275 research outputs found
Quantum confined electronic states in atomically well-defined graphene nanostructures
Despite the enormous interest in the properties of graphene and the potential
of graphene nanostructures in electronic applications, the study of quantum
confined states in atomically well-defined graphene nanostructures remains an
experimental challenge. Here, we study graphene quantum dots (GQDs) with
well-defined edges in the zigzag direction, grown by chemical vapor deposition
(CVD) on an iridium(111) substrate, by low-temperature scanning tunneling
microscopy (STM) and spectroscopy (STS). We measure the atomic structure and
local density of states (LDOS) of individual GQDs as a function of their size
and shape in the range from a couple of nanometers up to ca. 20 nm. The results
can be quantitatively modeled by a relativistic wave equation and atomistic
tight-binding calculations. The observed states are analogous to the solutions
of the text book "particle-in-a-box" problem applied to relativistic massless
fermions.Comment: accepted for publication in Phys. Rev. Let
Automated structure discovery in atomic force microscopy
Atomic force microscopy (AFM) with molecule-functionalized tips has emerged as the primary experimental technique for probing the atomic structure of organic molecules on surfaces. Most experiments have been limited to nearly planar aromatic molecules due to difficulties with interpretation of highly distorted AFM images originating from nonplanar molecules. Here, we develop a deep learning infrastructure that matches a set of AFM images with a unique descriptor characterizing the molecular configuration, allowing us to predict the molecular structure directly. We apply this methodology to resolve several distinct adsorption configurations of 1S-camphor on Cu(111) based on low-temperature AFM measurements. This approach will open the door to applying high-resolution AFM to a large variety of systems, for which routine atomic and chemical structural resolution on the level of individual objects/molecules would be a major breakthrough
Structure discovery in Atomic Force Microscopy imaging of ice
The interaction of water with surfaces is crucially important in a wide range
of natural and technological settings. In particular, at low temperatures,
unveiling the atomistic structure of adsorbed water clusters would provide
valuable data for understanding the ice nucleation process. Using
high-resolution Atomic Force Microscopy (AFM) and Scanning Tunnelling
Microscopy, several studies have demonstrated the presence of water pentamers,
hexamers, heptamers (and of their combinations) on a variety of metallic
surfaces, as well the initial stages of 2D ice growth on an insulating surface.
However, in all these cases, the observed structures were completely flat,
providing a relatively straightforward path to interpretation. Here, we present
high-resolution AFM measurements of several new water clusters on Au(111) and
Cu(111), whose understanding presents significant challenges, due to both their
highly 3D configuration and to their large size. For each of them, we use a
combination of machine learning, atomistic modelling with neural network
potentials and statistical sampling to propose an underlying atomic structure,
finally comparing its AFM simulated images to the experimental ones. These
results provide new insights into the early phases of ice formation, which is a
ubiquitous phenomenon ranging from biology to astrophysics
Bistability in the Tunnelling Current through a Ring of Coupled Quantum Dots
We study bistability in the electron transport through a ring of N coupled
quantum dots with two orbitals in each dot. One orbital is localized (called b
orbital) and coupling of the b orbitals in any two dots is negligible; the
other is delocalized in the plane of the ring (called d orbital), due to
coupling of the d orbitals in the neighboring dots, as described by a
tight-binding model. The d orbitals thereby form a band with finite width. The
b and d orbitals are connected to the source and drain electrodes with a
voltage bias V, allowing the electron tunnelling. Tunnelling current is
calculated by using a nonequilibrium Green function method recently developed
to treat nanostructures with multiple energy levels. We find a bistable effect
in the tunnelling current as a function of bias V, when the size N>50; this
effect scales with the size N and becomes sizable at N~100. The temperature
effect on bistability is also discussed. In comparison, mean-field treatment
tends to overestimate the bistable effect.Comment: Published in JPSJ; minor typos correcte
Muonium in nano-crystalline II-VI semiconductors
http://www.sciencedirect.com/science/article/B6TVH-4V2NP2J-Y/2/3738c97c2d99528da5d86b486571793
Identification of Colletotrichum species associated with anthracnose disease of coffee in Vietnam
Colletotrichum gloeosporioides, C. acutatum, C. capsici and C. boninense associated with anthracnose disease on coffee (Coffea spp.) in Vietnam were identified based on morphology and DNA analysis. Phylogenetic analysis of DNA sequences from the internal transcribed spacer region of nuclear rDNA and a portion of mitochondrial small subunit rRNA were concordant and allowed good separation of the taxa. We found several Colletotrichum isolates of unknown species and their taxonomic position remains unresolved. The majority of Vietnamese isolates belonged to C. gloeosporioides and they grouped together with the coffee berry disease (CBD) fungus, C. kahawae. However, C. kahawae could be distinguished from the Vietnamese C. gloeosporioides isolates based on ammonium tartrate utilization, growth rate and pathogenictity. C. gloeosporioides isolates were more pathogenic on detached green berries than isolates of the other species, i.e. C. acutatum, C capsici and C. boninense. Some of the C. gloeosporioides isolates produced slightly sunken lesion on green berries resembling CBD symptoms but it did not destroy the bean. We did not find any evidence of the presence of C. kahawae in Vietnam
Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots
We examine the multiple exciton population dynamics in PbS quantum dots by
ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We
simultaneously probe the first three excitonic transitions over a broad
spectral range. Transient spectra show the presence of first order bleach of
absorption for the 1S_h-1S_e transition and second order bleach along with
photoinduced absorption band for 1P_h-1P_e transition. We also report evidence
of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures
of carrier multiplication (multiexcitons for the single absorbed photon) from
analysis of the first and second order bleaches, in the limit of low absorbed
photon numbers (~ 10^-2), at pump energies from two to four times the
semiconductor band gap. The multiexciton generation efficiency is discussed
both in terms of a broadband global fit and the ratio between early- to
long-time transient absorption signals.. Analysis of population dynamics shows
that the bleach peak due to the biexciton population is red-shifted respect the
single exciton one, indicating a positive binding energy.Comment: 16 pages, 5 figure
Charge transport through single molecules, quantum dots, and quantum wires
We review recent progresses in the theoretical description of correlation and
quantum fluctuation phenomena in charge transport through single molecules,
quantum dots, and quantum wires. A variety of physical phenomena is addressed,
relating to co-tunneling, pair-tunneling, adiabatic quantum pumping, charge and
spin fluctuations, and inhomogeneous Luttinger liquids. We review theoretical
many-body methods to treat correlation effects, quantum fluctuations,
nonequilibrium physics, and the time evolution into the stationary state of
complex nanoelectronic systems.Comment: 48 pages, 14 figures, Topical Review for Nanotechnolog
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