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The isotope selectivity of computed tomography (CT) imaging based on nuclear resonance fluorescence (NRF) transmission method using a quasi-monochromatic laser Compton scattering (LCS) gamma-ray beam in the MeV region was demonstrated at the Ultra Violet Synchrotron Orbital Radiation–III (UVSOR-III) Synchrotron Radiation Facility (Institute of Molecular Science, National Institute of Natural Science) for two enriched lead isotope rods (206Pb and 208Pb) implanted in an aluminum cylinder. Since these two rods show the same gamma-ray attenuation in atomic processes, it is impossible to differentiate between them using a standard Gamma-CT technique based on atomic attenuation of gamma rays. The LCS gamma-ray beam had a maximum energy of 5.528 MeV and an intensity of approximately 5.5 photons/s/eV at the resonance energy ( $J^{\pi } = 1^{-}$ at 5.512 MeV in 208Pb). A lead collimator with a hole diameter of 1 mm was used to define the size of the LCS gamma-ray beam at the CT target. The CT image of the 208Pb rod was selectively obtained with a 2-mm pixel size resolution, which was determined by the horizontal step size of the CT stage.

A complete theoretical framework for photon elastic scattering has been implemented in GEANT4 and is validated against experimental results. The implementation covers all atomic numbers and energy range between 0.3 MeV and 20 MeV. The total elastic scattering cross section is calculated based on scattering amplitudes to account for interferences between coherent processes. New scattering amplitudes are calculated for Rayleigh scattering to extend the cross section data table and avoid interpolation and extrapolation. Validation on a significant number of test cases showed good agreement between our implementation and experimental data.

Two analysis methods for $$\textit{Z}_\mathrm{eff}$$ evaluation were explored using both experimental and simulated gamma-ray attenuation data. Using particle-capture reactions on composite targets to generate multi-monoenergetic gamma rays between 1 and 12 MeV, we demonstrate the advantage of using neural networks for effective Z evaluation of shielded materials in single-pixel measurements. Furthermore, we extend the analysis to 2D processing of transmission radiography and by using Geant4-simulated data we prove the superiority of artificial neural networks in terms of image quality and material discrimination against classical methods.

Carotenoids and chlorophyll are essential parts of plant leaves and are involved in photosynthesis, a vital biological process responsible for the origin of life on Earth. Here, we investigate how β-carotene and chlorophyll-a form mixed molecular phases on a Au(111) surface using low-temperature scanning tunneling microscopy and molecular manipulation at the single-molecule level supported by density functional theory calculations. By isolating individual molecules from nanoscale molecular clusters with a scanning tunneling microscope tip, we are able to identify five β-carotene conformations including a structure exhibiting a three-dimensional conformation. Furthermore, molecular resolution images enable direct visualization of β-carotene/chlorophyll-a clsuters, with intimate structural details highlighting how they pair: β-carotene preferentially positions next to chlorophyll-a and induces switching of chlorophyll-a from straight to several bent tail conformations in the molecular clusters.

Bremsstrahlung based dual-energy attenuation is the most widely used methodology for high-energy cargo inspection systems and Z e f f mapping. Currently, the inspection systems do not exploit the spectroscopic component of the transmitted spectrum, relying mostly on energy integrated attenuation signals. In this work, we propose a neural-network based approach to determine Z e f f from energy resolved transmitted spectra using single high-energy attenuations. The neural-network approach based on a regression algorithm was benchmarked against the dual-energy attenuation method. The evaluation is made using Geant4 simulated data covering the full range of Z values and indicates potential improvements in terms of imaging speed and Z e f f discriminating power for the neural-network based approach.

We developed the isotope-specific Computed Tomography imaging technique based on the Nuclear Resonance Fluorescence absorption method at BL1U of UVSOR-III by using high flux and quasi-monochromatic gamma-ray beam for a sample target containing two enriched lead isotope rods (206Pb and 208Pb). The maximum Laser Compton Scattering gamma-ray beam energy was 5528 keV and the gamma-rays flux was greater than 107 photons/s after collimation. A two-dimensional image of 208Pb isotope distribution was selectively obtained using an exited state in 208Pb at 5512 keV.

Compton γ -ray sources have been in operation for over 30 years with new facilities being under construction or proposed. The gamma beam system under implementation at the Extreme Light Infrastructure - Nuclear Physics facility in Romania will deliver brilliant γ -ray beams with energies up to 19.5 MeV. Several instruments for measuring the parameters of the γ -ray beam are under development at ELI-NP. One of these instruments based on a High Purity Germanium detector is routinely used for beam energy measurements at other facilities. Here we investigate the use of a High Purity Germanium detector to continuously monitor the intensity of the ELI-NP gamma beam by measuring the inelastic scattering of photons. This method relies on both experimental and simulated data and it has been successfully tested during a recent experiment at the High Intensity γ -ray Source facility.

Here we report on the characterization of one of the large-volume LaBr$_{3}$:Ce detectors for the ELIGANT project at ELI-NP. The main focus of this work is the response function for high-energy $\gamma$ rays of such detectors. In particular, we compare a selection of unfolding methods to resolve small structures in $\gamma$-ray spectra with high-energies. Three methods have been compared using $\gamma$-ray spectra with energies up to 12 MeV obtained in an experiment at the 3 MV Tandetron\texttrademark\ facility at IFIN-HH. The results show that the iterative unfolding approach gives the best qualitative reproduction of the emitted $\gamma$-ray spectrum. Furthermore, the correlation fluctuations in high-energy regime from the iterative method are two orders of magnitude smaller than when using the matrix inversion approach with second derivative regularization. In addition, the iterative method is computationally faster as it does not contain large matrix inversions. The matrix inversion method does, however, give more consistent results over the full energy range and in the low-statistics limit. Our conclusion is that the performance of the iterative approach makes it well suitable for semi-online analysis of experimental data. These results will be important, both for experiments with the ELIGANT setup, and for on-line diagnostics of the energy spread of the $\gamma$-ray beam which is under implementation at ELI-NP., Comment: 12 pages, 6 figures, 1 table

Non-destructive inspection carried out by using nuclear resonances excited by an MeV energy region gamma ray is a promising method. The high penetrability of MeV gamma ray of nuclear resonant energy makes possible the detection of nuclides surrounded by massive materials. As an application of this method, computed tomography imaging of nuclear distribution inside objects can be reconstructed from transmission factor of gamma rays. We have studied the image reconstruction of the nuclear distribution using Monte-Carlo simulations to estimate the gamma-ray transmission factor assuming the ELI-NP facility where about 3 order higher intensity of quasi-monoenergetic gamma rays will be available., CoE on Sustainable Energy System (Thai-Japan), Faculty of Engineering, Rajamangala University of Technology Thanyaburi (RMUTT), Thailand

Metalated molecules are the ideal building blocks for the bottom-up fabrication of, e.g., two-dimensional arrays of magnetic particles for spintronics applications. Compared to chemical synthesis, metalation after network formation by an atom beam can yield a higher degree of control and flexibility and allows for mixing of different types of magnetic atoms. We report on successful metalation of tetrapyridyl-porphyrins (TPyP) by Co and Cr atoms, as demonstrated by scanning tunneling microscopy experiments. For the metalation, large periodic networks formed by the TPyP molecules on a Ag(111) substrate are exposed in situ to an atom beam. Voltage-induced dehydrogenation experiments support the conclusion that the porphyrin macrocycle of the TPyP molecule incorporates one transition metal atom. The newly synthesized Co-TPyP and Cr-TPyP complexes exhibit striking differences in their electronic behavior, leading to a magnetic character for Cr-TPyP only as evidenced by Kondo resonance measurements. ispartof: Journal of Physical Chemistry Letters vol:6 issue:6 pages:1048-1052 ispartof: location:United States status: published

We demonstrate the effective confinement of surface electrons in the pores of molecular networks formed by dehydrobenzo[12]annulene (DBA) molecules with butoxy groups (DBA-OC4) on Au(111). Investigation of the network formation starting from single molecules reveals a considerable interaction of the molecules with the substrate, which is at the origin of the observed confinement.

The electronic properties of nanometer size Au(111) vacancy islands are modified by covering the islands with an atomically thin NaCl(001) layer. The large-scale commensurability between the atomic lattices of the cover layer and the islands gives rise to various Moire-effect-induced standing wave patterns of which the periodicity and orientation depend on the relative angle between the lattices. The observed standing wave patterns are accompanied by the formation of multiple band gaps resulting from the Moire-effect-related periodic potential. Moreover, the energy level quantization and the coverlayer-induced energy shift give rise to partial depopulation of the NaCl/Au interface state. Targeted use of a thin covering layer and the resulting Moire-effect-related potential offers yet unexploited possibilities to conveniently modify the electronic structure of nanoparticles.

We probe the electron-phonon coupling for in situ engineered porphyrin-based magnetic molecular layers supported on weakly reactive surfaces. Using high-resolution scanning tunneling microscopy and spectroscopy at 4.5 K we show that the electronic and magnetic properties of the engineered molecules are the result of interplay between many-body spin-flip excitations and electron-phonon interactions. crosscheck: This document is CrossCheck deposited related_data: Supplementary Information copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal copyright_licence: This article is freely available. This article is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported Licence (CC BY-NC 3.0) history: Received 7 May 2016; Accepted 24 August 2016; Accepted Manuscript published 25 August 2016; Advance Article published 31 August 2016; Version of Record published 15 September 2016 ispartof: Chemical Communications vol:52 issue:76 pages:11359-11362 ispartof: location:England status: published

The Extreme Light Infrastructure - Nuclear Physics facility is dedicated to nuclear physics studies with the use of extreme electromagnetic radiation. One of the main research system to be installed and operated in the facility is an outstanding high brilliance gamma beam system. The Gamma Beam System of ELI–NP will produce intense, quasi–monochromatic gamma beams via inverse Compton scattering of short laser pulses on relativistic electron beam pulses. The gamma beams available at ELI–NP will allow for the performance of photo-nuclear reactions aiming to reveal the intimate structure of the atomic nucleus. Nuclear Resonance Fluorescence, photo-fission, photo-disintegration reactions above the particle threshold will be used to study the dipole response of nuclei, the structure of the Pygmy resonances, nuclear processes relevant for astrophysics, production and study of exotic neutron–rich nuclei.

A range of artificial molecular systems has been created that can exhibit controlled linear and rotational motion. In the further development of such systems, a key step is the addition of communication between molecules in a network. Here, we show that a two-dimensional array of dipolar molecular rotors can undergo simultaneous rotational switching when applying an electric field from the tip of a scanning tunnelling microscope. Several hundred rotors made from porphyrin-based double-decker complexes can be simultaneously rotated when in a hexagonal rotor network on a Cu(111) surface by applying biases above 1 V at 80 K. The phenomenon is observed only in a hexagonal rotor network due to the degeneracy of the ground-state dipole rotational energy barrier of the system. Defects are essential to increase electric torque on the rotor network and to stabilize the switched rotor domains. At low biases and low initial rotator angles, slight reorientations of individual rotors can occur, resulting in the rotator arms pointing in different directions. Analysis reveals that the rotator arm directions are not random, but are coordinated to minimize energy via crosstalk among the rotors through dipolar interactions. Hundreds of molecular rotors in a 2D network can be simultaneously rotated by applying an electric field from the tip of a scanning tunnelling microscope.

Cl-functionalized scanning tunneling microscopy (STM) tips are fabricated by modifying a tungsten STM tip in situ on islands of ultrathin NaCl(100) films on Au(111) surfaces. The functionalized tips are used to achieve clear atomicresolution imaging of NaCl(100) islands. In comparison with bare metal tips, the chemically modified tips yield drastically enhanced spatial resolution as well as contrast reversal in STM topographs, implying that Na atoms, rather than Cl atoms, are imaged as protrusions. STM simulations based on a Green’s function formalism reveal that the experimentally observed contrast reversal in the STM topographs is due to the highly localized character of the Cl-pz states at the tip apex. An additional remarkable characteristic of the modified tips is that in dI/dV maps, a Na atom appears as a ring with a diameter that depends crucially on the tip-sample distance., The research in Leuven has been supported by the Research Foundation – Flanders (FWO, Belgium) and the Flemish Concerted Action research program (BOF KU Leuven, GOA/14/007). Z. L. thanks the China Scholarship Council for financial support (No. 2011624021). K. S. and V. I. are postdoctoral researchers of the FWO. J. C. acknowledges financial support from the Spanish Ministry of Innovation and Science under contract NOs. MAT2010-18432 and MAT2013-47878-C2-R.

Single magnetic Co atoms are deposited on atomically thin NaCl films on Au(111). Two different adsorption sites are revealed by high-resolution scanning tunneling microscopy (STM), i.e., at Na and at Cl locations. Using density functional based simulations of the STM images, we show that the Co atoms substitute with either a Na or Cl atom of the NaCl surface, resulting in cationic and anionic Co dopants with a high thermal stability. The dependence of the magnetic coupling between neighboring Co atoms on their separation is investigated via spatially resolved measurement of the local density of states. ispartof: Physical Review Letters vol:112 issue:2 pages:1-5 ispartof: location:United States status: published

Nowadays molecular nanoporous networks have numerous uses in surface nanopatterning applications and in studies of host-guest interactions. Trimesic acid (TMA), a benzene derivative with three carboxylic groups, is a marvelous building block for forming 2D H-bonded porous networks. Here, we report a low-temperature study of the nanoporous "chicken-wire" superstructure formed by TMA molecules adsorbed on a Au(111) surface. Distinct preferential orientations of the porous networks on Au(111) lead to the formation of peculiar TMA polymorphs that are stabilized only at the boundary between rotational molecular domains. Scanning tunneling microscopy (STM) and spectroscopy are used to investigate the electronic properties of both the molecular building blocks and the pores. Sub-molecular-resolution imaging and spatially resolved electronic spectroscopy reveal a remarkable change in the appearance of the molecules in the STM images at energies in the range of the lowest unoccupied molecular orbital, accompanied by highly extended molecular wave functions into the pores. The electronic structure of the pores reflects a weak confinement of surface electrons by the TMA network. Our experimental observations are corroborated by density-functional-theory-based calculations of the nanoporous structure adsorbed on Au(111). ispartof: Langmuir vol:29 issue:37 pages:11593-11599 ispartof: location:United States status: published

Quasi-one-dimensional YSi(2) nanostructures are formed via self-assembly on the Si(100) surface. These epitaxial nanowires are metastable and their formation strongly depends on the growth parameters. Here, we explore the various stages of yttrium silicide formation over a range of metal coverages and growth temperatures, and establish a rudimentary phase diagram for these novel and often coexisting nanophases. We identify, in addition to previously identified stoichiometric wires, several new nanowire systems. These nanowires exhibit a variety of surface reconstructions, which sometimes coexist on a single wire. From a comparison of scanning tunneling microscopy images, tunneling spectra, and first-principles density functional theory calculations, we determine that these surface reconstructions arise from local orderings of yttrium vacancies. Nanowires often agglomerate into nanowire bundles, the thinnest of which are formed from single wire pairs. The calculations show that such bundles are energetically favored compared to well-separated single wires. Thicker bundles are formed at slightly higher temperature. They extend over several microns, forming a robust network of conducting wires that could possibly be employed in nanodevice applications.

The Nuclear Physics oriented pillar of the pan-European Extreme Light Infrastructure (ELI-NP) will host an ultra-bright, energy tunable, and quasi-monochromatic gamma-ray beam system in the range of 0.2–19.5 MeV produced by laser Compton backscattering. This gamma beam satisfies the criteria for large-size product investigations with added capabilities like isotope detection through the use of nuclear resonance fluorescence (NRF) and is ideal for non-destructive testing applications. Two major applications of gamma beams are being envisaged at ELI-NP: industrial applications based on NRF and industrial radiography and tomography. Both applications exploit the unique characteristics of the gamma beam to deliver new opportunities for the industry. Here, we present the experimental setups proposed at ELI-NP and discuss their performance based on analytical calculations and GEANT4 numerical simulations. One of the main advantages of using the gamma beam at ELI-NP for applications based on NRF is the availability of an advanced detector array, which can enhance the advantages already provided by the high quality of the gamma beam.

An extensive redistribution of spin density in TBrPP-Co molecules adsorbed on a Cu(111) surface is investigated by monitoring Kondo resonances at different locations on single molecules. Remarkably, the width of the Kondo resonance is found to be much larger on the organic ligands than on the central cobalt atom-reflecting enhanced spin-electron interactions on molecular orbitals. This unusual effect is explained by means of first-principles and numerical renormalization-group calculations highlighting the possibility to engineer spin polarization by exploiting interfacial charge transfer.

Titanium dioxide (TiO2) is widely recognized as one of the most promising photocatalysts for solar energy utilization and environmental cleanup, but because of its wide bandgap, pure TiO2 can only absorbs ultraviolet light, which represents 4% of the solar spectrum1-6. Here we establish a conceptually novel approach, termed non-compensated n-p codoping, to narrow the bandgap of TiO2 and shift the optical response into the visible spectral range where a much larger fraction of the solar spectrum can be captured. The concept embodies two key ingredients: The electrostatic attraction within the n-p dopant pair enhances the thermodynamic and kinetic solubility in substitutional doping, and the non-compensated nature ensures the creation of broadened intermediate electronic states that effectively narrow the bandgap. The concept is demonstrated quantitatively within first-principles density functional theory. The experimental evidence for bandgap narrowing is obtained in the forms of direct measurements of the density of states by scanning tunneling spectroscopy, dramatically redshifted and increased optical absorbance, and enhanced photoactivity manifested by efficient hole-electron separation in the visible spectral region. These findings represent the first crucial steps toward development of a new class of titania-based photocatalysts with greatly enhanced efficiency of solar energy conversion facilitating environmentally friendly applications ofrenewablemore » energy.« less

Exceptionally long and uniform YSi(2) nanowires are formed via self-assembly on Si(001). The in-plane width of the thinnest wires is known to be quantized in odd multiples of the silicon lattice constant. Here, we identify a class of nanowires that violates the "odd multiple" rule. The structure of the thinnest wire in this category is determined by comparing scanning tunneling spectroscopy measurements with the calculated surface density of states of candidate models by means of the Pendry R-factor analysis. The relative stability of the odd and even wire systems is analyzed via first-principles calculations.

We report the manipulation of a Kondo resonance originating from the spin-electron interactions between a two-dimensional molecular assembly of TBrPP-Co molecules and a Cu(111) surface at 4.6 K. By manipulating nearest-neighbor molecules with a scanning tunneling microscope tip we are able to tune the spin-electron coupling of the center molecule inside a hexagonal molecular assembly in a controlled step-by-step manner. The Kondo temperature increases from 105 to 170 K with decreasing the number of nearest neighbor molecules from six to zero. The scattering of surface electrons by the molecules located at edges of the molecular layer reduces the spin-electron coupling strength for the molecules inside the layer. Investigations of different molecular arrangements indicate that the observed Kondo resonance is independent on the molecular lattice.

Single chlorophyll-a molecules, a vital resource for the sustenance of life on Earth, have been investigated by using scanning-tunneling-microscope manipulation and spectroscopy on a gold substrate at 4.6 K. The chlorophyll-a binds on Au(111) via its porphyrin unit while the phytyl-chain is elevated from the surface by a support of four CH3 groups. By injecting tunneling electrons from the STM-tip, we are able to bend the phytyl-chain, which enable switching of four molecular conformations in a controlled manner. Statistical analyses and structural calculations reveal that all reversible switching mechanisms are initiated by a single tunnelling-electron energy-transfer process, which induces bond rotation within the phytyl-chain., 5 pages, 4 figures

Electron donor-acceptor molecular charge transfer complexes (CTCs) formed by $\ensuremath{\alpha}$-sexithiophene (6T) and tetrafluoro-tetracyano-quinodimethane (${\mathrm{F}}_{4}\mathrm{TCNQ}$) on a Au(111) surface are investigated by scanning tunneling microscopy, spectroscopy, and spectroscopic imaging at 6 K. New hybrid molecular orbitals are formed in the CTCs, and the highest occupied molecular orbital of the CTC is mainly located on the electron accepting ${\mathrm{F}}_{4}\mathrm{TCNQ}$ while the lowest unoccupied molecular orbital is predominantly positioned on the electron donating 6T. We observed the conductance switching of ${\mathrm{F}}_{4}\mathrm{TCNQ}$ inside CTCs, which may find potential applications in novel molecular device operations.

Deviating from the common growth mode of molecular films of organic molecules where the adsorbates remain intact, we observe an essentially different growth behavior for metallocenes with a low temperature scanning tunneling microscope. Ferrocene molecules adsorb dissociatively and form a two layer structure. The top layer unit cell is composed of two tilted cyclopentadienyl (cp) rings, while the first layer consists of ferrocene molecules and cp-Fe complexes. Surprisingly a fourfold symmetry is observed for the top layer while the first layer displays threefold symmetry elements. It is this symmetry mismatch which induces an incommensurability between these layers in all except one surface direction. The top layer is weakly bonded and has an antiferromagnetic ground state as calculated by local spin density functional approximation.

Mathematics and art converge in the fractal forms that also abound in nature. We used molecular self-assembly to create a synthetic, nanometer-scale, Sierpinski hexagonal gasket. This nondendritic, perfectly self-similar fractal macromolecule is composed of bis -terpyridine building blocks that are bound together by coordination to 36 Ru and 6 Fe ions to form a nearly planar array of increasingly larger hexagons around a hollow center.

Two conformations of isolated single TBrPP-Co molecules on a Cu(111) surface are switched by applying +2.2 V voltage pulses from a scanning tunneling microscope tip at 4.6 K. The TBrPP-Co has a spin-active cobalt atom caged at its center and the interaction between the spin of this cobalt atom and free electrons from the Cu(111) substrate can cause a Kondo resonance. Tunneling spectroscopy data reveal that switching from the saddle to a planar molecular conformation enhances spin-electron coupling, which increases the associated Kondo temperature from 130 K to 170 K. This result demonstrates that the Kondo temperature can be manipulated just by changing molecular conformation without altering chemical composition of the molecule., Comment: To appear in Nano Lett (2006)