82
5 Adatoms, Defects, and Kink Edges
crystal. Size derivacy refers properties that the parent bulk never demonstrate. For
instance, non-magnetic metals manifest magnetism at the nanoscale [1–3]; phase
transition from conductor to insulator occurs at the size of a few nanometers [4];
nanoscale gold meets the demand of local surface plasmonics [5, 6]; the catalytic
ability of gold for CO oxidation is greatly enhanced at small sizes [7–9]. Other
intriguing properties for applications include RNA-delivery, localized surface plasma
resonance for enhancing Raman spectroscopy signals [10, 11], laser applications in
medication [12], enhancement of photoluminescence [13], etc. Accompanied with
the structure evolution from the fcc bulk to the strained structures such as icosahedral
or decahedral [14, 15], substantial bond contraction occurs to the outermost atomic
shells [16] that can be described as an elastic sheet of “skin” covering the bulk body
[17].
Besides the bond length contraction, potential trap depression, charge and energy
density elevation, and electronic configurations occur to nanoparticles. Singleelectron tunneling spectroscopy revealed the generation of an energy gap whose
width is inversely proportional to the diameter of Au and Pd crystals [4, 18]. STM/S
also revealed that the valence DOS of Au monomer and dimer [19], Au-Au chain
[19, 20], and Au nanowire [21] moves up in energy, being indication of strong local
polarization. Fascinating properties demonstrated by such even undercoordinated
atoms are associated with Coulomb blockade [4], size-dependent reaction dynamics
[18] and standing wave formation at edges [22].
Atoms with even fewer CNs at sites like terrace edges, point defects, or adatoms
modify the CLS significantly and irregularly. Figure 5.1 shows typical XPS spectra for Rh and Pt adatoms. Rh adatoms modify the spectral at energies between
306.6 and 307.1 eV with features belong to neither positive nor negative CLS order
[23]. Pt adatoms enhance, however, the intensity of the 71.0 eV peak other than the
70.5 eV peak with positive shift [24]. Terrace edges of W(110) vicinal surfaces [25],
Re surfaces [26, 27], Rh(111) vicinal surfaces [28], and Rh(110) missing-row type
reconstructed surfaces [29] also modify the XPS profiles irregularly. However, signal
from such lower coordinated atoms is weaker compared to that of the skin. Direct
spectral decomposition is hardly certain and less reliable.
The thickness dependence of the InSe/Pt in Fig. 5.2 shows the same coordination
trend of Pt adatoms. For the Pt 4f 3/2 , redshift from 71.6 to 71.1 eV occurs as the
thickness increases from 0.05 to 7 ML (left panel). The In 4d and Se 3d levels also
undergo redshift as the thickness increases from 0.03 ML to 2.2 ML. CN reduction
induced core-level entrapment holds globally true and it is element independent.
However, polarization is quite subjective [31].
5.1.2 STM Observation
Interaction of metallic adatoms on metal oxide supports forms a key area of research
in catalysis. In particular, gold and platinum nanoparticles supported on TiO 2 and
5 Adatoms, Defects, and Kink Edges
crystal. Size derivacy refers properties that the parent bulk never demonstrate. For
instance, non-magnetic metals manifest magnetism at the nanoscale [1–3]; phase
transition from conductor to insulator occurs at the size of a few nanometers [4];
nanoscale gold meets the demand of local surface plasmonics [5, 6]; the catalytic
ability of gold for CO oxidation is greatly enhanced at small sizes [7–9]. Other
intriguing properties for applications include RNA-delivery, localized surface plasma
resonance for enhancing Raman spectroscopy signals [10, 11], laser applications in
medication [12], enhancement of photoluminescence [13], etc. Accompanied with
the structure evolution from the fcc bulk to the strained structures such as icosahedral
or decahedral [14, 15], substantial bond contraction occurs to the outermost atomic
shells [16] that can be described as an elastic sheet of “skin” covering the bulk body
[17].
Besides the bond length contraction, potential trap depression, charge and energy
density elevation, and electronic configurations occur to nanoparticles. Singleelectron tunneling spectroscopy revealed the generation of an energy gap whose
width is inversely proportional to the diameter of Au and Pd crystals [4, 18]. STM/S
also revealed that the valence DOS of Au monomer and dimer [19], Au-Au chain
[19, 20], and Au nanowire [21] moves up in energy, being indication of strong local
polarization. Fascinating properties demonstrated by such even undercoordinated
atoms are associated with Coulomb blockade [4], size-dependent reaction dynamics
[18] and standing wave formation at edges [22].
Atoms with even fewer CNs at sites like terrace edges, point defects, or adatoms
modify the CLS significantly and irregularly. Figure 5.1 shows typical XPS spectra for Rh and Pt adatoms. Rh adatoms modify the spectral at energies between
306.6 and 307.1 eV with features belong to neither positive nor negative CLS order
[23]. Pt adatoms enhance, however, the intensity of the 71.0 eV peak other than the
70.5 eV peak with positive shift [24]. Terrace edges of W(110) vicinal surfaces [25],
Re surfaces [26, 27], Rh(111) vicinal surfaces [28], and Rh(110) missing-row type
reconstructed surfaces [29] also modify the XPS profiles irregularly. However, signal
from such lower coordinated atoms is weaker compared to that of the skin. Direct
spectral decomposition is hardly certain and less reliable.
The thickness dependence of the InSe/Pt in Fig. 5.2 shows the same coordination
trend of Pt adatoms. For the Pt 4f 3/2 , redshift from 71.6 to 71.1 eV occurs as the
thickness increases from 0.05 to 7 ML (left panel). The In 4d and Se 3d levels also
undergo redshift as the thickness increases from 0.03 ML to 2.2 ML. CN reduction
induced core-level entrapment holds globally true and it is element independent.
However, polarization is quite subjective [31].
5.1.2 STM Observation
Interaction of metallic adatoms on metal oxide supports forms a key area of research
in catalysis. In particular, gold and platinum nanoparticles supported on TiO 2 and