'Floating' graphene on a bed of calcium atoms

Date:
September 17, 2020
Source:
ARC Centre of Excellence in Future Low-Energy Electronics
Technologies
Summary:
Adding calcium to graphene creates an extremely-promising
superconductor, but where does the calcium go? In a new study,
a Monash-led team has for the first time confirmed what actually
happens to those calcium atoms.

Surprising everyone, the calcium goes underneath both the upper
graphene sheet and a lower 'buffer' sheet, 'floating' the graphene
on a bed of calcium atoms.



FULL STORY ========================================================================== Adding calcium to graphene creates an extremely-promising superconductor,
but where does the calcium go?

========================================================================== Adding calcium to a composite graphene-substrate structure creates a
high transition-temperature (Tc) superconductor.

In a new study, an Australian-led team has for the first time confirmed
what actually happens to those calcium atoms: surprising everyone,
the calcium goes underneath both the upper graphene sheet and a lower
'buffer' sheet, 'floating' the graphene on a bed of calcium atoms.

Superconducting calcium-injected graphene holds great promise for energy- efficient electronics and transparent electronics.

STUDYING CALCIUM-DOPED GRAPHENE: THROWING OFF THE DUVET Graphene's
properties can be fine-tuned by injection of another material (a process
known as 'intercalation') either underneath the graphene, or between
two graphene sheets.



==========================================================================
This injection of foreign atoms or molecules alters the electronic
properties of the graphene by either increasing its conductance,
decreasing interactions with the substrate, or both.

Injecting calcium into graphite creates a composite material (calcium- intercalated graphite, CaC6) with a relatively 'high' superconducting transition temperature (Tc). In this case, the calcium atoms ultimately
reside between graphene sheets.

Injecting calcium into graphene on a silicon-carbide substrate also
creates a high-Tc superconductor, and we always thought we knew where
the calcium went in this case too...

Graphene on silicon-carbide has two layers of carbon atoms: one graphene
layer on top of another 'buffer layer': a carbon layer (graphene-like
in structure) that forms between the graphene and the silicon-carbide
substrate during synthesis, and is non-conducting due to being partially
bonded to the substrate surface.

"Imagine the silicon carbide is like a mattress with a fitted sheet
(the buffer layer bonded to it) and a flat sheet (the graphene),"
explains lead author Jimmy Kotsakidis.

Conventional wisdom held that calcium should inject between the two carbon layers (between two sheets), similar to injection between the graphene
layers in graphite. Surprisingly, the Monash University-led team found
that when injected, the calcium atoms' final destination location instead
lies between buffer layer and the underlying silicon-carbide substrate
(between the fitted sheet and the mattress!).

"It was quite a surprise to us when we realised that the calcium was
bonding to the silicon surface of the substrate, it really went against
what we thought would happen," explains Kotsakidis.

Upon injection, the calcium breaks the bonds between the buffer layer and substrate surface, thus, causing the buffer layer to 'float' above the substrate, creating a new, quasi-freestanding bilayer graphene structure
(Ca- QFSBLG).

This result was unanticipated, with extensive previous studies not
considering calcium intercalation underneath the buffer layer. The
study thus resolves long-standing confusion and controversy regarding
the position of the intercalated calcium.


========================================================================== Story Source: Materials provided by ARC_Centre_of_Excellence_in_Future_Low-Energy_Electronics
Technologies. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jimmy C. Kotsakidis, Antonija Grubisić-Čabo, Yuefeng
Yin, Anton
Tadich, Rachael L. Myers-Ward, Matthew DeJarld, Shojan P. Pavunny,
Marc Currie, Kevin M. Daniels, Chang Liu, Mark T. Edmonds, Nikhil
V. Medhekar, D. Kurt Gaskill, Amadeo L. Va'zquez de Parga, Michael
S. Fuhrer.

Freestanding n-Doped Graphene via Intercalation of Calcium and
Magnesium into the Buffer Layer-SiC(0001) Interface. Chemistry of
Materials, 2020; 32 (15): 6464 DOI: 10.1021/acs.chemmater.0c01729 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200917105343.htm

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