ATOMIC SCALE INVESTIGATIONS OF SURFACE REACTION MECHANISM BETWEEN TRANSITION METAL PHTHALOCYANINE WITH SMALL GAS MOLECULES

Ph.DDOCTOR OF PHILOSOPH

On-Surface Synthesis of Nitrogen-Substituted Gold-Phosphorus Porous Network

Porous metal-organic frameworks (MOFs) have become one of the fastest growing fields with broad applications due to the presence of various active sites (e.g., cavities, metal nodes, and organic linkers). Notably, an on-surface synthesis represents a facile method for the fabrication of two-dimensional (2D) MOFs, where the substrate not only supports and dictates the symmetry of the surface nanostructures but also provides the metal atoms to form the metal nodes. However, the realization of on-surface synthesized 2D metal-inorganic porous networks is rarely explored, except for a recently reported gold-phosphorus network. Here, we integrate one more dimension into the growth process (activated gas phase atoms) and demonstrate the on-surface synthesis of ternary metal-inorganic gold-phosphorus-nitrogen porous networks on Au(111). The atomic structure is precisely identified through a combination of low-temperature scanning tunneling microscopy, density functional theory calculations, and X-ray photoelectron spectroscopy. Using the thermal energy from the heated Au(111) substrate, the phosphorus precursor can simultaneously grab gold atoms from the surface and nitrogen atoms from activated nitrogen gas to build up a long-range ordered 2D surface porous structure. Our study opens a completely new platform for the on-surface synthesis of tailored metal-inorganic frameworks. </p

Pressure-dependent band-bending in ZnO: A near-ambientpressure X-ray photoelectron spectroscopy study

Journal of Energy Chemistr

Fluorination-guided Li-anchoring behaviors on phthalocyanines

Understanding the interactions between metallic lithium (Li) and the anchoring sites/groups is essential for the design of stable host materials and artificial interphases in lithium metal batteries (LMBs). Here, we investigate the interactions of lithium with the polar organic functional groups in copper(II) hexadecafluorophthalocyanine (F16CuPc) and copper(II) phthalocyanine (CuPc) through the combination of in-situ X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), synchrotron-based near-edge X-ray absorption fine structures (NEXAFS), and density functional theory (DFT) calculations. It is revealed that the highly polar C-F bonds can anchor the Li atom via ionic Li-F interaction around the outer aza bridge N atoms in F16CuPc, while Li tends to interact with the inner pyrrolic N atoms around the central Cu in CuPc. The central Cu(II) ions in both molecules are reduced to Cu(I) upon interaction with Li. Electrons are transferred from Li to the lowest unoccupied molecular orbitals (LUMO) of both F16CuPc and CuPc molecules, as revealed by the UPS and NEXAFS measurements. Our systematic study can shed light on the design of anode materials by adding polar functional groups for applications in lithium metal batteries (LMBs).Ministry of Education (MOE)The authors acknowledge the financial support from the Natural Science Foundation of China (U2032147), Singapore MOE Tier II grant R143-000-A29-112, and Academic Research Fund Tie I grant RG104/18

An investigation on the relationship between the stability of lithium anode and lithium nitrate in electrolyte

Understanding the relationship between lithium anode and electrolyte is important to develop a more compatible lithium/electrolyte system for stable and safe cycling of lithium-metal based batteries. However, to date, there has not been any work to quantify the effects of electrolyte on the performance of electrode due to the complexity. Herein, we quantify the relationship between an electrolyte additive, LiNO3, and the stability of lithium anode. It is found that, with increasing the amount of LiNO3, the cyclability of lithium anode rises linearly and the risk of dendrite-induced short circuits can be reduced. Low Coulombic efficiency (CE) and short circuits tend to occur in tandem. LiNO3 is found to be continuously consumed upon electrochemical cycling, which leads to a low CE and a high risk of short circuits.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Published versio

Designing Kagome lattice from potassium atoms on phosphorus-gold surface alloy

Materials with flat bands are considered as ideal platforms to explore strongly correlated physics such as the fractional quantum hall effect, high-temperature superconductivity, and more. In theory, a Kagome lattice with only nearest-neighbor hopping can give rise to a flat band. However, the successful fabrication of Kagome lattices is still very limited. Here, we provide a new design principle to construct the Kagome lattice by trapping atoms into Kagome arrays of potential valleys, which can be realized on a potassium-decorated phosphorus-gold surface alloy. Theoretical calculations show that the flat band is less correlated with the neighboring trivial electronic bands, which can be further isolated and dominate around the Fermi energy with increased Kagome lattice parameters of potassium atoms. Our results provide a new strategy for constructing Kagome lattices, which serve as an ideal platform to study topological and more general flat band phenomena.</p

Abnormal near-infrared absorption in 2D black phosphorus induced by Ag nanoclusters surface functionalization

Black phosphorus (BP), as a fast emerging 2D material, shows promising potential in near-infrared (NIR) photodetection owing to its relatively small direct thickness-dependent bandgaps. However, the poor NIR absorption due to the atomically thin nature strongly hinders the practical application. In this study, it is demonstrated that surface functionalization of Ag nanoclusters on 2D BP can induce an abnormal NIR absorption at ≈746 nm, leading to ≈35 (138) times enhancement in 808 (730) nm NIR photoresponse for BP-based field-effect transistors. First-principles calculations reveal that localized bands are introduced into the bandgap of BP, serving as the midgap states, which create new transitions to the conduction band of BP and eventually lead to the abnormal absorption. This work provides a simple yet effective method to dramatically increase the NIR absorption of BP, which is crucial for developing high-performance NIR optoelectronic devices.MOE (Min. of Education, S’pore

Reversible oxidation of blue phosphorus monolayer on Au(111)

Practical applications of two-dimensional (2D) black phosphorus (BP) are limited by its fast degradation under ambient conditions, for which many different mechanisms have been proposed; however, an atomic level understanding of the degradation process is still hindered by the absence of bottom-up methods for the growth of large-scale few-layer black phosphorus. Recent experimental success in the fabrication of single-layer blue phosphorus provides a model system to probe the oxidation mechanism of two-dimensional (2D) phosphorene down to single-layer thicknesses. Here, we report an atomic-scale investigation of the interaction between molecular oxygen and blue phosphorus. The atomic structure of blue phosphorus and the local binding sites of oxygen have been precisely identified using qPlus-based noncontact atomic force microscopy. A combination of low-temperature scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements reveal a thermally reversible oxidation process of blue phosphorus in a pure oxygen atmosphere. Our study clearly demonstrates the essential role of oxygen in the initial oxidation process, and it sheds further light on the fundamental pathways of the degradation mechanism

Alkali metal storage mechanism in organic semiconductor of perylene-3,4,9,10-tetracarboxylicdianhydride

Organic semiconductor-based electrode materials are promising candidates for energy storage devices due to their high capacity, excellent flexibility, low cost and resource sustainability. The alkali metal storage mechanisms on various active functional groups of the organic materials, however, are still not clear at the molecular scale. It is essential to conduct systematic mechanism studies for the alkali storage behaviors in organic electrode materials. Here, the chemical and electronic structure evolutions upon the deposition of lithium (Li) and sodium (Na) on a model organic semiconductor electrode material of perylene-3,4,9,10-tetracarboxylicdianhydride (PTCDA), have been investigated by in-situ x-ray photoemission spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), near edge x-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory (DFT) calculations. It reveals that Li/Na can react with the carbonyl oxygen and increase the electron density within the PTCDA perylene. Moreover, the band-bending like features are observed on PTCDA film upon Li/Na interaction. Our experimental results and theoretical calculations indicate that reactions on carbonyl groups and charge redistribution are crucial for the Li/Na storage process, which shed light on comprehensive insights for the Li/Na storage behaviors on organic semiconductor-based electrode materials.Ministry of Education (MOE)Authors acknowledge the financial support from Singapore MOE grant R143-000-A29-112 and Academic Research Fund Tier 1 (RG104/ 18), as well as the computing resources from National Supercomputing Centre Singapore