Silicene nanoribbons as carbon monoxide nanosensors with ... · Silicene Nanoribbons as Carbon Monoxide Nanosensors with Molecular Resolution Tim H. Osborn, Amir A. Farajian ( ) Department
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Nano Res
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Silicene nanoribbons as carbon monoxide nanosensorswith molecular resolution
Tim H. Osborn and Amir A. Farajian (), Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0454-7
Oxygen, on the other hand, interacts strongly with
the pristine silicene nanoribbon with adsorption
energy of 2.96 eV/O2. Upon relaxation, the O2
molecule splits apart in favor of individual Si-O
bonds. This large adsorption energy seems to
indicate that pristine silicene nanoribbons would
easily oxidize at ambient conditions. However,
Padova et al. have experimentally demonstrated
that silicene nanoribbons (on a silver substrates)
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resist oxidation up to exposures of 1000 L [34].
Undoubtedly, substrate effects and kinetics are of
interest here and will need to be resolved if silicene
nanosensors (and other devices) are to be realized
for use in oxygen environments. This exploration is
beyond the scope of this work.
Figure 5. Quantum conductance modulation
resulting from environmental gas molecules
adsorptions on nanoribbon: N2 (blue), O2 (red), CO2
(green) and H2O (purple). Middle of the gap is set at
zero.
To explore environmental gas effects on the
conductance of the nanoribbon we calculate
conductance before and after the adsorption of O2
and N2 molecules. The conduction curves depicted
in the top panel of Figure 5 confirm the inert
behavior of N2 (blue curve) showing conductance
nearly identical to that of the pristine nanoribbon.
For oxygen, we see that conductance is significantly
reduced (red curve) while the 0.09 eV band gap is
preserved. These results indicate that although CO
sensing capability of silicene nanoribbons may
diminish in oxygen-containing atmosphere, the
capability is preserved in nitrogen-containing
atmosphere.
The effects of CO2 and H2O adsorption are also
investigated. The conductance results are shown in
the middle panel of Figure 5 and the structures are
presented in the bottom panel of the same figure.
For H2O adsorption the minimum energy
configuration results from water splitting [35] and
subsequent attachment of H and OH at the edge
(with a binding energy of 1.62 eV) while CO2
adsorbs via physisorption (with a binding energy of
0.46 eV). Similar to the case of oxygen adsorption,
owing to the destructive effect on the nanoribbons
structure, water molecules should also be removed
from the environment for proper CO sensor
functionality.
Interestingly, comparing carbon oxides sensing
by graphene nanoribbons [3] and by silicene
nanoribbons, we notice that detection of CO is
much more feasible in the presence of CO2 for
silicene nanoribbons as compared with graphene
nanoribbons. This is because CO2 physisorbs on
both nanoribbon types whereas CO physisorbs on
graphene nanoribbons but chemisorbs on silicene
nanoribbons.
3.4 Silver Contacts
As previously mentioned, a silicene nanosensor
needs to be connected to leads via contacts. Here we
explore connection to the Ag(001) surface as a
contact for the nanosensor. Because silicene has
already been successfully grown on Ag(110) and
Ag(001), this contact would likely be realizable
experimentally. Here we model the top surface of
a bulk silver contact with a single fixed layer of
Ag(001) . (Figure 6)
To explore potential CO adsorptions on the
bulk Ag(100) contact itself, we use lowest energy
geometry for CO adsorption on Ag(100) as
calculated by Qin et al. [36] They report the most
energetically favorable configuration to be a vertical
C-down adsorption on top of a silver atom with a
C-Ag distance of 3.44 Ang. Using this geometry, we
calculate the conduction for a pristine Ag ribbon
(contact) and an Ag ribbon with a single CO
adsorption to obtain the conduction curves shown
in Figure 6. Here we see that the overall conduction
is only slightly reduced by the presence of a single
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CO molecule. This demonstrates that CO molecule
adsorptions on the contact surface itself may
slightly alter the conductance reading of the silicene
nanoribbon sensor, however, the conductance
change is much less significant than the one arising
from CO adsorption on a silicene nanoribbon. This
result is expected based on weaker physisorption
interaction (0.19 eV/CO) [36] and smaller charge
transfer (0.123 |e|) [36] from the CO to similar
silver surfaces as reported by Qin et al.
Figure 6. Conductance of fixed contact based on
surface Ag(100) layer without (blue) and with
CO-adsorbed molecule (red). Fermi energy is set at
zero.
3.5 Silicene on Silver Contacts
To combine the Ag-contact and the silicene
nanoribbon, the Si-Ag interface is relaxed
independently to fixed contacts at both ends and
then rejoined to the pristine silicene junction region,
as depicted in Figure 7 (red box). Initial geometries
were taken from He et al. [37]. Upon relaxation, we
see that the periodicity of the silicene nanoribbon is
broken by the interaction with the Ag contact edge,
i.e. a single boat-like link is formed within the red
box shown in Figure 7. To verify the existence of
this boat-like link we re-relax the entire silicene
ribbon while fixing the ends to the contact and find
that it is indeed preserved.
Figure 7. Conductance of pristine silicene
nanorobbon attached to Ag(100) contacts (blue),
and silicene nanoribbon with CO-Center (red) and
CO-Edge adsorption (green). Red box indicates
interface that was relaxed on the fixed Ag-contact.
Fermi energy is set at zero.
Next, we calculate the conductance for the
silver contacted silicene nanoribbon. The
conduction results show that over a small bias
range (1 eV) near Fermi energy, conductance is
modulated but not in a systematic or detectable way.
The ribbon-plus-contact system is shown here to be
metallic. We consider this as a manifestation of
interface states penetrating the silicene bridge as a
result of the close proximity of the two Ag contacts.
This effectively masks the conductance modulation
effects of the CO-nanoribbon interaction. By
extending the length of the silicene nanoribbon the
behavior will approach that of the pristine
nanoribbon that we described earlier, for which
detectable conductance modulation effects are
expected upon CO adsorption.
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4. Conclusions
We have shown that a single carbon monoxide
molecule detectably modulates the quantum
conductance of silicene nanoribbons. The weak
chemisorption of CO on the silicene nanoribbon
would enable the silicene sensor to be recovered
upon heating. We have also shown that individual
environmental gas molecule adsorption events
modulate the conduction of pristine silicene
nanoribbons in a differentiable manner from CO;
while N2 and CO2 essentially do not affect
conductance, O2 and H2O can strongly chemisorb
and diminish silicene's capability to detect CO.
Proper functionality of such basic silicene
nanosensors may therefore require removing oxygen
and water from silicene environment. From another
perspective, among the gas molecules that we
considered (CO, CO2, O2, N2, H2O), CO, O2 and H2O
cause the most significant changes in conductance,
and are therefore predicted to be detectable. Effects
of CO adsorption density and edge-dangling bond
defects are also investigated. Silver-contact-coupled
sensors are considered and shown not to work for
short (~ 1 nm) silicene segments as they do not
exhibit any systematic change in conductance due to
effects of silver contacts in close proximity. This
highlights the importance of using long-enough
silicene nanoribbons in which interface states are
confined away from the functional CO-adsorption
region. Overall, these results indicate that long
silicene nanoribbons could provide a unique
nanosensor capable of single molecule resolution
Acknowledgements
This research was supported by the National Science
Foundation grant ECCS-0925939
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