Realization of quantum nanomagnets in metal-free porphyrins

Quantum nanomagnets exhibit collective quantum behaviors beyond the usual long range ordered states due to the interplay of low dimension, competing interactions and strong quantum fluctuations. Despite numerous theoretical works treating quantum magnetism, the experimental study of individual quantum nanomagnets remains very challenge, greatly hindering the development of this cutting-edge field. Here, we demonstrate an effective strategy to realize individual quantum nanomagnets in metal-free porphyrins by using combined on-surface synthesis and atom manipulation approaches, with the ultimate ability to arrange coupled spins one by one as envisioned by Richard Feynman 60 years ago. A series of metal-free porphyrin nanomagnets have been constructed on Au(111) and their collective magnetic properties have been thoroughly characterized on the atomic scale by scanning probe microscopy together with theoretical calculations. Our results reveal that the constructed S=1/2 antiferromagnets host a gapped excitation in consistent with isotropic Heisenberg antiferromagnets S=1/2 model, while the S=1 antiferromagnets with odd-number units exhibit two zero-mode end states due to quantum fluctuations. Our achieved strategy not only provides a unique testing bed to study the strongly correlated effects of quantum magnetism in purely organic materials, but expands the functionalities of porphyrins with implications for quantum technological applications.

Atomic Control of Active Site Ensembles in Ordered Alloys to Enhance Hydrogenation Selectivity

Intermetallic compounds offer unique opportunities for atom-by-atom manipulation of catalytic ensembles through precise stoichiometric control. The [Pd, (M), Zn] γ-brass phase allows for controlled synthesis of Pd-M-Pd catalytic sites (M = Zn, Pd, Cu, Ag and Au) isolated in an inert Zn matrix. These multi-atom heteronuclear active sites are catalytically distinct from Pd single atoms and fully coordinated Pd. We quantify the unexpectedly large effect of active site composition (i.e., identity of M atom in Pd-M-Pd sites) on ethylene selectivity during acetylene semi-hydrogenation. Subtle stoichiometric control demonstrates Pd-Pd-Pd sites are active for ethylene hydrogenation, whereas Pd-Zn-Pd sites show no measurable ethylene to ethane conversion. Agreement between experimental and density functional theory predicted activities and selectivities demonstrates precise control of Pd-M-Pd active site composition. The diversity and well-defined structure of intermetallics can be utilized to design active sites assembled with atomic-level precision.

Overview

This chapter presents the basic designs and working principles of STM and AFM, as well as an elementary theory of tunneling and the imaging mechanism of atomic resolution. Three elementary theories of tunneling are presented: the one-dimensional Schrödinger’s equation in vacuum, the semi-classical approximation, and the Landauer formalism. The relation between the decay constant and the work function, and a general expression of tunneling conductance versus tip-sample distance are derived. A brief summary of experimental facts on the mechanism of atomic resolution STM and AFM is presented, which leads to a picture of interplay between the atomic states of the tip and the sample, as well as the role of partial covalent bonds formed between those electronic states. Four illustrative applications are presented, including imaging self-assembed molecules on solid-liquid interfaces, electrochemical STM, catalysis research, and atom manipulation.

Atomic Forces and Tunneling

This chapter presents a unified theory of tunneling phenomenon and covalent bond force, as a result of the similarity between the Bardeen theory of tunneling and the Herring-Landau theory of the covalent bond. Three general theoretical treatments are presented, which show that tunneling conductance is proportional to the square of the covalent bond interaction energy, or equivalently, the square of covalent bond force. The constant of proportionality is related to the electronic properties of the materials. For the case of a metal tip and a metal sample, an explicit equation contains only measurable physical quantities is derived. Several experimental verifications are presented. The equivalence of covalent bond energy and tunneling conductance provides a theoretical explanation of the threshold resistance observed in atom-manipulation experiments, and points to a method of predicting the threshold resistance for atom manipulation. Theory of imaging wavefunctions with AFM is discussed.

Single hydrogen atom manipulation for reversible deprotonation of water on a rutile TiO2 (110) surface

The discovery of hydrogen atoms on the TiO2 surface is crucial for many practical applications, including photocatalytic water splitting. Electronically activating interfacial hydrogen atoms on the TiO2 surface is a common way to control their reactivity. Modulating the potential landscape is another way, but dedicated studies for such an activation are limited. Here we show the single hydrogen atom manipulation, and on-surface facilitated water deprotonation process on a rutile TiO2 (110) surface using low temperature atomic force microscopy and Kelvin probe force spectroscopy. The configuration of the hydrogen atom is manipulated on this surface step by step using the local field. Furthermore, we quantify the force needed to relocate the hydrogen atom on this surface using force spectroscopy and density functional theory. Reliable control of hydrogen atoms provides a new mechanistic insight of the water molecules on a metal oxide surface.

Novel Slow Release Nanocomposite Fertilizers

Nanotechnology deals with atom-by-atom manipulation and the strategies and products developed are quite precise. Despite the fact that the nanotechnology is noticeably exploited in the subject of energy, environment and health, the research is agricultural sciences had just scratched the surface. However, the potentials of nanotechnology in agricultural sciences had been reviewed. Among the applications, nanofertilizers technology is very revolutionary and known to exhibit economic advantage if the products advanced are economically feasible and socially sustainable. These nano fertilizers are pronounced to reduce nutrient loss due to leaching, emissions, and long-term incorporation by soil microorganisms.