Motivation
Rapid developments in nanoscience and nanotechnology make it essential to be able to make spectroscopic measurements at the nanometer scale. The existence of quantum confinement effects in nanometer scale structures is a large part of what drives interest in this field; the strong dependence of these effects on individual structure sizes and the potential for strong coupling of levels from closely spaced structures renders the overall performance of assemblies of small structures complex, and difficult to predict. In addition, segregation at interfaces, particularly those to nanostructures will dramatically affect both electronic and transport properties across them. Many of the technologies earmarked for significant growth in the coming decades are critically dependent on credible nanometer-scale measurements and analysis. These certainly include high-density electronics, biosensors, high throughput experimentation, separations, and catalysis. Development of robust, nanoscale measurement tools is critically important if the full potential of these technologies is to be realized.
The spatial resolution available in a spectroscopic measurement depends both on the size of the probe beam, and on the spreading of the excited region due to either carrier diffusion or energy transfer through a system. In the case of incident visible or UV light, the diffraction limit until recently seemingly prevented resolution at much below 0.1 um. Focused x-ray sources provided insufficient incident flux to make nm scale spectroscopy possible. Electron beams can be focused to sub-nm dimensions, but only at high energies, so that the electrons must undergo a large number of inelastic scatterings to drop into a bound energy level, again leading up until recently to resolution no better than 0.1 um. A number of approaches have been invented in recent years to avoid these limitations. Important examples are: Near-field scanning optical microscopy beats the diffraction limit through the use of an aperture. The advent of ultra-bright third generation synchrotron sources now allows focusing of x-ray beams to a few tens of nm with exceedingly high incident fluxes. The use of the scanning tunneling microscope allows injection of electrons or holes into a structure with 0.1 nm incident beam diameter, and at energies close to resonance with bound levels, greatly reducing their diffusion lengths.
Scope of the Workshops
The development of powerful new
techniques and the modification of existing techniques for the measurement of
spectroscopy with spatial resolution approaching the nanometer scale in recent
years bring forth an unprecedented level of understanding as to the
relationship between size/structure/composition/strain in nanometer sized
structures, and the resultant electronic structure. Briefly put, these techniques make possible
the interrogation of individual nanostructures, or small numbers of coupled
nanostructures, rather than ensembles, so that the above mentioned
relationships and the coupling of energy levels between neighboring structures
can be systematically studied. These
workshop serve as a forum for scientists from the
One
of the focus areas sessions at NSS3 was interfaces to nanostructures, and how
to characterize them. Presentations in
this area described results of transport though nanotubes/metal, metal/molecule
and semiconductor/metal interfaces, including vibrational spectroscopy derived
from these measurements. Measurements of
this type contribute to our understanding of the nature of the transport across
these interfaces. A
crucial issue for application of nanostructures in logic, or sensing
applications is our ability to produce reliable, low impedance contacts to
these structures, to allow transport of signals into and out of them. It has
been recognized at least since the seminal work of Landauer that transport through a nanoscale structure
is determined not merely by its intrinsic conductance but also by the
transmission probabilities across the interfaces bounding it. Recent comparisons of measurements and
calculations of the conductance profile for a simple case, that of single
dithiol molecules between gold contacts show that these transmission
probabilities can dominate the transport, resulting in measured values orders
of magnitude below that for the molecule itself. Measured electrical transport through carbon
nanotubes has also indicated that the barriers formed at the contacts determine
the electrical character of devices based upon these structures. Indeed, this has led to a pronounced
sensitivity the way in which the contacts are formed and subsequently
processed, e.g. annealing in vacuum.
Direct measurement of the electrostatic potential across metallic
nanowires by scanned gate microscopy in fact indicates that the resistance at
the contacts can account for almost the entire potential drop across them. This session included contributions both from
experimentalists and from theorists doing first principles calculations of
transport through these structures, and provided for a very lively discussion
on this crucial issue.
A second focus area
addressed critical issues in quantum dots, including characterization of size,
shape, composition and correlation to electronic and optical properties,
requiring advanced nanoscale spectroscopies such as optical near-field
spectroscopy. Knowledge gained from such
measurements is needed to feed back into the development of functionalized
quantum dot materials that can enable new technologies such as spintronics and
quantum computing. Additional important issues involve achieving electronic
control of individual quantum dots using local external fields, for example,
with an STM tip or an optical laser pulse.
Coherent optical control of the wavefunction of individual quantum dots
using nano-optical techniques could eventually enable solid state quantum
computing.
A
third focus area was single molecule spectroscopies (SMS), which are of great
interest at present. From a scientific viewpoint, the greatest strength of single molecule spectroscopy
lies in our ability to resolve structural and dynamic heterogeneity masked by
conventional ensemble measurements. SMS methods are particularly suited
for the investigation of complex kinetic processes, determination of structural
heterogeneity of complex systems, and materials characterization
on molecular length scales. SMS techniques can be applied in a host
of sample matrices from glassy materials to living cells and promise to be
important in addressing unanswered questions such
as the origin of the observed “blinking” phenomenon in quantum dots and the
molecular nature of "hot spots" in surface
enhanced Raman scattering
The development of new techniques which allow
probing of individual nanostructures or small numbers of nanostructures
continues to shed new experimental light on the interrelationship between
synthesis, structure and properties of materials, enhancing our understanding
and driving further development of theoretical models to explain unexpected
observations. At the heart of the
excitement of nanoscience is the fact that it provides working models in which
one of the great intellectual feats of the twentieth century, the development
of quantum mechanics, can be tested. The
goal of research in nanoscale spectroscopy is to extend our understanding of
fundamental issues in nanoscale science.
These include the relationship between structure and electronic energy
levels at the nanoscale, the coupling of energy levels in neighboring mesoscale
structures, the mechanisms which limit the coherence length in coupled
structures, and how transport occurs across interfaces between nanostructures.
NSS3 provided a forum for discussion of new experimental observations, how
these can be understood within a theoretical framework, and how they require
the modification of existing models of phenomena at the nanoscale.
Program
Participation was from
approximately 70 scientists and students from the
|
Stefan Eisebitt |
Bessy II |
Lensless X-ray Imaging |
|
Hendrik Ohldag |
SSRL, Stanford |
XPEEM from Antiferromagnets |
|
Barry Barker |
Lab for Physical Sciences |
STS at and Below 4K |
|
Harald Ade |
NC |
Soft X-ray characterization
in real and reciprocal space |
|
Saw-Wai Hla |
|
Single Atom Manipulation |
|
Markus Morgenstern |
RWTH |
Wavefunction Mapping in different dimensions |
|
Adrian Wetzel |
|
Time of flight SPM |
|
Jun Cheng |
|
Coherent Optical Spectroscopy of single quantum dots |
|
Allan Bracker |
Naval Research Labs |
Optical pumping of spin in single charged quantum dots |
|
David Fromm |
|
Optical Properties of Nanoscale Metallic Structures |
|
Bernd Kabius |
|
Energy-Filtered TEM, and the TEAM instrument |
|
Andy Lupini |
|
STEM-based EELS and Applications to Nanoscience |
|
Vladimir Oleshko |
|
PEEL Spectroscopy and EFTEM |
|
Chris Davis |
|
Optical Properties of
Nanohole arrays |
|
Igor Smolyaninov |
|
Far field microscope with
nm resolution |
|
Lukas Novotny |
|
Nanoscale Optical Spectroscopy |
|
Bennett Goldberg |
|
Resonant microRaman and
light spectroscopy from Nanotubes |
|
Paul Barbara |
Univ. of Texas-Austin |
Single Molecule Spectroscopy |
|
Ken Shih |
Univ. of Texas-Austin |
Rabi flopping on a single quantum dot |
|
James Kushmerick |
Naval Research Labs |
Understanding Charge Transport in Molecular
Electronics and Vibrational
Spectroscopy |
|
Massimilliano DiVentra |
|
First Principles
Calculations of Transport across interfaces to molecules & nanostructures |
|
Michael Fuhrer |
|
Probing Energy States of
Molecules and Nanotubes through Transport |
|
Igor Smolyninov |
|
Far-field optical microscope with nanometer-scale
resolution |
|
Yoichi Uehara |
|
Light Emission STM |
|
Yoshio Watanabe |
NTT-Japan |
SPELEEM observation of individual single-walled carbon
nanotubes |
|
Emil Zolotoyabko |
Technion |
Stroboscopic X-Ray Imaging |
Invited speakers who participated in NSS3.
Organization
The local organization
committee consisted of Ray
Phaneuf, Dennis
Drew, Doug
English, and Janice
Reutt-Robey (University of Maryland), Stephan
Stranick (NIST), and Dan Gammon (NRL).The international program committee
for the workshop consists of Ray Phaneuf, (University of Maryland), Doug
English, (University of Maryland), Stephan Stranick, (NIST), Giancarlo Salviati
(IMEM-CNR, Italy), Stefan Heun (Laboratorio TASC-INFM), Marcello
Colocci (University of Firenze, Italy), and from Japan: Yoichi
Uehara (Tohoku University, Japan), Takashi Sekiguchi
(NIMS, Japan) and Yoshio
Watanabe (NTT, Japan).
Photos from the Third International
Workshop
(Click here for additional photos)
The workshop was held at the
Laboratory for Physical Sciences (LPS),
at the
Previous Workshops
The International Workshops on Nanoscale
Spectroscopy were created to serve as a forum for the exchange of information
about applications and new developments of these techniques in the rapidly
growing field of nanoscience and technology.
The first workshop on Nanoscale
Spectroscopy and its Applications to Semiconductor Research was held at the
Acknowledgements
Support was generously
provided by LPS, by a National Science Foundation Materials Research and
For more information on NSS3, contact:
Department of Materials Science and Engineering
Tel.: 1-301-935-6473
Fax: 1-301-935-6723
Organizer, Third International Workshop on Nanoscale Spectroscopy and Nanotechnology