22 research outputs found
Is it time to reappraise blood pressure thresholds and targets? Statement from the international society of hypertension - a global perspective
No abstract available
Self-Assembly and Conformation of Tetrapyridilporphyrin on the Ag(111) Surface
We present a low-temperature scanning tunneling microscopy (STM) study on the supramolecular ordering of tetrapyridyl-porphyrin (TPyP) molecules on Ag(111). Vapor deposition in a wide substrate temperature range reveals that TPyP molecules easily diffuse and self-assemble into large, highly ordered chiral domains. We identify two mirror-symmetric unit cells, each containing two differently oriented molecules. From an analysis of the respective arrangement it is concluded that lateral intermolecular interactions control the packing of the layer, while its orientation is induced by the coupling to the substrate. This finding is corroborated by molecular mechanics calculations. High-resolution STM images recorded at 15 K allow a direct identification of intramolecular features. This makes it possible to determine the molecular conformation of TPYP on Ag(111). The pyridyl groups are alternately rotated out of the porphyrin plane by an angle of 60°
Sub-cycle optical control of current in a semiconductor: from the multiphoton to the tunneling regime
Nonlinear interactions between ultrashort optical waveforms and solids can be
used to induce and steer electric current on a femtosecond (fs) timescale,
holding promise for electronic signal processing at PHz frequencies [Nature
493, 70 (2013)]. So far, this approach has been limited to insulators,
requiring extremely strong peak electric fields and intensities. Here, we show
all-optical generation and control of directly measurable electric current in a
semiconductor relevant for high-speed and high-power (opto)electronics, gallium
nitride (GaN), within an optical cycle and on a timescale shorter than 2 fs, at
intensities at least an order of magnitude lower than those required for
dielectrics. Our approach opens the door to PHz electronics and metrology,
applicable to low-power (non-amplified) laser pulses, and may lead to future
applications in semiconductor and photonic integrated circuit technologies
Gate control of Mott metal-insulator transition in a 2D metal-organic framework
Strong electron-electron Coulomb interactions in materials can lead to a vast
range of exotic many-body quantum phenomena, including Mott metal-insulator
transitions, magnetic order, quantum spin liquids, and unconventional
superconductivity. These many-body phases are strongly dependent on band
occupation and can hence be controlled via the chemical potential. Flat
electronic bands in two-dimensional (2D) and layered materials such as the
kagome lattice, enhance strong electronic correlations. Although theoretically
predicted, correlated-electron phases in monolayer 2D metal-organic frameworks
(MOFs) - which benefit from efficient synthesis protocols and tunable
properties - with a kagome structure have not yet been realised experimentally.
Here, we synthesise a 2D kagome MOF comprised of 9,10-dicyanoanthracene
molecules and copper atoms on an atomically thin insulator, monolayer hexagonal
boron nitride (hBN) on Cu(111). Scanning tunnelling microscopy (STM) and
spectroscopy reveal an electronic energy gap of ~200 meV in this MOF,
consistent with dynamical mean-field theory predictions of a Mott insulating
phase. By tuning the electron population of kagome bands, via either
template-induced (via local work function variations of the hBN/Cu(111)
substrate) or tip-induced (via the STM probe) gating, we are able to induce
Mott metal-insulator transitions in the MOF. These findings pave the way for
devices and technologies based on 2D MOFs and on electrostatic control of
many-body quantum phases therein.Comment: 19 pages, 4 figure
Non-Drude THz conductivity of graphene due to structural distortions
The remarkable electrical, optical and mechanical properties of graphene make
it a desirable material for electronics, optoelectronics and quantum
applications. A fundamental understanding of the electrical conductivity of
graphene across a wide frequency range is required for the development of such
technologies. In this study, we use terahertz (THz) time-domain spectroscopy to
measure the complex dynamic conductivity of electrostatically gated graphene,
in a broad 0.1 - 7 THz frequency range. The conductivity of doped
graphene follows the conventional Drude model, and is predominantly governed by
intraband processes. In contrast, undoped charge-neutral graphene exhibits a
THz conductivity that significantly deviates from Drude-type models. Via
quantum kinetic equations and density matrix theory, we show that this
discrepancy can be explained by additional interband processes, that can be
exacerbated by electron backscattering. We propose a mechanism where such
backscattering -- which involves flipping of the electron pseudo-spin -- is
mediated by the substantial vector scattering potentials that are associated
with structural deformations of graphene. Our findings highlight the
significant impact that structural distortions and resulting electrostatic
vector scattering potentials can have on the THz conductivity of charge-neutral
graphene. Our results emphasise the importance of the planar morphology of
graphene for its broadband THz electronic response.Comment: 74 pages, 21 figure
Molecular Nanoscience and Engineering on Surfaces
Molecular engineering of low-dimensional materials exploiting controlled self-assembly and positioning of individual atoms or molecules at surfaces opens up new pathways to control matter at the nanoscale. Our research thus focuses on the study of functional molecules and supramolecular architectures on metal substrates. As principal experimental tools we employ low-temperature scanning tunneling microscopy and spectroscopy. Here we review recent studies in our lab at UBC: Controlled manipulation of single CO molecules, self-assembled biomolecular nanogratings on Ag(111) and their use for electron confinement, as well as the organisation, conformation, metalation and electronic structure of adsorbed porphyrins
Direct observation of narrow electronic energy band formation in 2D molecular self-assembly
Surface-supported molecular overlayers have demonstrated versatility as platforms for fundamental research and a broad range of applications, from atomic-scale quantum phenomena to potential for electronic, optoelectronic and catalytic technologies. Here, we report a structural and electronic characterisation of self-assembled magnesium phthalocyanine (MgPc) mono and bilayers on the Ag(100) surface, via low-temperature scanning tunneling microscopy and spectroscopy, angle-resolved photoelectron spectroscopy (ARPES), density functional theory (DFT) and tight-binding (TB) modeling. These crystalline close-packed molecular overlayers consist of a square lattice with a basis composed of a single, flat-adsorbed MgPc molecule. Remarkably, ARPES measurements at room temperature on the monolayer reveal a momentum-resolved, two-dimensional (2D) electronic energy band, 1.27 eV below the Fermi level, with a width of ∼20 meV. This 2D band results from in-plane hybridization of highest occupied molecular orbitals of adjacent, weakly interacting MgPc's, consistent with our TB model and with DFT-derived nearest-neighbor hopping energies. This work opens the door to quantitative characterisation – as well as control and harnessing – of subtle electronic interactions between molecules in functional organic nanofilms
Zwitterionic Self-Assembly of L-Methionine Nanogratings on the Ag(111) Surface
The engineering of complex architectures from functional molecules on surfaces provides new pathways to control matter at the nanoscale. In this article, we present a combined study addressing the self-assembly of the amino acid L-methionine on Ag(111). Scanning tunneling microscopy data reveal spontaneous ordering in extended molecular chains oriented along high-symmetry substrate directions. At intermediate coverages, regular biomolecular gratings evolve whose periodicity can be tuned at the nanometer scale by varying the methionine surface concentration. Their characteristics and stability were confirmed by helium atomic scattering. X-ray photoemission spectroscopy and high-resolution scanning tunneling microscopy data reveal that the L-methionine chaining is mediated by zwitterionic coupling, accounting for both lateral links and molecular dimerization. This methionine molecular recognition scheme is reminiscent of sheet structures in amino acid crystals and was corroborated by molecular mechanics calculations. Our findings suggest that zwitterionic assembly of amino acids represents a general construction motif to achieve biomolecular nanoarchitectures on surfaces
Designing optoelectronic properties by on-surface synthesis: formation and electronic structure of an iron-terpyridine macromolecular complex
Supramolecular chemistry protocols applied on surfaces offer compelling
avenues for atomic scale control over organic-inorganic interface structures.
In this approach, adsorbate-surface interactions and two-dimensional
confinement can lead to morphologies and properties that differ dramatically
from those achieved via conventional synthetic approaches. Here, we describe
the bottom-up, on-surface synthesis of one-dimensional coordination
nanostructures based on an iron (Fe)-terpyridine (tpy) interaction borrowed
from functional metal-organic complexes used in photovoltaic and catalytic
applications. Thermally activated diffusion of sequentially deposited ligands
and metal atoms, and intra-ligand conformational changes, lead to Fe-tpy
coordination and formation of these nanochains. Low-temperature Scanning
Tunneling Microscopy and Density Functional Theory were used to elucidate the
atomic-scale morphology of the system, providing evidence of a linear tri-Fe
linkage between facing, coplanar tpy groups. Scanning Tunneling Spectroscopy
reveals highest occupied orbitals with dominant contributions from states
located at the Fe node, and ligand states that mostly contribute to the lowest
unoccupied orbitals. This electronic structure yields potential for hosting
photo-induced metal-to-ligand charge transfer in the visible/near-infrared. The
formation of this unusual tpy/tri-Fe/tpy coordination motif has not been
observed for wet chemistry synthesis methods, and is mediated by the bottom-up
on-surface approach used here