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Free
standing atomically thin crystals have long been thought to be
impossible due to predictions made by Landau and others on the
absence of a true long-range order in two dimensions.
However, the experimental discovery of graphene, a single atomic
sheet of single-crystal graphite demonstrated that both crystalline order
and associated electronic band structure can be preserved
sufficiently well atomically thin, mesoscopic size flakes. This
provides a playground to study collective phenomena in low
dimensions, such as superconductivity, magnetic ordering, or density
waves.
We
are currently focused on atomically thin
crystals of NbSe2 and other chalcogenides that can be
prepared following the
same mechanical exfoliation technique developed in graphene research. Using
the so called “scotch tape” method one first cleaves a single
crystal of NbSe2, the cleaved NbSe2 is pressed against a substrate of doped Si covered
with 300 nm thermally grown SiO2 and
then separated. This
process deposits a wide range of thin crystals on the substrate.
We identify the thinnest crystal flakes using an optical
microscope under which they show different colors and faintness. By correlating the optical identification with atomic force microscopy (AFM) and Raman
spectroscopy measurements a “color code” is developed, allowing for rapid
identification of flake thickness by optical means.
Once identified, electrically contacts to individual crystal
flakes are made. We
previously developed a lithography
free technique [1] using a thin quartz filament as a shadow mask
placed directly on the crystal of interest to prepare three-terminal devices.
This technique allows for rapid fabrication, as well as
protecting the crystal from possible contamination and damage from
the wide range of solvents required by standard lithographic
fabrication. For other devices, such as Hall bars, a standard e-beam lithography
fabrication technique is employed.
Our recent work on atomically thin NbSe2
has demonstrated that superconductivity survives down to a single
unit cell of 2H-NbSe2 which has a reduced onset Tc of
up to 2.5K. In these
thinnest crystalline NbSe2 flakes we have modulated the
superconducting transition temperature by
200mK (8%) using an electric field produced by 300-nm thermally
grown SiO2 as gate dielectric. This is higher than what is expected from a simple BCS model.
We attribute this to either the multiple-band
superconductivity, or enhanced Coulomb repulsion in atomically thin NbSe2
[2].
Contacts:
Neal Staley, nes151 @ psu.edu
Publications:
[1]N.
E. Staley et al., “Lithography-free fabrication of graphene
devices,” Appl. Phys. Lett. 90,
143518 (2007).
[2]Neal E. Staley, Jian Wu, Peter
Eklund, Ying Liu, Linjun Li and Zhuan Xu, “Electric field effect
on superconductivity in atomically thin flakes of NbSe2,”
Submitted to PRB (2008)
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Optical image of NbSe2 flakes showing
the color variation as thickness is reduced. Gold colored is
thickest, while light purple is thinnest in this image.
Atomic force microscopy (AFM) image of single unit
cell NbSe2 flakes. Note the distance that the flake
lies above the substrate is sample dependant, therefore folds within
the flake are the only way to measure thickness.
Ebeam lithography patterned hall bar of NbSe2.
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