The in-situ growth phenomenon of metal nanoparticles directly from an oxide support, known as ‘ex-solution’, has recently been reported by multiple researchers in the fields of high-temperature catalysis and renewable energy. When perovskite oxides (ABO3) are used as posting frameworks, precious or transition metals can be dissolved as cations in the B-sites of the perovskite lattice under oxidizing conditions; they can also be ex-solved partially upon subsequent reduction as nano-sized metallic particles decorating oxide surface. Compared to traditional nanoparticle synthesis and dispersion techniques, this process is faster, more cost-effective and allows finer and better particle distribution without coalescence. Furthermore, its reversibility indicates that catalyst agglomeration can be avoided through re-oxidation, significantly enhancing the lifetime of the supported catalysts. Here, we are quantifying the mechanisms of ex-solution of metal catalytic particles using in situ TEM with a gas flow holder system and a heating holder.
Relevant publications: Advanced Materials (2020) in press, Energy & Environmental Science (2020) in press, ACS Applied Materials & Interfaces 12, 24039-24047 (2020), Journal of the American Chemical Society 141, 6690-6697 (2019).
Metallic nanoparticles with specific shapes such as spiky nanoparticles (SNPs) have attracted intense attention due to their unique plasmonic resonance. In particular, the size and shape-dependent physicochemical properties of SNPs enable various interdisciplinary research via such functionalities as surface enhanced Raman scattering detection, surface plasmon resonance sensing, bionanosensing, target drug delivery, and catalysis. Typically, to efficiently functionalize the SNP via tuning its diameter and surface roughness, the reactivity of metal ions needs to be manipulated by controlling the amount of reducing agents with the assistance of capping agents which promote the anisotropic growth of branches. Nevertheless, a judicious control over the diameter and surface roughness of the SNP can only be achieved by understanding the growth mechanism and kinetics, and morphology evolution. Here, we are using in situ liquid cell TEM to provide direct insights into the growth mechanism and kinetics of single and multiple SNPs under different electron beam cylinder sizes, dose rates and solution concentrations.
Relevant publications: Journal of the American Chemical Society 141, 12601-12609 (2019).
(Fig. 2) A series of TEM bright field images of an individual spiky Au nanoparticle growing via radiolysis and the kinetic data of the particles with relevant UV-vis results.
Photo-sensing has attracted extensive interest in modern multifunctional technologies such as thermal imaging, biomedical imaging, night vision, information communication, military facilities, etc. To achieve outstanding detection performances, a variety of materials and mechanisms have been developed as follow: the 2D materials (graphene, MoS2, GaS etc) that possess excellent electrical properties, the hybrid structures (graphene/QDs, MoS2/QDs, SWCNT/QDs etc) that enable efficient charge separation and faster carrier mobility, and the heterojunction nanostructures (GaN/AlN, Ge/Si, P3HT:ZnO etc) whose excellent crystallinity with the capability for charge separation have led the rapid development of photo-detectors. Despite such efforts, further advancement has been impeded due to the choice of substrates for monolayer growth, the band gap opening of graphene, and the complexity of forming heterojunctions in nanostuructures. Recently, vanadium dioxide (VO2) has presented a promising platform for next-generation photo sensors due to its unique property of undergoing a metal(R phase)-insulator(M phase) transition (MIT) at a critical temperature (Tc) of 68 °C, accompanied by a large change in resistivity (∆ρ) over three orders, occurring within a few tens of femtoseconds. Taking such advantages of VO2, we are investigating and developing the photo-sensors of hydrogen doped VO2 wires using in situ TEM with a gas flow holder system, and in situ optics system.
Relevant publications: Nano Letters 20, 2733-2740 (2020), Chemistry of Materials 32, 4013-4023 (2020), Small 16, 1906109 (2020), Nano Letters 17, 7737-7743 (2017), Scientific Reports 5, 10861 (2015), Nano Letters 13, 1822-1828 (2013).
(Fig. 3) Applications of UV and IR sensors (left), Structural and electrical transitions of VO2 (right)
Developing and integrating reliable low-k materials is a key challenge for next generation interconnect and wireless communication technologies. Lowering the dielectric constant k which has commonly been enabled by introducing pores into the dielectric material decreases the resistance-capacitance (RC) delay, reduces power consumption, reduces cross-talk between nearby interconnects in integrated circuits (ICs), and increases the bandwidths in antennas by preventing surface wave propagation. Lowering the k through increasing porosity often degrades intrinsic electrical reliability, and leads to a higher leakage current and lower dielectric breakdown voltages along with other physical properties including mechanical integrity, thermal stability, and dielectric loss. Moreover, the electrical and dielectric reliabilities of low-k materials under stress require characterization due to the increasing popularity of thin and flexible electronic devices in a wide-variety of technological areas, such as batteries, transistors, electrodes, displays, biosensors and biomedical instruments. These devices are required to undergo large mechanical deformation in use that includes bending, stretching, twisting, and folding while maintaining reliable high performance of the devices. Porous low-k films have shown a large degradation in both reliabilities under mechanical stress, and after unloading the stress, both leakage current and dielectric constant were not recovered. To tackle these problems, we are developing unique designs of ceramic nanolattices using two-photon lithography, and measuring their mechanical and electrical properties using an in situ SEM and a high voltage in situ optics system.
Relevant publications: Nano Letters 19, 5689-5696 (2019), Nano Letters 17, 7737-7743 (2017).
(Fig. 4) Fabrication process of a nanolattice capacitor using two-photon lithography (upper) and in situ mechanical measurements of the nanolattice (lower).
