5 research outputs found
Temperature-Driven Reversible Rippling and Bonding of a Graphene Superlattice
In order to unravel the complex interplay between substrate interactions and film configuration, we investigate and characterize graphene on a support with non-three-fold symmetry, the square Ir(100). Below 500 °C, distinct physisorbed and chemisorbed graphene phases coexist on the surface, respectively characterized by flat and buckled morphology. They organize into alternating domains that extend on mesoscopic lengths, relieving the strain due to the different thermal expansion of film and substrate. The chemisorbed phase exhibits exceptionally large one-dimensional ripples with regular nanometer periodicity and can be reversibly transformed into physisorbed graphene in a temperature-controlled process that involves surprisingly few C–Ir bonds. The formation and rupture of these bonds, rather than ripples or strain, are found to profoundly alter the local electronic structure, changing graphene behavior from semimetal to metallic type. The exploitation of such subtle interfacial changes opens new possibilities for tuning the properties of this unique material
Magnetic Patterning by Electron Beam-Assisted Carbon Lithography
We
report on the proof of principle of a scalable method for writing
the magnetic state by electron-stimulated molecular dissociative adsorption
on ultrathin Co on Re(0001). Intense microfocused low-energy electron
beams are used to promote the formation of surface carbides and graphitic
carbon through the fragmentation of carbon monoxide. Upon annealing
at the CO desorption temperature, carbon persists in the irradiated
areas, whereas the clean surface is recovered elsewhere, giving origin
to chemical patterns with nanometer-sharp edges. The accumulation
of carbon is found to induce an in-plane to out-of-plane spin reorientation
transition in Co, manifested by the appearance of striped magnetic
domains. Irradiation at doses in excess of 1000 L of CO followed by
ultrahigh vacuum annealing at 380 °C determines the formation
of a graphitic overlayer in the irradiated areas, under which Co exhibits
out-of-plane magnetic anisotropy. Domains with opposite magnetization
are separated here by chiral Neél walls. Our fabrication protocol
adds lateral control to spin reorientation transitions, permitting
to tune the magnetic anisotropy within arbitrary regions of mesoscopic
size. We envisage applications in the nano-engineering of graphene-spaced
stacks exhibiting the desired magnetic state and properties
Unraveling the Structural and Electronic Properties at the WSe<sub>2</sub>–Graphene Interface for a Rational Design of van der Waals Heterostructures
WSe<sub>2</sub> thin films grown by chemical vapor deposition on
graphene on SiC(0001) are investigated using photoelectron spectromicroscopy
and electron diffraction. By tuning of the growth conditions, micrometer-sized
single or multilayer WSe<sub>2</sub> crystalline islands preferentially
aligned with the main crystallographic directions of the substrate
are obtained. Our experiments suggest that the WSe<sub>2</sub> islands
nucleate from defective WSe<sub><i>x</i></sub> seeds embedded
in the support. We explore the electronic properties of prototypical
van der Waals heterostructures by performing μ-angle resolved
photoemission spectroscopy on WSe<sub>2</sub> islands of varying thickness
(mono- and bilayer) supported on single layer, bilayer, and trilayer
graphene. The experiments are substantiated by DFT calculations indicating
that the interaction between WSe<sub>2</sub> and graphene is weak
and the electronic properties of the resulting heterostructures are
unaffected by the thickness of the supporting graphene layer or by
the crystallographic orientation. Yet the WSe<sub>2</sub>–graphene
distance and the WSe<sub>2</sub>/WSe<sub>2</sub> interlayer separation
strongly influence the electronic band alignment at the high symmetry
points of the Brillouin zone. The values of technology relevant quantities
such as splitting of spin polarized bands and effective mass of electrons
at band valleys are extracted from experimental angle resolved spectra.
These findings establish further strategies for tuning the morphology
and electronic properties of artificially fabricated van der Waals
heterostructures that may be used in the fields of nanoelectronics
and valleytronics
Unraveling the Structural and Electronic Properties at the WSe<sub>2</sub>–Graphene Interface for a Rational Design of van der Waals Heterostructures
WSe<sub>2</sub> thin films grown by chemical vapor deposition on
graphene on SiC(0001) are investigated using photoelectron spectromicroscopy
and electron diffraction. By tuning of the growth conditions, micrometer-sized
single or multilayer WSe<sub>2</sub> crystalline islands preferentially
aligned with the main crystallographic directions of the substrate
are obtained. Our experiments suggest that the WSe<sub>2</sub> islands
nucleate from defective WSe<sub><i>x</i></sub> seeds embedded
in the support. We explore the electronic properties of prototypical
van der Waals heterostructures by performing μ-angle resolved
photoemission spectroscopy on WSe<sub>2</sub> islands of varying thickness
(mono- and bilayer) supported on single layer, bilayer, and trilayer
graphene. The experiments are substantiated by DFT calculations indicating
that the interaction between WSe<sub>2</sub> and graphene is weak
and the electronic properties of the resulting heterostructures are
unaffected by the thickness of the supporting graphene layer or by
the crystallographic orientation. Yet the WSe<sub>2</sub>–graphene
distance and the WSe<sub>2</sub>/WSe<sub>2</sub> interlayer separation
strongly influence the electronic band alignment at the high symmetry
points of the Brillouin zone. The values of technology relevant quantities
such as splitting of spin polarized bands and effective mass of electrons
at band valleys are extracted from experimental angle resolved spectra.
These findings establish further strategies for tuning the morphology
and electronic properties of artificially fabricated van der Waals
heterostructures that may be used in the fields of nanoelectronics
and valleytronics
Imaging Phase Segregation in Nanoscale Li<sub><i>x</i></sub>CoO<sub>2</sub> Single Particles
LixCoO2 (LCO)
is a common
battery cathode material that has recently emerged as a promising
material for other applications including electrocatalysis and as
electrochemical random access memory (ECRAM). During charge–discharge
cycling LCO exhibits phase transformations that are significantly
complicated by electron correlation. While the bulk phase diagram
for an ensemble of battery particles has been studied extensively,
it remains unclear how these phases scale to nanometer dimensions
and the effects of strain and diffusional anisotropy at the single-particle
scale. Understanding these effects is critical to modeling battery
performance and for predicting the scalability and performance of
electrocatalysts and ECRAM. Here we investigate isolated, epitaxial
LiCoO2 islands grown by pulsed laser deposition. After
electrochemical cycling of the islands, conductive atomic force microscopy
(c-AFM) is used to image the spatial distribution of conductive and
insulating phases. Above 20 nm island thicknesses, we observe a kinetically
arrested state in which the phase boundary is perpendicular to the
Li-planes; we propose a model and present image analysis results that
show smaller LCO islands have a higher conductive fraction than larger
area islands, and the overall conductive fraction is consistent with
the lithiation state. Thinner islands (14 nm), with a larger surface
to volume ratio, are found to exhibit a striping pattern, which suggests
surface energy can dominate below a critical dimension. When increasing
force is applied through the AFM tip to strain the LCO islands, significant
shifts in current flow are observed, and underlying mechanisms for
this behavior are discussed. The c-AFM images are compared with photoemission
electron microscopy images, which are used to acquire statistics across
hundreds of particles. The results indicate that strain and morphology
become more critical to electrochemical performance as particles approach
nanometer dimensions