment of the NPs’ long axes (yellow dashed lines) with the circumference of the nanodroplet (white dashed lines). (D) Schematic illustration and (E) time-series TEM images of platinum NP rearrangement around two fusing nanodroplets. Adapted with permission from ref 75. Copyright 2016 American Chemical Society. 1309



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highlight how direct observation of the assembly process can reveal different pathways that lead to the same product. C. Assembly of Nanoparticle Rings Using Binary Liquid Mixtures as Templates. Soft nanoscale templates such as viruses,70 soft block copolymer,71 and phase-separated droplets72,73 in solutions can serve as scaffolds for the NP organization. Nonetheless, the mechanisms by which these scaffolds interact with NPs and guide their assembly are not well-understood, and in the case of two-phase liquid interfaces, it is also not clear how NPs populate these interfaces.74 Here, we look at the assembly of platinum NPs into welldefined rings around the phase separated EDTA nanodroplets in water.75 EDTA disperses into fluid-like aggregates (nanodroplets) when dissolved in water at a pH of 7−8. The assembly dynamics of platinum NPs via attachment to these EDTA nanodroplets is illustrated in Figure 8A, B. Our real-time observations reveal that there are two modes of NP attachment: 1) NPs in solution can directly bind to (orange arrows: Figure 8B) and align with equatorial circumference of the EDTA nanodroplet (Figure 8C), or 2) NP can be inserted into a position between two droplet-bound NPs by pushing these bound NPs apart (red arrows: Figure 8B). The attachment of new NPs and subsequent assembly ceases when the platinum NPs fully cover the nanodroplet’s circumference. The ability to visualize the assembly of NP rings also allowed us to identify another assembly mode where two (or more) partially formed NP ring assemblies coalesce into a larger one, as shown (Figure 8D, E). The NPs remain bound to the fusing nanodroplets throughout the nanodroplet coalescence and rearrange to form a larger ring on the circumference of the newly formed nanodroplet. As this transient pathway results in the same final morphology as in Figure 8B, it would be almost impossible to capture with ex situ approaches.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

See Wee Chee: 0000-0003-0095-3242 Guanhua Lin: 0000-0003-0645-2916 Utkur Mirsaidov: 0000-0001-8673-466X Notes

The authors declare no competing financial interest. Biographies Shu Fen Tan received her B.S. and Ph.D., both in Chemistry, from National University of Singapore (2011 and 2016, respectively), working with Associate Professor Christian Nijhuis. She is now working as a postdoctoral researcher with Assistant Professor Utkur Mirsaidov. Her research interests include surface chemistry, in situ liquid cell electron microscopy, NP self-assembly, and plasmonics. See Wee Chee received his Ph.D. in Materials Science and Engineering from the University of Illinois at Urbana−Champaign in 2008. He was first introduced to in situ TEM during his postdoctoral training with Dr. Renu Sharma at Arizona State University. Later, he went on to work with Dr. Frances M. Ross (IBM Watson Research Center) and Prof. Robert Hull (Rensselaer Polytechnic Institute). Currently, he is a senior research fellow with the Mirsaidov group. Guanhua Lin received his Ph.D. in Physical Chemistry from Chinese Academy of Sciences, China, in 2011. He then worked as a postdoctoral fellow at Nicolaus Copernicus University in Poland, in 2012. He joined the group of Assistant Professor Utkur Mirsaidov as a research fellow in 2013. His research interests focus on the self-assembly and growth of inorganic nanostructures using in situ TEM. Utkur Mirsaidov is an Assistant Professor in the Departments of Physics and Biology at the National University of Singapore. He received his Ph.D. in Physics from the University of TexasAustin followed by postdoctoral trainings at the University of Illinois at Urbana−Champaign (2006−2009) and National University of Singapore (2009−2013).

Concluding Remarks

LC-TEM is emerging as a powerful approach to probe a broad range of nanoscale processes that occur in solution directly. It can reveal never-seen-before transient stages of material growth and NP self-assembly. To gain deeper insight into self-assembly, models that can describe many-body interactions and account for nonadditive interactions between NPs in solution are needed.7 Direct nanoscale dynamic studies that capture how each interacting NPs in an ensemble behaves at any given time will aid in the development of these models. There are several challenges associated with the LC-TEM based study of nanoscale processes. One primary challenge is that effects associated with the electron beam may limit the straightforward interpretation of experimental observations.76 For example, high energy electrons may cause the insulating SiNx to develop a positive charge38,39 or change the solution chemistry through radiolysis.16 The diffusive motion of NPs may also be modified by confinement and surface effects within these liquid cells.19 Studies that combine in situ LC-TEM approach with carefully designed control experiments is essential in distinguishing these artifacts. Looking forward, faster and more electron sensitive detectors should allow us to capture some of the fast intermediate stages that presently elude us, and at lower electron dose rates, which would minimize electron beam induced effects and improve our quantification of interaction forces. Future liquid cell designs may include nanoscale patterning of the freestanding membrane windows that will be critical to the study of templated NP assembly, an area where the assembly dynamics remain to be explored.

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ACKNOWLEDGMENTS This work was supported by Academic Research Fund (MOE2015-T2-1-07) from the Singapore Ministry of Education and the NUS Young Investigator Award (NUSYIA-FY14-P17) from the National University of Singapore.

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REFERENCES

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