Zainul Aabdin

Nanoparticle Dynamics in a Nanodroplet

We describe the dynamics of 3-10 nm gold nanoparticles encapsulated by ∼30 nm liquid nanodroplets on a flat solid substrate and find that the diffusive motion of these nanoparticles is damped due to strong interactions with the substrate. Such damped dynamics enabled us to obtain time-resolved observations of encapsulated nanoparticles coalescing into larger particles. Techniques described here serve as a platform to study chemical and physical dynamics under highly confined conditions.

Hydration Layer-Mediated Pairwise Interaction of Nanoparticles

When any two surfaces in a solution come within a distance the size of a few solvent molecules, they experience a solvation force or a hydration force when the solvent is water. Although the range and magnitude of hydration forces are easy to characterize, the effects of these forces on the transient steps of interaction dynamics between nanoscale bodies in solution are poorly understood. Here, using in situ transmission electron microscopy, we show that when two gold nanoparticles in water approach each other at a distance within two water molecules (∼5 Å), which is the combined thickness of the hydration shell of each nanoparticle, they form a sterically stabilized transient nanoparticle dimer. The interacting surfaces of the nanoparticles come in contact and undergo coalescence only after these surfaces are fully dehydrated. Our observations of transient steps in nanoparticle interactions, which reveal the formation of hydration layer mediated metastable nanoparticle pairs in solution, have significant implications for many natural and industrial processes.

Nanodroplet-Mediated Assembly of Platinum Nanoparticle Rings in Solution

Soft fluidlike nanoscale objects can drive nanoparticle assembly by serving as a scaffold for nanoparticle organization. The intermediate steps in these template-directed nanoscale assemblies are important but remain unresolved. We used real-time in situ transmission electron microscopy to follow the assembly dynamics of platinum nanoparticles into flexible ringlike chains around ethylenediaminetetraacetic acid nanodroplets dispersed in solution. In solution, these nanoring assemblies form via sequential attachment of the nanoparticles to binding sites located along the circumference of the nanodroplets, followed by the rearrangement and reorientation of the attached nanoparticles. Additionally, larger nanoparticle ring assemblies form via the coalescence of smaller ring assemblies. The intermediate steps of assembly reported here reveal how fluidlike nanotemplates drive nanoparticle organization, which can aid the future design of new nanomaterials.

Nanodroplet Depinning from Nanoparticles.

Nanoscale defects on a substrate affect the sliding motion of water droplets. Using in situ transmission electron microscopy imaging, we visualized the depinning dynamics of water nanodroplets from gold nanoparticles on a flat SiNx surface. Our observations showed that nanoscale pinning effects of the gold nanoparticle oppose the lateral forces, resulting in stretching, even breakup, of the water nanodroplet. Using continuum long wave theory, we modeled the dynamics of a nanodroplet depinning from a nanoparticle of comparable length scales, and the model results are consistent with experimental findings and show formation of a capillary bridge prior to nanodroplet depinning. Our findings have important implications on surface cleaning at the nanoscale.

Transient Clustering of Reaction Intermediates during Wet Etching of Silicon Nanostructures

Wet chemical etching is a key process in fabricating silicon (Si) nanostructures. Currently, wet etching of Si is proposed to occur through the reaction of surface Si atoms with etchant molecules, forming etch intermediates that dissolve directly into the bulk etchant solution. Here, using in situ transmission electron microscopy (TEM), we follow the nanoscale wet etch dynamics of amorphous Si (a-Si) nanopillars in real-time and show that intermediates generated during alkaline wet etching first aggregate as nanoclusters on the Si surface and then detach from the surface before dissolving in the etchant solution. Molecular dynamics simulations reveal that the molecules of etch intermediates remain weakly bound to the hydroxylated Si surface during the etching and aggregate into nanoclusters via surface diffusion instead of directly diffusing into the etchant solution. We confirmed this model experimentally by suppressing the formation of nanoclusters of etch intermediates on the Si surfaces by shielding the hydroxylated Si sites with large ions. These results suggest that the interaction of etch intermediates with etching surfaces controls the solubility of reaction intermediates and is an important parameter in fabricating densely packed clean 3D nanostructures for future generation microelectronics.

Effect of Electron Beam on Nanoparticle Dynamics in Solution during in situ TEM Observation

1. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore, 117551. 2. Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546. 3. Centre for BioImaging Sciences, Department of Biological Sciences, National University of Singapore, Science Drive 4, Singapore, 117543. 4. Nanocore, 4 Engineering Drive 3, National University of Singapore, Singapore 117576.

Nanoscale Dynamics in Ultrathin Liquids Visualized with TEM

Nanoscale imaging of frozen aqueous specimens and solid materials with transmission electron microscopes (TEM) has revolutionized our understanding in biological and material sciences. However, there is an ample number of important problems in life and physical sciences that occur only in liquid environments. Therefore, there is an incredible advantage of being able to image nanoscale processes directly in liquids [1-3]. I will describe our recent work on development of platform for imaging soft materials and biological samples in liquids using TEM [4-6]. We use this platform to study liquid properties at nanoscale. Here we show that the properties of fluid at nanoscale dominated by its interfacial interaction with the solid substrate surface and drastically differ from the expected bulk behavior. For example, the diffusive movement and rotation of nanocrystals within liquid nanodroplets are severely dampened when compared with macroscopic fluids. We will describe dynamic processes in nanoscale fluids such as condensation of nanodroplets and flow of nanodroplets. Imaging nanoscale fluids also enabled us to observe nanocrystal nucleation through nanocluster aggregation that differs from predictions of classical nucleation theory. We observe that crystals form amorphous aggregates.

Study of the Anisotropic Wet Etching of Nanoscale Structures in Alkaline Solutions

A qualitative and semi quantitative analysis of anisotropic etching of silicon nanostructures in alkaline solutions was done. Dedicated nanostructures were fabricated on 300mm wafers and their geometric change during wet etching was analyzed, stepwise, by top down SEM or TEM. We challenge the previously described wagon wheel technique towards nanodimensions and describe the pros and cons of the technique using relevant experimental conditions. The formation of specific geometric patterns are explained by the face-specificity of the etch rates. Clear differences in anisotropy were revealed between pillars etched in KOH or in TMAH, and for wagon wheels etched in TMAH or in NH4OH. Finally etch rates were extracted for the different types of crystal planes and compared.

Liquid Cell Platform to Directly Visualize Bottom-Up Assembly and Top-Down Etch Processes inside TEM

The assembly process of nanostructures from nanoparticles in solution is fundamental for bottom-up fabrication of functional materials and devices. In a similar way, bottom-down fabrication approach requires etching of materials. We take advantages of emerging in-situ liquid cell transmission electron microscopy (TEM) technique and explored several liquid processes such as: (1) nanoparticle-nanoparticle interaction in thin fluid layer, (2) wet etching of nanostructures and (3) pinning and de-pinning of nanodroplets at solid surface. Our approach is to directly visualize nanoscale liquid process, which is important for development of new nanofabrication processes for the design of next generation nanoscale devices. Our finding of nanodroplet pinning has important implications on surface cleaning at the nanoscale.

Visualizing Nanoscale Assembly in Solution Using In Situ TEM

Using dynamic in situ TEM imaging [1-4] in liquids, I will describe how nanoparticles form in solution and how these nanoparticles interact with each other. First, I will discuss how phase separation of a solution containing Au ions into solute-rich and solute-poor phases leads to formation of Au nanocrystal through a pathway that does not follow classical nucleation theory (CNT). Namely, I will show that there are multiple steps that lead to formation of nuclei from which nanocrystal grow (Figure 1A). These steps are: 1) phase separation of liquid solution into solute-poor and solute-rich phases, from which 2) an amorphous nanoparticles which serve as a precursor for nuclei emerges. This is followed by 3) crystallization of amorphous nanoparticle into a crystalline nuclei.