101 research outputs found
Direct evidence for efficient ultrafast charge separation in epitaxial WS/graphene heterostructure
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to
investigate ultrafast charge transfer in an epitaxial heterostructure made of
monolayer WS and graphene. This heterostructure combines the benefits of a
direct gap semiconductor with strong spin-orbit coupling and strong
light-matter interaction with those of a semimetal hosting massless carriers
with extremely high mobility and long spin lifetimes. We find that, after
photoexcitation at resonance to the A-exciton in WS, the photoexcited holes
rapidly transfer into the graphene layer while the photoexcited electrons
remain in the WS layer. The resulting charge transfer state is found to
have a lifetime of \,ps. We attribute our findings to differences in
scattering phase space caused by the relative alignment of WS and graphene
bands as revealed by high resolution ARPES. In combination with spin-selective
excitation using circularly polarized light the investigated WS/graphene
heterostructure might provide a new platform for efficient optical spin
injection into graphene.Comment: 28 pages, 14 figure
Direct evidence for efficient ultrafast charge separation in epitaxial WS<sub>2</sub>/graphene heterostructures
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS2 and graphene. This heterostructure combines the benefits of a direct-gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS2, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electrons remain in the WS2 layer. The resulting charge-separated transient state is found to have a lifetime of ∼1 ps. We attribute our findings to differences in scattering phase space caused by the relative alignment of WS2 and graphene bands as revealed by high-resolution ARPES. In combination with spin-selective optical excitation, the investigated WS2/graphene heterostructure might provide a platform for efficient optical spin injection into graphene
a route towards defined surface functionalization
We investigate the surface-catalyzed dissociation of the archetypal molecular
switch azobenzene on the Cu(111) surface. Based on X-ray photoelectron
spectroscopy, normal incidence X-ray standing waves and density functional
theory calculations a detailed picture of the coverage-induced formation of
phenyl nitrene from azobenzene is presented. Furthermore, a comparison to the
azobenzene/Ag(111) interface provides insight into the driving force behind
the dissociation on Cu(111). The quantitative decay of azobenzene paves the
way for the creation of a defect free, covalently bonded monolayer. Our work
suggests a route of surface functionalization via suitable azobenzene
derivatives and the on surface synthesis concept, allowing for the creation of
complex immobilized molecular systems
Laser Induced Creation of Antiferromagnetic 180 Degree Domains in NiO Pt Bilayers
The antiferromagnetic order in heterostructures of NiO Pt thin films can be modified by optical pulses. After the irradiation with laser light, the optically induced creation of antiferromagnetic domains can be observed by imaging the created domain structure utilizing the X ray magnetic linear dichroism effect. The effect of different laser polarizations on the domain formation can be studied and used to identify a polarization independent creation of 180 domain walls and domains with 180 different N el vector orientation. By varying the irradiation parameters, the switching mechanism can be determined to be thermally induced. This study demonstrates experimentally the possibility to optically create antiferromagnetic domains, an important step towards future functionalization of all optical switching mechanisms in antiferromagnet
A Comparison of the Pitfall Trap, Winkler Extractor and Berlese Funnel for Sampling Ground-Dwelling Arthropods in Tropical Montane Cloud Forests
Little is known about the ground-dwelling arthropod diversity in tropical montane cloud forests (TMCF). Due to unique habitat conditions in TMCFs with continuously wet substrates and a waterlogged forest floor along with the innate biases of the pitfall trap, Berlese funnel and Winkler extractor are certain to make it difficult to choose the most appropriate method to sample the ground-dwelling arthropods in TMCFs. Among the three methods, the Winkler extractor was the most efficient method for quantitative data and pitfall trapping for qualitative data for most groups. Inclusion of floatation method as a complementary method along with the Winkler extractor would enable a comprehensive quantitative survey of ground-dwelling arthropods. Pitfall trapping is essential for both quantitative and qualitative sampling of Diplopoda, Opiliones, Orthoptera, and Diptera. The Winkler extractor was the best quantitative method for Psocoptera, Araneae, Isopoda, and Formicidae; and the Berlese funnel was best for Collembola and Chilopoda. For larval forms of different insect orders and the Acari, all the three methods were equally effective
The 2021 ultrafast spectroscopic probes of condensed matter roadmap
In the 60 years since the invention of the laser, the scientific community has developed numerous fields of research based on these bright, coherent light sources, including the areas of imaging, spectroscopy, materials processing and communications. Ultrafast spectroscopy and imaging techniques are at the forefront of research into the light–matter interaction at the shortest times accessible to experiments, ranging from a few attoseconds to nanoseconds. Light pulses provide a crucial probe of the dynamical motion of charges, spins, and atoms on picosecond, femtosecond, and down to attosecond timescales, none of which are accessible even with the fastest electronic devices. Furthermore, strong light pulses can drive materials into unusual phases, with exotic properties. In this roadmap we describe the current state-of-the-art in experimental and theoretical studies of condensed matter using ultrafast probes. In each contribution, the authors also use their extensive knowledge to highlight challenges and predict future trends
Submonolayer growth of CuPc on noble metal surfaces
The understanding of growth mechanisms and electronic properties is a key issue for improving the performance of small organic devices, in which the metal-organic interface and its properties play a crucial role. In this context we investigated the adsorption behavior and the electronic properties of copper-II-phthalocyanine (CuPc) within the first adsorbate layer on Au(111) and Cu(111). Together with recent results published for CuPc/Ag(111) [Kroger et al., New J. Phys. 12, 083038 (2010)] this leads to a comprehensive understanding of the adsorption of CuPc on noble metal surfaces: On Cu(111) the molecule-surface interaction is the strongest. The molecules chemisorb on the surface and form one-dimensional chains or two-dimensional islands, depending on coverage. This behavior indicates an attractive intermolecular interaction. In contrast, on Au(111) CuPc is only weakly physisorbed and behaves like a two-dimensional gas in a wide coverage regime. Only when densely packed do the molecules form ordered structures, which are scarcely influenced by the structure of the metallic surface. Molecule-molecule interaction is also very weak, but in contrast to CuPc on Ag(111) no clear indications for a repulsive interaction are found. Regarding the adsorption strength, this latter system represents an (possibly unique) intermediate case which enables the unusual intermolecular repulsion found recently. Our results highlight the special role of this model system, since the interaction of CuPc with the metal can be "tuned" in any order of the adsorption scenarios observed by selecting the right substrate material
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