13 research outputs found
Recrystallization of MBE‐Grown MoS2 Monolayers Induced by Annealing in a Chemical Vapor Deposition Furnace
A systematic study of MoS2 grown by a combination of physical vapor deposition and post-growth annealing treatment has been conducted. Hereby, MoS2 thin films with thicknesses between 1 and 2 layers are first grown on sapphire by molecular beam epitaxy at different growth temperatures and then transferred to S environment inside a tube furnace for an annealing process. Depending on the growth temperature, the as-grown layers are either amorphous or form a crystalline structure composed of closely packed nanometer-size grains. The annealing process leads to recrystallization of these layers significantly increasing the size of the MoS2 crystalline domains to the range of 50–100 nm. While the originally amorphous layer displays rotational domains after annealing, recrystallization of samples grown at high temperatures yields single crystalline layers. All samples display an increase of the crystallite dimension, which is accompanied by the disappearance of the defect-related peaks in the Raman spectra, sharpening of the excitonic signatures in absorption, and strong enhancement of the photoluminescence yield. The results represent a promising way to combine advantages of physical vapor deposition and a post-growth annealing in a chemical vapor deposition furnace toward fabrication of wafer-scale single crystalline transition metal dichalcogenide mono- and multilayer films on non-van der Waals substrates.Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659Peer Reviewe
Y‐Stabilized ZrO2 as a Promising Wafer Material for the Epitaxial Growth of Transition Metal Dichalcogenides
Y-stabilized ZrO2 (YSZ) as a promising single-crystal wafer material for the epitaxial growth of transition metal dichalcogenides applicable for both physical (PVD) and chemical vapor deposition (CVD) processes is used. MoS2 layers grown on YSZ (111) exhibit sixfold symmetry and in-plane epitaxial relationship with the wafer of (1010) MoS2 || (211) YSZ. The PVD-grown submonolayer thin films show nucleation of MoS2 islands with a lateral size of up to 100 nm and a preferential alignment along the substrate step edges. The layers exhibit a strong photoluminescence yield as expected for the 2H-phase of MoS2 in a single monolayer limit. The CVD-grown samples are composed of triangular islands of several micrometers in size in the presence of antiparallel domains. The results represent a promising route toward fabrication of wafer-scale single-crystalline transition metal dichalcogenide layers with a tunable layer thickness on commercially available wafers.Deutsche Forschungsgemeinschaft
http://dx.doi.org/10.13039/501100001659Interreg
http://dx.doi.org/10.13039/100013276Peer Reviewe
Revealing the Competing Contributions of Charge Carriers, Excitons, and Defects to the Non-Equilibrium Optical Properties of ZnO
Due to its wide band gap and high carrier mobility, ZnO is an attractive
material for light-harvesting and optoelectronic applications. Its functional
efficiency, however, is strongly affected by defect-related in-gap states that
open up extrinsic decay channels and modify relaxation timescales. As a
consequence, almost every ZnO sample behaves differently, leading to
irreproducible or even contradicting observations. Here, a complementary set of
time-resolved spectroscopies is applied to two ZnO samples of different defect
density to disentangle the competing contributions of charge carriers,
excitons, and defects to the non-equilibrium dynamics after photoexcitation:
Time-resolved photoluminescence, excited state transmission, and electronic sum
frequency generation. Remarkably, defects affect the transient optical
properties of ZnO across more than eight orders of magnitude in time, starting
with photodepletion of normally occupied defect states on femtosecond
timescales, followed by the competition of free exciton emission and exciton
trapping at defect sites within picoseconds, photoluminescence of defect-bound
and free excitons on nanosecond timescales, and deeply trapped holes with
microsecond lifetimes. These findings do not only provide the first
comprehensive picture of charge and exciton relaxation pathways in ZnO, but
also uncover the microscopic origin of previous conflicting observations in
this challenging material and thereby offer means of overcoming its
difficulties
Strong coupling of monolayer WS2 excitons and surface plasmon polaritons in a planar Ag/WS2 hybrid structure
Monolayer (1L) transition metal dichalcogenides (TMDC) are of strong interest
in nanophotonics due to their narrow-band intense excitonic transitions
persisting up to room temperature. When brought into resonance with surface
plasmon polariton (SPP) excitations of a conductive medium opportunities for
studying and engineering strong light-matter coupling arise. Here, we consider
a most simple geometry, namely a planar stack composed of a thin silver film,
an Al2O3 spacer and a monolayer of WS2. We perform total internal reflection
ellipsometry which combines spectroscopic ellipsometry with the
Kretschmann-Raether-type surface plasmon resonance configuration. The combined
amplitude and phase response of the reflected light at varied angle of
incidence proves that despite the atomic thinness of 1L-WS2, the strong
coupling (SC) regime between A excitons and SPPs propagating in the thin Ag
film is reached. The phasor representation of rho corroborates SC as rho
undergoes a topology change indicated by the occurrence of a double point at
the cross over from the weak to the strong coupling regime. Our findings are
validated by both analytical transfer matrix method calculations and numerical
Maxwell simulations. The findings open up new perspectives for applications in
plasmonic modulators and sensors benefitting from the tunability of the optical
properties of 1L-TMDCs by electric fields, electrostatic doping, light and the
chemical environment.Comment: 15 pages, 3 figure
Tunable intersubband transitions in ZnO/ZnMgO multiple quantum wells in the mid infrared spectral range
We report on controllable tuning of intersubband transitions in ZnO/Zn0.60Mg0.40O multiple quantum well structures grown by molecular beam epitaxy on sapphire. The transitions from the first to the second electronic energy state within the conduction band are directly observed by infrared spectroscopy. By variation of the quantum well width, the intersubband transition energies are tuned from 290 to 370 meV. The experimental results are in good agreement with theoretical calculations assuming the presence of internal electric fields of 2 MV·cm−1
Demonstration of hyperbolic metamaterials at telecommunication wavelength using Ga-doped ZnO
Nanoscale transport of surface excitons at the interface between ZnO and a molecular monolayer
Structure of <i>p</i>‑Sexiphenyl Nanocrystallites in ZnO Revealed by High-Resolution Transmission Electron Microscopy
The
structure of <i>para</i>-sexiphenyl (6P) nanocrystallites
embedded in ZnO single crystals is resolved by cross-sectional high-resolution
transmission electron microscopy (HRTEM) combined with image contrast
simulations and X-ray diffraction measurements. The hybrid structures
are prepared by subsequent physical vapor deposition of 6P on ZnO(1010) templates followed by overgrowth with ZnO. Application
of ultramicrotomy for HRTEM specimen preparation and imaging under
different focus conditions provides direct access to the atomic and
molecular structure of the hybrid interface and the organic inclusion.
The hybrid stacks reveal a high structural perfection. The 6P nanocrystallites
maintain a structure as in the bulk crystal. Individual 6P lattice
planes can be traced up to the lateral and top interfaces with ZnO,
indicating that all interfaces are defined on an atomic/molecular
level. Further evaluation of the HRTEM images reveals peculiarities
of 6P growth on ZnO(1010). The common 6P β-phase
coexists here with the rarely reported γ-phase. The ZnO surface
structure induces two mirror-symmetric in-plane preferential orientations
of the 6P nanocrystallites. The ZnO surface topography, on the other
hand, is critical for the structural perfection of 6P. Although conformal
growth is observed, ZnO step edges induce characteristic stacking
faults in 6P nanocrystallites