Chapter 1. Introduction, perspectives, and aims. On the science
.
of simulation and modelling. Modelling at bulk, meso, and nano
scale. (2 hours).
Chapter 2. Experimental Techniques in Nanotechnology. Theory
and Experiment: “Two faces of the same coin” (2 hours).
Chapter 3. Introduction to Methods of the Classic and Quantum
Mechanics.
Force
Fields,
Semiempirical,
Plane-Wave
pseudopotential calculations. (2 hours)
Chapter 4. Intoduction to Methods and Techniques of Quantum
Chemistry, Ab initio methods, and Methods based on Density
Functional Theory (DFT). (4 hours)
Chapter 5. Visualization codes, algorithms and programs.
GAUSSIAN, CRYSTAL, and VASP. (6 hours).
.
Chapter 6. Calculation of physical and chemical properties of
nanomaterials. (2 hours).
Chapter 7. Calculation of optical properties. Photoluminescence.
(3 hours).
Chapter
8.
Modelization
of
the
growth
mechanism
of
nanomaterials. Surface Energy and Wullf architecture (3 hours)
Chapter
9. Heterostructures Modeling. Simple and complex
metal oxides. (2 hours)
Chapter 10. Modelization of chemical reaction at surfaces.
Heterogeneous catalysis. Towards an undertanding of the
Nanocatalysis. (4 hours)
Chapter 2. Experimental Techniques in
Nanotechnology. Theory and Experiment: “Two
faces of the same coin”
Juan Andrés y Lourdes Gracia
Departamento de Química-Física y Analítica
Universitat Jaume I
Spain
&
CMDCM, Sao Carlos
Brazil
Sao Carlos, Novembro 2010
Experiment and Theory:
New strategies and
methodologies
Simulation
Oxford Dictionary Definition
“… produce a computer
model of (a process) “
Powerful and indispensable tools for
nanoscience/nanotechnology
SYNTHESIS
Obtaining tiny slabs that serve as precisely controlled
mockups of the real world catalysts.
1.
2.
3.
4.
5.
6.
7.
8.
Vapor Liquid Solid (VLS)
Chemical Vapor deposition (CVD)
Solid Vapor Deposition (SVD)
Single Source Chemical Vapor Deposition (SSCVD)
Litography
Laser Ablation
Sol-Gel
Template-Assisted Methods
M. L. Curri, R. Comparelli, M. Striccolia and A. Agostiano Phys. Chem. Chem. Phys., 2010, 12, 1119
Powerful and indispensable tools for
nanoscience/nanotechnology
EXPERIMENTS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Scanning Tunneling Microscopy (STM)
Scanning Electron Microscopy (SEM)
Energy-Dispersive X-ray Spectroscopy (EDX)
Transmission Electron Microscopy (TEM)
Selected Area Electron Diffraction (SAED)
X-ray Photoelectron Spectroscopy (XPS)
Powder X-ray Diffraction (XRD)
Electron Energy Loss Spectroscopy (EELS)
Raman Spectroscopy
Photolumuniscence (PL)
Cathodoluminiscence (CL)
In the last 30 years, we have seen an extraordinary
experimental advance on the techniques to produce, in a
controlled way, smaller and smaller structures, even in atomic
scale.
Parallel to these achievements, characterization
techniques have also matured in order to better understand
the properties of these materials.
Altogether, these factors are responsible for the rising of
nanoscience and nanotechnology.
Schwartz, D. A.; Norberg, N. S.; Nguyen, Q. P.; Parker, J. M.; Gamelin, D. R. J. Am. Chem. Soc. 2003, 125,
13205.
Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2001, 404,
59.
Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3620
Powerful and indispensable tools for
nanoscience/nanotechnology
Last but not least, theorists are employing ab initio
schemes or density functional theory to calculate
how molecules will stick to the nanoparticles and
interact.
THEORY
History (1)
All of Chemistry revolves around
swapping electrons, and theoretical and
computational methods and techniques
forecasting how atoms and molecules
will rearrange themselves and bond as
the electrons they share shift to
minimize energy.
History (2)
G.Whitesides
What Will Chemistry Do in the Next Twenty
Years?
Angew. Chem Int. Ed. Engl., 29, 1209 (1990)
The path of Chemistry in the future will be
determined by its generation of new ideas
through four basic research Areas
.Materials Chemistry
.Biological Chemistry
.Computational Chemistry
.Chemistry exploring the limits of size
and speed in chemical phenomena
Materials Chemistry
Biological Chemistry
Polymers
Molecular recognition
Surfaces and Interfaces
Evolution and self-assembly
Functional and “smart”materials
Bioenergetics
Materials for manufacturing
Environmentally compatible
materials
Computational Chemistry
Chemistry exploring the
Increasing power
limits of size and speed in
New architectures: massively
chemical phenomena.
parallel machines and neural nets
Exploring the limits : very small;
very fast; very large
Somorjai, G. A.; Levine, R. D. “The Changing
Landscape of Physical Chemistry at the Beginning of
the 21st Century” J. Phys. Chem. B 109, 9853 (2005).
“Now enter the nanosciences, which again are also
“There
changes
in the
way research
is
drivenare
bymajor
the environmental
needs
of occurring
technologies,
which
provide
“These
include
chemistry,
problems
of size
performed
chemistry.
This isofinmatter
part due
our
challengesinin
tophysical
learn
the
manipulation
on to
the
reduction
microelectronics
that led to the
rise of
success
in providing
an ever-increasing
science
component
nanoscale:
connecting
molecules and
studying
nanoscience
and
nanotechnologies,
and the
designtheir
of drugs
toself-assembly,
existing and emerging
technologies
that accelerates
their
optical,
electronic,
magnetic,
and implant devices
thatchemical,
extend human
life span
and sustain
need
for
even more.
Our ability
to study thethemes
science
and
mechanical
properties.
The centralizing
of of
the
health
of the human
body.”
chemical
us todominant
target major
physicalcomplexity
chemistry permitted
again become
at thescientific
start
and
societal
problems
require
interdisciplinary
of the
21st century,
justthat
as they
wereandominant
in the
approach”
early decades of the 20th century.”
THE ROYAL SWEDISH ACADEMY OF SCIENCES
The discovery of carbon
atoms bound in the form of
a ball is rewarded
Nanostructures
1996
THE ROYAL SWEDISH ACADEMY OF SCIENCES
Development of
computational methods in
chemistry awarded
Quantum Chemistry
1998
THE ROYAL SWEDISH ACADEMY OF SCIENCES
Femtochemistry
1999
For showing that it is possible with rapid laser
technique to see how atoms in a molecule move
during a chemical reaction.
“Chemistry is not solely an
experimental science
anymore”
Nobel Prize in Physics 2010
Andre Geim and Konstantin Novoselov
for their "groundbreaking experiments
regarding the two-dimensional material
graphene
M. Pumera, Chem. Soc. Rev., 2010, 39, 4146–4157
Nobel Prize in Chemistry 2010
Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki
winners for "developing new, more efficient
ways of linking carbon atoms together to
build the complex molecules that are
improving our everyday lives."
This was a reaction that was possible with other metals, but it did
not work very well. With palladium it worked much better.
One of the main features of the reactions is that they are
catalytic processes that allow synthetic chemists to do things which
they could not previously do - to join carbon atoms together in a
new way,'
To illustrate the diversity and importance of the palladium-catalysed
cross coupling reactions:
- Total synthesis of the anticancer drug Taxol (paclitaxel)
- The Heck reaction is also used to make a strategic bond in a
synthesis of morphine.
- Negishi coupling was key to the laboratory synthesis of the natural
product hennoxazole A, a marine antiviral compound.
- Suzuki coupling is used to prepare the antiviral bromoindole
alkaloid dragmacidin.
These are merely a handful of examples of palladium-catalysed
cross coupling, which has been used - and continues to be used - in
the synthesis of thousands of important compounds, from the most
complex natural products to tonne-scale industrial intermediates.
“Over the last years, firstprinciples calculations have
become recognized as an
outstanding tool so as to
elucidate the electronic
structure of crystalline
materials”
“Theory and experimentation combine today
in the search for understanding of the inner
structure of matter”
W. Kohn, Rev. Mod. Phys, 1999, 71, 1253 (Nobel Lecture)
Electronic structure of matter-wave functions and density functionals
“.....for his development of the density-functional theory.....”
J. A. Pople, Rev. Mod. Phys, 1999, 71, 1267 (Nobel Lecture)
Quantum chemical models
“.....for his development of computational methods in
quantum chemistry.....”
Basic Challenges
Since chemistry concerns the study of properties of substances or
molecular systems in terms of atoms, the basic challenge facing
computational chemistry is to describe or even predict.
1. the structure and stability of a molecular system.
concerns prediction of which state of system has the lowest energy.
2. the (free) energy of different states of a molecular system.
involves prediction of the relative (free)energy of different states.
3. reaction processes within molecular systems in terms
of interactions at the atomic level.
involves prediction of the dynamic process of change of states.
1<2<3
Increasing difficulty
Theory vs. Experiment
Modern research in the chemical sciences seeks not only to make
useful molecules and materials but to understand, design, and control
their properties. Theory is at the very center of this effort, providing
the framework for an atomic and molecular level description of
chemical structure and reactivity that forms the basis for interpreting
experimental data and provides guidance toward new experimental
directions.
Theoretical and computational chemistry has developed into an
important tool in almost all areas of chemistry. Their methods and
techniques have found its way into the everyday work of many
experimental chemists. Calculations can predict the outcome of chemical
reactions, afford insight into reaction mechanisms, and be used to
interpret structure and bonding in molecules. Thus, contemporary
theory offers tremendous opportunities in experimental chemical
research.
Theory vs. Experiment
Combined experimental and computational studies of chemical
reactivity can yield remarkable insight into reaction mechanisms and
kinetics. This is particularly true for chemical reaction taking place in very
tight places, involving unusual mechanistic features.
Physics-based simulations complement experiments in building a
molecular-level understanding: they can test hypotheses and interpret
and analyse experimental data in terms of interactions at the atomic level
not available experimentally.
The joint use of both theoretical and experimental results also
suggests additional experiments and simulations that can further increase
our knowledge.
The insights gained from simulation are synergistic with those that
arise from new experiments, and sometimes they lead the way on
problems where experiments are not available.
Feymann, R. P.
Eng. Sci. 23, 22 (1960).
“The principle of Physics as far as I can see, do
not speak against the possibility of maneuvering
things atom by atom .”
Quantum Mechanics
Theoretical and Computational Chemistry
THEORETICAL AND COMPUTATIONAL CHEMISTRY
Fundaments
Methods
Techniques
Quantum Mechanics
Classical
Statistical
Mechanics
Classification of Molecular Systems
CRYSTALINE
SOLID STATE
LIQUID STATE
MACROMOLECULES
GAS PHASE
Possible
Possible
Easy
Easy
Easy
Trivial
Quantum
Mechanics
Classical
Statistical
Mechanics
REDUCTION
to the few degrees
of freedom by
symmetry
Essential
many-particle
system
REDUCTION
to few particles
by
dilution
Points 1. 2. & 3. allows us to say
“we can actually start to observe
phenomena at the atomic scale
under realistic conditions.”
The
dream
of
Richard
Feynman (in 1960’s) is fulfilled!
P.
Methods & Techniques of
Theoretical and Computational
Chemistry
 Prediction
 Interpretation
 Characterization
of chemical species of
difficult (in some cases)
experimental detection
 Understanding physical and chemical
properties at atomic level
Computational and Theoretical Chemistry
- Energy (DE, DG, DH and DS)
- Ionization potential (IP)
- Electron affinity (EA)
- Geometry (bond distance, bond angle and dihedral angle)
- Electronic properties
(molecular orbitals, density of states, band gap)
- Vibrational Frequencies, IR
(analysis of stationary points; R, P, I and TS structures)
- Analysis of Potential Energy Surfaces
(crossing points, valley ridge inflexion points, conical intersections)
- Electron density
(topological analysis: AIM, ELF)
Computational and Theoretical Chemistry
Software
GAUSSIAN (2009)
CRYSTAL (2009)
VASP
GAMESS
MOLCAS
ADF
XcrysDen
TopMod
Hardware
Silicon Graphics MIPS R14k 400MHz
PC/Linux Cluster, AMD +2200MP
Theoretical
work
Experimental
work
complementary tools
The cooperation between both
worlds is mandatory
Interaction between Experiments, Analytical Theories, and Computation
R. A. Marcus, J. Phys. Chem. C 2009, 113, 14598–14608
We all recognize that one of the main goals in research is to capture
the physical essence of a phenomenon and use it not only to interpret
but also to predict the results of new experiments. One view of theory,
demonstrated in the present article, is that experiments are primary,
often the source of new theory, and that the interaction of theory and
experiment is paramount, each stimulating the other.
Nevertheless, discerning basic theoretical problems in the wealth of
available experimental and computational results can be a major
hurdle and sometimes the development of the theory can be relatively
rapid once the existence of an experimental puzzle is known. The
writer continues to be impressed with this exciting interplay of
experiment and theory and with many experimental puzzles that exist
and that continue to arise in new experiments, when one keeps an eye
out for them. For the theoretically oriented students it is perhaps a
truism to add that the broader one’s background is in physics,
chemistry and mathematics, and the more one is familiar with the new
results and the potential and limitations of new techniques, the larger
the range of interesting problems that one can address.
Experiment and Theory in Harmony
Mark A. Johnson at Yale University discusses how the two sides of physical
chemistry have necessarily developed together, and looks at how their
synergy dictates the direction of contemporary research.
Equations such as
Schrödinger’s famous
contribution to quantum
mechanics underpin
much of physical chemistry.
Nature Chemistry, 1, 8 (2009)
Experiment and Theory in Harmony
Physical chemists seek to anchor the empirical rules of chemistry
to the laws of physics, and thus provide secure concepts to explain
the trends seen in reactivity and molecular structure.
A recurrent theme in contemporary physical chemistry is a
convergence of experimental and theoretical methods towards
sufficiently complex model systems. By this I mean systems
that not only reproduce ‘real’ chemical processes but also do so in
a fashion that reveals molecular level, quantum-mechanically
consistent pictures that are not greatly obscured by either thermal
or ensemble averaging.
Nature Chemistry, 1, 8 (2009)
Controlling the properties of nanostructures requires a detailed
understanding of structure, microstructure, and chemistry at everdecreasinglength scales. The modern day transmission electron microscope
has thus become an indispensable tool in the study of nanostructures. In this
Perspective,we present a brief account of the capabilities of the TEM with
some typical examples for characterizing nanostructures. The modern-day
TEM has moved from a simple characterization tool to a nanoscale
laboratory enabling in situ observation of several fundamental processes at
unprecedented resolution levels.
N. Ravishankar, J. Phys. Chem. Letters, 2010, 1, 1212–1220
technique (spatial resolution)
information
imaging
mass thickness contrast (>1 nm)
diffraction contrast (~1 nm);
bright field/dark field imaging
phase contrast (<0.1 nm); high
resolution imaging
Z-contrast (<0.1 nm); high-angle
annular dark field imaging
distinguishing particles with large
difference in average Z
phases, defects, orientation relationship,
growth direction, morphology
atomic structure of defect-free and
defect-containing crystals
atomic level distribution of high Z
elements
diffraction
selected area diffraction (500
nm) microdiffraction/nanobeam
diffraction (1-10 nm)
convergent beam electron
diffraction (1-10 nm)
orientation, crystal structure
orientation, local structure
point group/space group information
spectroscopy
X-ray energy-dispersive
spectroscopy (10 nm) electron
energy loss spectroscopy (<1
nm)
composition, elemental mapping
elemental mapping (including light
elements),
The future of nanotechnology rests upon approaches to making
new, useful nanomaterials and testing them in complex systems.
Currently, the advance from discovery to application is
constrained in nanomaterials relative to a mature market, as seen
in molecular and bulk matter. To reap the benefits of
nanotechnology, improvements in characterization are needed to
increase throughput as creativity outpaces our ability to confirm
results. The considerations of research, commerce, and regulation
are part of a larger feedback loop that illustrates a mutual need for
rapid, easy, and standardized characterization of a large property
matrix. Now, we have an opportunity and a need to strike a new
balance that drives higher quality research, simplifies commercial
exploitation, and allows reasoned regulatory approaches.
Erik K. Richman and James E. Hutchison VOL. 3 ▪ NO. 9 ▪ 2441–2446 ▪ 2009
Techniques for nanoscale structure
determination
Surface science techniques are characterized by their
ability to provide sensitivity to a slice of material with
nanoscale thicknesson top of a single-crystal substrate.
The blossoming of nanoscience and nanotechnology
requires adapting or developing appropriate techniques of
characterization with additional nanoscale resolution in one
or two of the other dimensions.
The challenge of detailed atomic-level structure (bond
lengths and bond angles) in such nanomaterials is even
more formidable, especially if we wish to keep a threedimensional spatial resolution in a single nanoparticle.
Techniques for nanoscale structure
determination
A recent overview of the issues involved is available in a recent
review article [4], so we will only discuss the most promising
techniques here, with special focus on STM and LEED since these
methods are not covered in any detail in that review.
At present, it appears that mainly STM, XRD (x-ray diffraction)
and high-energy electron diffraction are ready for the task of
detailed nanostructure determination, while XAFS may provide
such information in conjunction with other techniques: no other
technique can at this stage, to my knowledge, find the atomic
positions in an individual nanostructure with an accuracy of the
order of 0.1 Å = 0.01 nm.
Techniques for nanoscale structure
determination
Scanning tunneling microscopy (STM)
STM unquestionably dominates the field of structural analysis of
nanostructures [5]. However, a visual inspection of STM images is not
sufficient to extract bond lengths and angles, except those parallel to an
extended surface that provides some reference ‘‘yardstick” such as known
bulk lattice constants.
In fact, even in a qualitative sense, a visual inspection of an STM
image is known to often give incorrect answers: to equate ‘‘bumps” in
‘‘topographic” images with atoms or even electronic orbitals has been
shown in many cases to lead to gross errors of interpretation.
For example, oxygen atoms often appear as dips when one expects to
see bumps, as happens when they are adsorbed on various metal
surfaces; this and other examples are discussed in Ref. [6]. It is safe to
assume that a substantial fraction of published visual interpretations of
STM images are simply wrong in some atomic-scale conclusions.
Techniques for nanoscale structure
determination
Scanning tunneling microscopy (STM)
Many theoretical models of STM have been developed since the
early work of Tersoff and Hamann [7,8]. Most of the basic principles
governing the current or topographic contrast recorded in an STM
image are now well understood in terms of the electronic and atomic
structures of both the tip and surface being probed, together with
their interactions [9].
In particular, the STM tip has to be treated on an equal footing
with the sample to be probed: the geometry and electronic structure
of the tip can affect the image as much as the sample’s properties.
This becomes particularly important for nanostructures with corners
and edges, which can look like tips from the point of view of the STM
‘‘tip”.
Techniques for nanoscale structure
determination
Scanning tunneling microscopy (STM)
A theory which has revealed itself to be computationally fast,
convenient and remarkably realistic for calculating STM images is
the elastic-scattering quantum chemistry (ESQC) method [10–12], in
spite of the use of the simple Extended Hückel Theory (EHT) to
describe the electronic structure.
It has been successfully applied to a variety of surfaces, although
rarely to fit unknown atomic positions to experimental images. One
example of structural determination by such fitting is the case of S
atoms adsorbed on Mo(100), for which the S height above the Mo
substrate was obtained [13,14].
Techniques for nanoscale structure
determination
Scanning tunneling microscopy (STM)
More recently, ab initio based formalisms relying on
Bardeen’s approximation to the electron current have been
successfully applied to semiconductor and metal surfaces
[8,15].
Such codes, however, involve large computer resources
and become inefficient when dealing with complex systems for
which many possible structural configurations need to be
explored, especially when there is a need to simulate entire
images rather than just a few scan lines.
Techniques for nanoscale structure
determination
Scanning tunneling microscopy (STM)
While most STM computer codes are set up for periodic
twodimensional surfaces, their application to non-periodic systems
such as nanostructures is feasible to some degree through the
subterfuge of periodic boundary conditions (i.e. repeating tipnanostructure units), thanks to the relative locality of STM tunneling.
Converting periodic codes to non-periodic ones is also an
option, even though this may require more computing resources.
Techniques for nanoscale structure
determination
X-ray diffraction (XRD)
X-ray diffraction has recently started to be applied to
nanoparticles [16–21]. Several of these studies obtain the particle
shape and/or average lattice parameters, without determining
localatom-by-atom deviations from such average lattice constants.
The challenge with XRD is to obtain a measurable signal,
usually requiring enough identical nanoparticles, thus also
demanding sufficient uniformity of size and orientation.
Techniques for nanoscale structure
determination
X-ray diffraction (XRD)
A very successful example is the structure determination
of 102-atom gold clusters, each coated in a layer of pmercaptobenzoic acid molecules and then crystallized [18].
Their analysis revealed an unexpected chiral structure
with 5-fold axial symmetry: the Au core can be viewed as
five twinned face-centered cubic crystallites.
Techniques for nanoscale structure
determination
X-ray diffraction (XRD)
Another example concerns Co nano-islands on Cu(001) [21],
for which the so-called mesoscopic misfit was investigated. The
authors observe that small Co islands (1–2 nm in diameter)
deposited on Cu(001) at 170 K (the total coverage is in the 0.1–
0.5 ML range) show significant static disorder, i.e. many Co atoms
are positioned somewhat away from the ideal hollow sites.
This is due to the fact that the Co–Co distance is observed to
be sharply reduced (by up to 10%) in these islands as compared
to the bulk.
Techniques for nanoscale structure
determination
X-ray absorption spectroscopy (XAS)
A natural technique for investigating nanoparticles is x-ray
absorption spectroscopy (XAS), including in particular extended xray absorption fine structure (EXAFS), and x-ray absorption nearedge structure (XANES, also called near-edge x-ray absorption
fine structure or NEXAFS):
this technique inherently focuses on structure around a central
atom [22,23], and is thus less dependent on periodic ordering than
XRD or LEED.
Techniques for nanoscale structure
determination
X-ray absorption spectroscopy (XAS)
One example is provided by the study of carbon-supported Pt
nanoparticles in the 2–6 nm diameter range [22]. An fcc packing
of the Pt atoms and a hemispherical cluster shape were in this
case suggested by STEM (scanning transmission electron
microscopy) and supported by the EXAFS data.
Using temperature-dependent EXAFS data, it is possible to
distinguish static disorder (in this case atomic relaxations from
bulk-like positions) from thermal disorder (random displacements
due to thermal vibrations).
Techniques for nanoscale structure
determination
X-ray absorption spectroscopy (XAS)
This provides a distribution of interatomic distances in the
nanoparticles. However, it is difficult to assign specific
displacements to specific atoms, so that a relatively simple model
must be fit to the available data.
For the smallest nanoparticles (2 nm), one model yielded an
average first-nearest-neighbor distance reduced by 0.002 nm or
0.7% relative to bulk Pt and largest individual reductions about 6
times larger.
Techniques for nanoscale structure
determination
Low-energy electron diffraction and NanoLEED
LEED was developed over the last half century to
measure the atomic structure of an extended surface of
about a millimeter in size to a depth of about a
nanometer, as given by the cross-section of the typical
LEED beam and by the electron mean-free path,
respectively [24].
Techniques for nanoscale structure
determination
Low-energy electron diffraction and NanoLEED
In that well-tested and successful implementation, LEED
has solved some 1000 surface structures of great variety
[25].
This implementation also allows determining the structure
of some nanostructures, for example C60 buckyballs
adsorbed in a periodic (4 x 4) lattice on an extended Cu(111)
surface [26]: as long as the nanostructure is periodic with a
unit cell that is not too large, a conventional LEED analysis is
possible.
Techniques for nanoscale structure
determination
Low-energy electron diffraction and NanoLEED
A different approach for LEED would be to sample a
single – and thus non-periodic – nanostructure, for
instance a single nanodot, nanotube or nanowire
(attached to a surface or hanging from supports).
This could be achieved experimentally by narrow or
focused LEED beams: two approaches have already
been proposed, as discussed next.
Techniques for nanoscale structure
determination
Low-energy electron diffraction and NanoLEED
It should be possible to focus the incident LEED beam onto a
small area, as can currently be done in low-energy electron
microscopy (LEEM) on the scale of 250 nm, including in future
onto smaller areas.
Diffraction from single objects as small as a few nanometers is
conceivable. Electron beams have in fact been focusedto
dimensions in the 50 nm range in various applications [27].
Then the diffracted pattern can be recorded, either as angular
dependent intensity data or as energy-dependent data (‘‘I–V
curves”).
Techniques for nanoscale structure
determination
Low-energy electron diffraction and NanoLEED
This idea has been proposed theoretically in the form of
convergent- beam LEED (CBLEED) [28]. The angular spread
of the converging beam then implies a corresponding
broadening of the diffraction pattern.
In the case of an ordered structure, the sharp spots of
normal LEED would be replaced by disks delimited by the
angular spread of the convergent beam: these disks contain
angle- dependent intensities that provide additional structural
information not present in sharp spots.
Techniques for nanoscale structure
determination
Low-energy electron diffraction and NanoLEED
Even for a diffuse LEED pattern (without sharp spots
due to absence of long-range periodicity) this would still
be valuable if this spread is taken into account in the
calculation through convolution (as is already commonly
done, for example, in photoelectron diffraction to reflect
the angular aperture of the detector [29]).
Techniques for nanoscale structure
determination
Low-energy electron diffraction and NanoLEED
Another approach is to use as electron source an
STM tip located tens or hundreds of nanometers from
the nanostructure; this tip serves to emit a very narrow
beam with angular spread of only
about 5º [30].
Such an experiment has already produced LEED
patterns from areas as small as 400 lmacross, with
areas smaller than 50 nm across being possible.
Techniques for nanoscale structure
determination
Low-energy electron diffraction and NanoLEED
On the theoretical side, two new features must be
addressed to analyze measured LEED intensities: a
convergent incident beam and the greater structural
complexity of a nanostructure compared with typical
periodic unit cells on an extended surface.
These challenges are met in a new method, called
NanoLEED, that we have implemented in recent years
[31–34].
Techniques for nanoscale structure
determination
High-energy electron diffraction
Transmission electron microscopy (TEM) and high-resolution
electron microscopy (HREM) can certainly image nanostructure at
the atomic scale, including for single nanostructures, but do not
provide three- imensional structure.
However, a closely related approach, under the name
coherent electron diffraction (CED), has very recently been used to
determine relaxations of the surface of single Au nanocrystals
(supported by graphene) of diameter 3-5 nm [46].
Techniques for nanoscale structure
determination
High-energy electron diffraction
Thus, the 200 kV electron diffraction pattern of a
single 4 nm nanocrystal was fit with simple models of
atomic relaxation, giving radial bond length
contractions up to 8% relative to the interior of the
particle: the larger values occur for atoms that have
lower coordination, as expected.
The Chemical Structure of a Molecule Resolved by
Atomic Force Microscopy
Resolving individual atoms has always been the ultimate goal of surface
microscopy. The scanning tunneling microscope images atomic-scale
features on surfaces, but resolving single atoms within an adsorbed
molecule remains a great challenge because the tunneling current is
primarily sensitive to the local electron density of states close to the
Fermi level. We demonstrate imaging of molecules with unprecedented
atomic resolution by probing the short-range chemical forces with use of
noncontact atomic force microscopy. The key step is functionalizing the
microscope’s tip apex with suitable, atomically well-defined terminations,
such as CO molecules. Our experimental findings are corroborated by
ab initio density functional theory calculations. Comparison with theory
shows that Pauli repulsion is the source of the atomic resolution,
whereas van der Waals and electrostatic forces only add a diffuse
attractive.
Leo Gross, Fabian Mohn, Nikolaj Moll, Peter Liljeroth, Gerhard Meyer
SCIENCE 325 1110 (2009)
Top Catal (2010) 53:832–847
Ex-situ characterization
• Transmission electron microscopy (TEM)
• X-ray diffraction (XRD)
• Diffuse reflectance UV–Vis spectroscopy X-ray photoelectron
spectroscopy (XPS)
• Scanning electron microscopy (SEM)
• Chemisorption, physisorptionSmall angle X-ray scattering (SAXS)
• Energy dispersive X-ray analysis (EDX)
• Thermogravimetric analysis (TGA)
• Temperature programmed oxidation (TPO)
• Inductively coupled plasma–optical emission spectroscopy (ICP–
OES)
Top Catal (2010) 53:832–847
Spectroscopies
characterization
and
microscopy
for
in
situ
• High-pressure scanning tunneling microscopy (HP-STM)
• Sum frequency generation spectroscopy (SFG)
• Ambient-pressure X-ray photoelectron spectroscopy (APXPS)
• Diffuse reflectance infrared spectroscopy (DRIFTS)
• UV-Raman and surface enhanced raman spectroscopy (SERS)
• Transmission electron microscopy (TEM)
• Tapered element oscillating microbalance (TEOM)
• Thermogravimetric analysis (TGA)
• UV–Vis diffuse reflectance spectroscopy
• X-ray diffraction (XRD)
• Small-angle/wide-angle X-ray scattering (SAXS-WAXS)
• Near-edge X-ray absorption fine structure (NEXAFS)
• Extended X-ray absorption fine structure (EXAFS)