72 research outputs found
Ultrafast charge carrier separation in Potassium-intercalated endohedral metallofullerene ScN@C thin films
Molecular materials have emerged as highly tunable materials for photovoltaic
and light-harvesting applications. The most severe challenge of this class of
materials is the trapping of charge carriers in bound electron-hole pairs,
which severely limits the free charge carrier generation. Here, we demonstrate
a significant modification of the exciton dynamics of thin films of endohedral
metallofullerene complexes upon alkali metal intercalation. For the exemplary
case of ScN@C thin films, we show that potassium intercalation
results in an additional relaxation channel for the optically excited
charge-transfer excitons that prevents the trapping of excitons in a long-lived
Frenkel exciton-like state. Instead, K intercalation leads to an ultrafast
exciton dissociation coinciding most likely with the generation of free charge
carriers. In this way, we propose alkali metal doping of molecular films as a
novel approach to enhance the light to-charge carrier conversion efficiency in
photovoltaic materials
Topological States on the Gold Surface
Gold surfaces host special electronic states that have been understood as a
prototype of Shockley surface states (SSs). These SSs are commonly employed to
benchmark the capability of angle-resolved photoemission spectroscopy (ARPES)
and scanning tunneling spectroscopy. We find that these Shockley SSs can be
reinterpreted as topologically derived surface states (TDSSs) of a topological
insulator (TI), a recently discovered quantum state. Based on band structure
calculations, the Z2 topological invariant can be well defined to characterize
the nontrivial features of gold that we detect by ARPES. The same TDSSs are
also recognized on surfaces of other well-known noble metals (e.g., silver,
copper, platinum, and palladium). Besides providing a new understanding of
noble metal SSs, finding topological states on late transition metals provokes
interesting questions on the role of topological effects in surface-related
processes, such as adsorption and catalysis.Comment: 21 pages, 3 figure
Tailoring the ferromagnetic surface potential landscape by a templating two-dimensional metal-organic porous network
Two-dimensional metal-organic porous networks (2D-MOPNs) have been identified
as versatile nanoarchitectures to tailor surface electronic and magnetic
properties on noble metals. In this context, we propose a protocol to
redecorate a ferromagnetic surface potential landscape using a 2D-MOPN.
Ultrathin cobalt (Co) films grown on Au(111) exhibit a well-ordered surface
triangular reconstruction. On the ferromagnetic surface, the adsorbed
2,4,6-tris(4-pyridyl)-1,3,5triazine (T4PT) molecules can coordinate with the
native Co atoms to form a large-scale Co-T4PT porous network. The Co-T4PT
network with periodic nanocavities serves as a templating layer to reshape the
ferromagnetic surface potential. The subsequently deposited C60 molecules are
steered by the network porous potential and the neighboring C60 interactions.
The prototype of the ferromagnetic-supported 2D-MOPN is a promising template
for the tailoring of molecular electronic and spin properties
Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy
Molecular materials enable a vast variety of functionalities for novel
electronic and spintronic devices. The unique possibility to alter or
substitute organic molecules or metallic substrates offers the opportunity to
modify and optimize interfacial properties for almost any desired field of
application. For this reason, we extend the successful approach to control
molecular interfaces by surface alloying. We present a comprehensive
characterization of the structural and electronic properties of the interface
formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown
on an Ag(111) single crystal surface. We monitor the changes of adsorption
height of the surface alloy atoms and electronic valence band structure upon
adsorption of one layer of PTCDA using the normal incidence x-ray standing wave
technique in combination with momentum-resolved photoelectron spectroscopy. We
find that the vertical buckling and the surface band structure of the SnAg
surface alloy is not altered by the adsorption of one layer of PTCDA, in
contrast to our recent study of PTCDA on a PbAg surface alloy [Phys. Rev.
Lett. 117, 096805 (2016)] . In addition, the vertical adsorption geometry of
PTCDA and the interfacial energy level alignment indicate the absence of any
chemical interaction between the molecule and the surface alloy. We attribute
the different interactions at these PTCDA/surface alloy interfaces to the
presence or absence of local -bonds between the PTCDA oxygen atoms and
the surface atoms. Combining our findings with results from literature, we are
able to propose an empiric rule for engineering the surface band structure of
alloys by adsorption of organic molecules
Coherent and incoherent magnons induced by strong ultrafast demagnetization in thin permalloy films
Understanding spin dynamics on femto- and picosecond timescales offers new
opportunities for faster and more efficient spintronic devices. Here, we
experimentally investigate the coherent spin dynamics after ultrashort laser
excitation by time-resolved magneto optical Kerr effect (TR-MOKE) in thin
Ni80Fe20 films. We provide a detailed study of the magnetic field and pump
fluence dependence of the coherent precessional dynamics. We show that the
coherent precession lifetime increases with the applied external magnetic field
which cannot be understood by viscous Gilbert damping of the coherent magnons.
Instead, it can be explained by nonlinear magnon interactions and by the change
in the fraction of incoherent magnons. This interpretation is in agreement with
the observed trends of the coherent magnon amplitude and lifetime as a function
of the exciting laser fluence. Our results provide a new insight into the
magnetization relaxation processes in ferromagnetic thin films, which is of
great importance for further spintronic applications.Comment: 8 pages, 7 figure
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
Equivalence of RABBITT and streaking delays in attosecond-time-resolved photoemission spectroscopy at solid surfaces
Gebauer A, Neb S, Enns W, Stadtmüller B, Aeschlimann M, Pfeiffer W. Equivalence of RABBITT and streaking delays in attosecond-time-resolved photoemission spectroscopy at solid surfaces. Applied Sciences. 2019;9(3): 592.The dynamics of the photoelectric effect in solid-state systems can be investigated via attosecond-time-resolved photoelectron spectroscopy. This article provides a comparison of delay information accessible by the two most important techniques, attosecond streaking spectroscopy and reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) at solid surfaces, respectively. The analysis is based on simulated time-resolved photoemission spectra obtained by solving the time-dependent Schrödinger equation in a single-active-electron approximation. We show a continuous transition from the few-cycle RABBITT regime to the streaking regime as two special cases of laser-assisted photoemission. The absolute delay times obtained by both methods agree with each other, within the uncertainty limits for kinetic energies >10 eV. Moreover, for kinetic energies >10 eV, both streaking delay time and RABBITT delay time coincide with the classical time of flight for an electron propagating from the emitter atom to the bulk-vacuum interface, with only small deviations of less than 4 as due to quantum mechanical interference effects
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