70 research outputs found
The magnetic, electronic, and light-induced topological properties in two-dimensional hexagonal FeX2 (X = Cl, Br, I) monolayers
Topological materials are fertile ground for investigating topological phases
of matter and topological phase transitions. In particular, the quest for novel
topological phases in 2D materials is attracting fast growing attention. Here,
using Floquet-Bloch theory, we propose to realize chiral topological phases in
2D hexagonal FeX2 (X=Cl, Br, I) monolayers under irradiation of circularly
polarized light. Such 2D FeX2 monolayers are predicted to be dynamical stable,
and exhibit both ferromagnetic and semiconducting properties. To capture the
full topological physics of the magnetic semiconductor under periodic driving,
we adopt ab initio Wannier-based tight-binding methods for the Floquet-Bloch
bands, with the light-induced band gap closings and openings being obtained as
the light field strength increases. The calculations of slab with open
boundaries show the existence of chiral edge states. Interestingly, the
topological transitions with branches of chiral edge states changing from zero
to one and from one to two by tuning the light amplitude are obtained, showing
that the topological floquet phase of high Chern number can be induced in the
present Floquet-Bloch systems
The role of collective motion in the ultrafast charge transfer in van der Waals heterostructures.
The success of van der Waals heterostructures made of graphene, metal dichalcogenides and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that van der Waals heterostructues can exhibit ultrafast charge transfer despite the weak binding of these heterostructures. Here we find, using time-dependent density functional theory molecular dynamics, that the collective motion of excitons at the interface leads to plasma oscillations associated with optical excitation. By constructing a simple model of the van der Waals heterostructure, we show that there exists an unexpected criticality of the oscillations, yielding rapid charge transfer across the interface. Application to the MoS2/WS2 heterostructure yields good agreement with experiments, indicating near complete charge transfer within a timescale of 100 fs
Nanoscale imaging of He-ion irradiation effects on amorphous TaO toward electroforming-free neuromorphic functions
Resistive switching in thin films has been widely studied in a broad range of
materials. Yet the mechanisms behind electroresistive switching have been
persistently difficult to decipher and control, in part due to their
non-equilibrium nature. Here, we demonstrate new experimental approaches that
can probe resistive switching phenomena, utilizing amorphous TaO as a model
material system. Specifically, we apply Scanning Microwave Impedance Microscopy
(sMIM) and cathodoluminescence (CL) microscopy as direct probes of conductance
and electronic structure, respectively. These methods provide direct evidence
of the electronic state of TaO despite its amorphous nature. For example CL
identifies characteristic impurity levels in TaO, in agreement with first
principles calculations. We applied these methods to investigate He-ion-beam
irradiation as a path to activate conductivity of materials and enable
electroforming-free control over resistive switching. However, we find that
even though He-ions begin to modify the nature of bonds even at the lowest
doses, the films conductive properties exhibit remarkable stability with large
displacement damage and they are driven to metallic states only at the limit of
structural decomposition. Finally, we show that electroforming in a nanoscale
junction can be carried out with a dissipated power of < 20 nW, a much smaller
value compared to earlier studies and one that minimizes irreversible
structural modifications of the films. The multimodal approach described here
provides a new framework toward the theory/experiment guided design and
optimization of electroresistive materials
Substrate transfer and ex situ characterization of on-surface synthesized graphene nanoribbons
Recent progress in the on-surface synthesis of graphene nanoribbons (GNRs)
has given access to atomically precise narrow GNRs with tunable electronic band
gaps that makes them excellent candidates for room-temperature switching
devices such as field-effect transistors (FET). However, in spite of their
exceptional properties, significant challenges remain for GNR processing and
characterization. This contribution addresses some of the most important
challenges, including GNR fabrication scalability, substrate transfer,
long-term stability under ambient conditions and ex situ characterization. We
focus on 7- and 9-atom wide armchair graphene nanoribbons (i.e, 7-AGNR; and
9-AGNR) grown on 200 nm Au(111)/mica substrates using a high throughput system.
Transfer of both, 7- and 9-AGNRs from their Au growth sub-strate onto various
target substrates for additional characterization is accomplished utilizing a
polymer-free method that avoids residual contamination. This results in a
homogeneous GNR film morphology with very few tears and wrinkles, as examined
by atomic force microscopy. Raman spectroscopy indicates no significant
degradation of GNR quality upon substrate transfer, and reveals that GNRs have
remarkable stability under ambient conditions over a 24-month period. The
transferred GNRs are analyzed using multi-wavelength Raman spectroscopy, which
provides detailed insight into the wavelength dependence of the width-specific
vibrational modes. Finally, we characterize the optical properties of 7- and
9-AGNRs via ultra-violet-visible (UV-Vis) spectroscopyComment: 30 pages, 14 figure
- …