Editorial Overview
Surface Science from Nanostructures to Sensing
Kurt W. Kolasinski
Department of Chemistry
West Chester University
West Chester, PA 19383 USA
[email protected]
Submitted to Current Opinion in Solid State & Materials Science
The surface science approach seeks a molecular level understanding of
chemical behavior and materials properties. Computational methods developed
for extended bulk samples are complicated by the symmetry breaking presence
of the surface. Nanoscale systems further complicate matters because the
interesting size-dependent properties of the nanoscale regime are the
consequence of (i) the influence and eventual dominant contribution of
surface effects compared to bulk effects and (ii) quantum confinement.
Theoretical approaches are always limited by available computational
resources such that trade offs must be made regarding how many particles
can be followed explicitly and at what level of approximation. From the
chemical bonding point of view – proceeding from the Heitler-London
treatment of a bond, through Hartree-Fock methods, Slater orbitals and
configuration interaction – the molecular scientist's first inclination has
often been to use as many orbitals with as much precision as possible. The
hopes of this field continually ride on more computational horsepower and
more computationally friendly orbitals (and methods of integrating them).
A fundamentally different approach was offered by Kohn in 1964 [1] when
Hohenberg and he showed that that the total energy of a quantum mechanical
system can be calculated exactly if the spatial distribution of the
electrons (the electron density) is known. With further developments, the
method introduced by Kohn and Sham [2] is what is now called density
functional theory (DFT) and it is particularly well-suited for the
calculation of large systems. Nonetheless, the Kohn-Sham implementation of
DFT is practicably limited to a few hundred atoms. Even though DFT is not
dependent on orbitals, the most common implementations of DFT have almost
invariably started with the construction of orbitals. This need not be the
case, and Ho, Huang and Carter give us a summary of recent efforts to
introduce orbital-free formulations of DFT (OF-DFT), with particular
emphasis on the calculation of materials properties. The great advantage of
OF-DFT is that computation time scales linearly with the size of the system
for all sizes. This makes possible the accurate study of any property
determined by the ground state wavefunctions (structure, chemical bonding,
mechanical properties) for systems with as many as 105 atoms. By being able
to treat much larger samples, real materials issues – such as the study of
dislocations, cracking and long-range surface reconstructions – can be
treated with ab initio methods.
While the idea of structure sensitive reactions is a venerable concept
in heterogeneous catalysis [3, 4], the observation of high catalytic
activity for nanoscale gold clusters [5-8] was an extraordinary discovery.
What are the effects of particle size, quantum confinement and particle
morphology? Do interactions with the substrate or dopants activate the
entire particle or only certain atoms within the gold cluster? Chretien,
Buratto and Metiu take a look at recent experimental as well as theoretical
work pertaining to catalysis by gold nanoparticles. Because of the extreme
sensitivity of reactivity on size, it is important to study Au clusters in
a size selective fashion, which a mass-selective cluster deposition
technique allows.
The clusters used in these studies are so small (2–20 atoms), they are
termed molecular clusters, which makes them particularly amenable to
theoretical analysis. This size range is much smaller than the particles
found in conventional technical catalysts (usually with 100 or more atoms).
However, this size range is well suited to gradient corrected (generalized
gradient approximation or GGA) density functional theory (DFT)
calculations. Metiu and co-workers have applied such an approach to develop
understanding of trends in reactivity. By following trends, they are able
to avoid some of the difficulties that arise from trying to treat the
effects of oxide-support/catalyst-particle interactions.
On the synthetic side of nanoscience, scientists are constantly looking
for new pathways to nanostructures. Understanding the mechanisms of
structure formation is particularly important for complete control of these
synthetic methods. Kolasinski considers the solid-liquid phase transition
and how it has been involved in a number of techniques used to make
nanoscale structures. To understand structure formation during freezing,
one needs to understand the dynamics of melting and fusion as well as the
factors leading to fluctuations in the profile of a liquid. Thus, a brief
review of capillary waves and their properties as well as the ultrafast
dynamics of fusion is presented. All tolled, six mechanisms of structure
formation during the solid/liquid phase transition are considered: (i) the
Bally-Dorsey mechanism of solidification driven extrusion; (ii)
hydrodynamic sputtering; (iii) laser-induced periodic surface structures
(LIPSS); (iv) freezing of capillary waves; (v) the Mullins-Sekerka
instability; and (vi) laser zone texturing.
Detection with chemical specificity of monolayers has long been a
challenging aspect of surface science. This is all the more difficult when
one leaves the realm of small molecules in ultrahigh vacuum (UHV) and
attempts to detect larger molecules – even biomolecules – or performs the
detection in the presence of environmental interferences, as is necessary
in the field of sensors. Xing, Dementev and Borguet review recent work on
chemical labeling techniques, emphasizing and comparing detection with
fluorescence spectroscopy and the more conventional x-ray photoelectron
spectroscopy (XPS). Chemical labeling can facilitate more specific and
higher sensitivity detection of certain analytes once they are derivatized.
They report that fluorescence labeling of surface species (FLOSS) can
exhibit detection sensitivities as low as 109 cm2. FLOSS can also be
performed in essentially any environment, not just UHV, and is
nondestructive.
One method of increasing detection sensitivity of surface species is to
increase the surface area. Porous substrates lend themselves naturally to
this application. The review of Ozdemir and Gole reports on recent advances
in the use of porous silicon (por-Si) in small molecule (H2O, ethanol,
methanol, isopropanol, COx, NOx, NH3, O2, H2, HCl, SO2, H2S and PH3)
sensing. Porous silicon exhibits a number of properties, which lend
themselves to sensor applications. Transparent in the infrared, silicon is,
of course, compatible with integration into electronic circuits. Porous
silicon can also be made photo- or electroluminescent. Pore opening, pore
wall and layer dimensions are easily controlled [9], which also means that
properties such as the effective dielectric constant of the layer are
variable. These characteristics facilitate both electrical and optical
methods of detection [10-17]. Chemical functionalization can also be
applied to por-Si substrates [18-25]. The deposition of metals and metal
oxides can also influence and enhance sensing capabilities as reported by
Ozdemir and Gole. Importantly, they also demonstrate that as our
understanding and modeling of surface interactions improves, we can use
this knowledge to guide us in the choice and optimization of surface
coatings.
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