4
1 Introduction
1.1 Overview
1.1.1 Coordination Bonds and Energetic Electrons
Atomic undercoordination refers to atoms associated with grain boundaries, homogeneous adatoms, point defects, solid or liquid skins, terrace edges, and nanostructures of various sizes and dimensionalities such a monatomic chains and
monatomic sheet. Atomic hetero-coordination means those associated with alloys,
compounds, chemisorbed skins, dopants, impurities, and interfaces. Performance of
bonds between the irregularly coordinated atoms and associated electrons in various
bands are crucial to the properties of materials at these sites [1–8].
One can create a new substance with desired functionalities by breaking the old
bonds and forming new kinds of bonds between different atoms. Harnessing the
known properties of a substance by relaxing the bonds and the associated energetics and dynamics of electrons in localization, densification, polarization, and
transportation becomes therefore increasingly important [1]. For instance, materials at sites surrounding irregularly-coordinated atoms perform differently from
themselves in the bulk interior of elemental solids. Although they are traditionally
unwanted, such irregularly-coordinated atoms are of key importance to the advancement of condensed matter physics, solid-state chemistry, materials sciences, and
device technologies, in particular, at the nanometer scale [3].
Controllable relaxation of the coordination bonds and the energetic electrons at
such irregularly-coordinated atomic sites provides profound impact to many areas of
scientific and technological interest [4, 9]. These subject areas include, for instances,
adhesion [10], adsorption [11], alloy formation [12, 13], catalytic reaction [14, 15],
corrosion protection [16], decomposition [17], diffusion [18], doping [19] epitaxial
growth [20, 21], hydrophobic lubrication [7], glass formation [22], dielectric modulation [23, 24], mechanical strength [25–28], thermal elasticity [8, 29], photon and
electron emission and transportation [30], quantum friction [31], radiation protection [32], topological insulator conduction [33], superconductivity [34, 35], thermal
stability [36, 37], wettability [38, 39], water and ice skin supersolidity [40, 41], etc.
Electrons associated with point defects [42, 43], homogeneous adatoms [44],
adsorbates [45–47], terrace edges [48–50], monatomic chains and their ends [51,
52], and solid skins [53, 54] result in, for instance, new types of energy states that
enhance tremendously the site-selective catalytic ability of a substance even though
the bulk parent, like gold, is chemically inert [55]. For instance, the performance of a
semiconductor could be promisingly improved if flaws that were previously thought
irrelevant are reduced. However a specific defect impacts the ability of halide perovskite to hold energy derived from light in the form of electrons. Defects could be
good or bad in semiconductors. Dislocations negatively impact the carrier dynamics
of halide perovskite. Reducing dislocation densities by more than one order of magnitude is found to lead to an increase of electron lifetime by four times [56]. Defects
rich MoS 2 ultrathin nanosheets with additional active edge sites could enhance electrocatalytic hydrogen evolution [57]. Atomic undercoordination laid the foundation
1 Introduction
1.1 Overview
1.1.1 Coordination Bonds and Energetic Electrons
Atomic undercoordination refers to atoms associated with grain boundaries, homogeneous adatoms, point defects, solid or liquid skins, terrace edges, and nanostructures of various sizes and dimensionalities such a monatomic chains and
monatomic sheet. Atomic hetero-coordination means those associated with alloys,
compounds, chemisorbed skins, dopants, impurities, and interfaces. Performance of
bonds between the irregularly coordinated atoms and associated electrons in various
bands are crucial to the properties of materials at these sites [1–8].
One can create a new substance with desired functionalities by breaking the old
bonds and forming new kinds of bonds between different atoms. Harnessing the
known properties of a substance by relaxing the bonds and the associated energetics and dynamics of electrons in localization, densification, polarization, and
transportation becomes therefore increasingly important [1]. For instance, materials at sites surrounding irregularly-coordinated atoms perform differently from
themselves in the bulk interior of elemental solids. Although they are traditionally
unwanted, such irregularly-coordinated atoms are of key importance to the advancement of condensed matter physics, solid-state chemistry, materials sciences, and
device technologies, in particular, at the nanometer scale [3].
Controllable relaxation of the coordination bonds and the energetic electrons at
such irregularly-coordinated atomic sites provides profound impact to many areas of
scientific and technological interest [4, 9]. These subject areas include, for instances,
adhesion [10], adsorption [11], alloy formation [12, 13], catalytic reaction [14, 15],
corrosion protection [16], decomposition [17], diffusion [18], doping [19] epitaxial
growth [20, 21], hydrophobic lubrication [7], glass formation [22], dielectric modulation [23, 24], mechanical strength [25–28], thermal elasticity [8, 29], photon and
electron emission and transportation [30], quantum friction [31], radiation protection [32], topological insulator conduction [33], superconductivity [34, 35], thermal
stability [36, 37], wettability [38, 39], water and ice skin supersolidity [40, 41], etc.
Electrons associated with point defects [42, 43], homogeneous adatoms [44],
adsorbates [45–47], terrace edges [48–50], monatomic chains and their ends [51,
52], and solid skins [53, 54] result in, for instance, new types of energy states that
enhance tremendously the site-selective catalytic ability of a substance even though
the bulk parent, like gold, is chemically inert [55]. For instance, the performance of a
semiconductor could be promisingly improved if flaws that were previously thought
irrelevant are reduced. However a specific defect impacts the ability of halide perovskite to hold energy derived from light in the form of electrons. Defects could be
good or bad in semiconductors. Dislocations negatively impact the carrier dynamics
of halide perovskite. Reducing dislocation densities by more than one order of magnitude is found to lead to an increase of electron lifetime by four times [56]. Defects
rich MoS 2 ultrathin nanosheets with additional active edge sites could enhance electrocatalytic hydrogen evolution [57]. Atomic undercoordination laid the foundation