46 research outputs found
Strong transient magnetic fields induced by THz-driven plasmons in graphene disks
Strong circularly polarized excitation opens up the possibility to generate
and control effective magnetic fields in solid state systems, e.g., via the
optical inverse Faraday effect or the phonon inverse Faraday effect. While
these effects rely on material properties that can be tailored only to a
limited degree, plasmonic resonances can be fully controlled by choosing proper
dimensions and carrier concentrations. Plasmon resonances provide new degrees
of freedom that can be used to tune or enhance the light-induced magnetic field
in engineered metamaterials. Here we employ graphene disks to demonstrate
light-induced transient magnetic fields from a plasmonic circular current with
extremely high efficiency. The effective magnetic field at the plasmon
resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong
(~1{\deg}) ultrafast Faraday rotation (~ 20 ps). In accordance with reference
measurements and simulations, we estimated the strength of the induced magnetic
field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ
cm-2
Increasing the Rate of Magnesium Intercalation Underneath Epitaxial Graphene on 6H-SiC(0001)
Magnesium intercalated 'quasi-freestanding' bilayer graphene on 6H-SiC(0001)
(Mg-QFSBLG) has many favorable properties (e.g., highly n-type doped,
relatively stable in ambient conditions). However, intercalation of Mg
underneath monolayer graphene is challenging, requiring multiple intercalation
steps. Here, we overcome these challenges and subsequently increase the rate of
Mg intercalation by laser patterning (ablating) the graphene to form
micron-sized discontinuities. We then use low energy electron diffraction to
verify Mg-intercalation and conversion to Mg-QFSBLG, and X-ray photoelectron
spectroscopy to determine the Mg intercalation rate for patterned and
non-patterned samples. By modeling Mg intercalation with the Verhulst equation,
we find that the intercalation rate increase for the patterned sample is
4.51.7. Since the edge length of the patterned sample is 5.2
times that of the non-patterned sample, the model implies that the increased
intercalation rate is proportional to the increase in edge length. Moreover, Mg
intercalation likely begins at graphene discontinuities in pristine samples
(not step edges or flat terraces), where the 2D-like crystal growth of
Mg-silicide proceeds. Our laser patterning technique may enable the rapid
intercalation of other atomic or molecular species, thereby expanding upon the
library of intercalants used to modify the characteristics of graphene, or
other 2D materials and heterostructures.Comment: 24 pages, 4 figure
Freestanding n-Doped Graphene via Intercalation of Calcium and Magnesium into the Buffer Layer - SiC(0001) Interface
The intercalation of epitaxial graphene on SiC(0001) with Ca has been studied
extensively, yet precisely where the Ca resides remains elusive. Furthermore,
the intercalation of Mg underneath epitaxial graphene on SiC(0001) has not been
reported. Here, we use low energy electron diffraction, x-ray photoelectron
spectroscopy, secondary electron cut-off photoemission and scanning tunneling
microscopy to elucidate the physical and electronic structure of both Ca- and
Mg-intercalated epitaxial graphene on 6H-SiC(0001). We find that Ca
intercalates underneath the buffer layer and bonds to the Si-terminated SiC
surface, breaking the C-Si bonds of the buffer layer i.e. 'freestanding' the
buffer layer to form Ca-intercalated quasi-freestanding bilayer graphene
(Ca-QFSBLG). The situation is similar for the Mg-intercalation of epitaxial
graphene on SiC(0001), where an ordered Mg-terminated reconstruction at the SiC
surface and Mg bonds to the Si-terminated SiC surface are formed, resulting in
Mg-intercalated quasi-freestanding bilayer graphene (Mg-QFSBLG).
Ca-intercalation underneath the buffer layer has not been considered in
previous studies of Ca-intercalated epitaxial graphene. Furthermore, we find no
evidence that either Ca or Mg intercalates between graphene layers. However, we
do find that both Ca-QFSBLG and Mg-QFSBLG exhibit very low workfunctions of
3.68 and 3.78 eV, respectively, indicating high n-type doping. Upon exposure to
ambient conditions, we find Ca-QFSBLG degrades rapidly, whereas Mg-QFSBLG
remains remarkably stable.Comment: 58 pages, 10 figures, 4 tables. Revised text and figure
Mechanism of periodic height variations along self-aligned VLS-grown planar nanostructures
In this study we report in-plane nanotracks produced by molecular-beam-epitaxy (MBE) exhibiting lateral self-assembly and unusual periodic and out-of-phase height variations across their growth axes. The nanotracks are synthesized using bismuth segregation on the GaAsBi epitaxial surface, which results in metallic liquid droplets capable of catalyzing GaAsBi nanotrack growth via the vaporâliquidâsolid (VLS) mechanism. A detailed examination of the nanotrack morphologies is carried out employing a combination of scanning electron and atomic force microscopy and, based on the findings, a geometric model of nanotrack growth during MBE is developed. Our results indicate diffusion and shadowing effects play significant roles in defining the interesting nanotrack shape. The unique periodicity of our lateral nanotracks originates from a rotating nucleation âhot spotâ at the edge of the liquidâsolid interface, a feature caused by the relative periodic circling of the non-normal ion beam flux incident on the sample surface, inside the MBE chamber. We point out that such a concept is divergent from current models of crawling mode growth kinetics and conclude that these effects may be utilized in the design and assembly of planar nanostructures with controlled non-monotonous structure
Chemical composition of nanoporous layer formed by electrochemical etching of p-type GaAs
Abstract : We have performed a detailed characterization study of electrochemically etched p-type GaAs in a hydrofluoric acid-based electrolyte. The samples were investigated and characterized through cathodoluminescence (CL), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). It was found that after electrochemical etching, the porous layer showed a major decrease in the CL intensity and a change in chemical composition and in the crystalline phase. Contrary to previous reports on p-GaAs porosification, which stated that the formed layer is composed of porous GaAs, we report evidence that the porous layer is in fact mainly constituted of porous As2O3. Finally, a qualitative model is proposed to explain the porous As2O3 layer formation on p-GaAs substrate
Manipulating Surface Energy to form Compound Semiconductor Nanostructures
Nanostructures have been lauded for their quantum confinement capabilities and potential applications in future devices. Compound semiconductor nanostructures are being integrated into the next generation of photovoltaic and light emitting devices to take advantage of their unique optical characteristics. Despite their promise, adoption of nanostructure based devices has been slow. This is due in large part to difficulties in effective fabrication and processing steps. By manipulating the surface energy of various components during growth, we can control the final structure and corresponding optoelectronic characteristics. Specifically I will present on GaSb quantum dots embedded in GaAs and GaAs nanowires using novel substrate and catalyst materials.
GaSb quantum dots embedded in a GaAs matrix are ideal for devices that require capture of minority carriers as they exhibit a type II band offset with carrier concentration in the valence band. However, during GaAs capping, there is a strong driving force for the dot to demolish into a distribution of intact dots, rings, and GaSb material clusters. We demonstrate the ability to mitigate this effect using both chemical and kinetic means: we alter the surface chemistry via the addition of aluminum, and use droplet epitaxy as an alternative quantum dot formation method. Secondly, the growth of high quality GaAs on silicon has always been restricted due to material incompatibilities. With the emergence of increasingly smaller low power electronics, there is a demand to integrate optoelectronic devices directly on the surface of CMOS sensor stacks. Utilizing the vapor-liquid-solid growth mechanism we are able to demonstrate the growth of high quality GaAs nanowires on polycrystalline substrates at low temperatures. This allows for the growth of III-V nanowire based devices directly on the metal pads of pre-packaged CMOS chips. We also investigate the potential use of bismuth as an alternative to gold for catalyzing nanowire growth.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135758/1/mdejarld_1.pd