11 research outputs found
Monolayer Solid-State Electrolyte for Electric Double Layer Gating of Graphene Field-Effect Transistors
The
electrostatic gating of graphene field-effect transistors is
demonstrated using a monolayer electrolyte. The electrolyte, cobalt
crown ether phthalocyanine (CoCrPc) and LiClO<sub>4</sub>, is deposited
as a monolayer on the graphene channel, essentially creating an additional
two-dimensional layer on top of graphene. The crown ethers on the
CoCrPc solvate lithium ions and the ion location is modulated by a
backgate without requiring liquid solvent. Ions dope the channel by
inducing image charges; the doping level (<i>i</i>.<i>e</i>., induced charge density) can be modulated by the backgate
bias with the extent of the surface potential change being controlled
by the magnitude and polarity of the backgate bias. With a crown ether
to Li<sup>+</sup> ratio of 5:1, programming tests for which the backgate
is held at −<i>V</i><sub>BG</sub> shift the Dirac
point by ∼15 V, corresponding to a sheet carrier density on
the order of 10<sup>12</sup> cm<sup>–2</sup>. This charge carrier
density agrees with the packing density of monolayer CoCrPc on graphene
that would be expected with one Li<sup>+</sup> for every five crown
ethers (at the maximum possible Li<sup>+</sup> concentration, 10<sup>13</sup> cm<sup>–2</sup> is predicted). The crown ethers provide
two stable states for the Li<sup>+</sup>: one near the graphene channel
(low-resistance state) and one ∼5 Å away from the channel
(high-resistance state). Initial state retention measurements indicate
that the two states can be maintained for at least 30 min (maximum
time monitored), which is 10<sup>6</sup> times longer than polymer-based
electrolytes at room temperature, with at least a 250 Ω μm
difference between the channel resistance in the high- and low-resistance
states
Magnetic Alignment of Gamma (Core)–Alpha (Shell) Fe<sub>2</sub>O<sub>3</sub> Nanorods in a Solid Polymer Electrolyte for Li-Ion Batteries
The temperature-dependent ionic conductivity
and thermal properties
are characterized for a solid polymer electrolyte of poly(ethylene
oxide) (PEO) and LiClO<sub>4</sub> filled with 1 wt % γ-phase
core (maghemite) and α-phase shell (hematite) Fe<sub>2</sub>O<sub>3</sub> nanorods. Samples are solvent-cast in the absence and
presence of a 0.5 T magnetic field, dried at room temperature under
vacuum for 72 h, and measured under nitrogen. Vibrating sample magnetometry
indicates that the magnetic treatment aligns the nanorods to some
extent in the desired orientation normal to the electrode surface.
For samples with an ether oxygen to lithium ratio (EO/Li) of 10:1,
the nanorods induce sample-to-sample variability in the ionic conductivity.
The magnetic treatment eliminates this variability, and differential
scanning calorimetry data support the observation that the magnetic
treatment increases the structural homogeneity of the electrolyte.
For samples with an EO/Li of 3:1, the ionic conductivity is 3 orders
of magnitude larger for samples containing 5 times more of the crystal
structure, (PEO)<sub>6</sub>/LiClO<sub>4</sub>. This result is surprising
because an inverse relationship between crystallinity and conductivity
is normally observed for semicrystalline, solid polymer electrolytes.
When the crystal fraction is increased by a factor of 8 via the combination
of nanorods and magnetic treatment, the conductivity does not continue
to increase, showing that the effect does not persist beyond a critical
fraction of (PEO)<sub>6</sub>/LiClO<sub>4</sub>. The results demonstrate
that field-effect alignment of magnetic nanorods increases the crystal
fraction and homogeneity of PEO/LiClO<sub>4</sub>, but does not affect
the ionic conductivity in the range of salt and nanorod concentrations
investigated
Increasing the Room-Temperature Electric Double Layer Retention Time in Two-Dimensional Crystal FETs
Poly(vinyl
alcohol) (PVA) and LiClO<sub>4</sub>, a solid polymer
electrolyte with a glass transition temperature (<i>T</i><sub>g</sub>) of 80 °C, is used to electrostatically gate graphene
field-effect transistors. The ions in PVA:LiClO<sub>4</sub> are drifted
into place by field-effect at <i>T</i> > <i>T</i><sub>g</sub>, providing <i>n</i>- or <i>p</i>-type doping, and when the device is cooled to room temperature,
the polymer mobility and, hence ion mobility are arrested and the
electric double layer (EDL) is “locked” into place in
the absence of a gate bias. Unlike other electrolytes used to gate
two-dimensional devices for which the <i>T</i><sub>g</sub>, and therefore the “locking” temperature, is well
below room temperature, the electrolyte demonstrated in this work
provides a route to achieve room-temperature EDL stability. Specifically,
a 6 orders of magnitude increase in the room temperature EDL retention
time is demonstrated over the commonly used electrolyte, poly(ethylene
oxide) (PEO) and LiClO<sub>4</sub>. Hall measurements confirm that
large sheet carrier densities can be achieved with PVA:LiClO<sub>4</sub> at top gate programming voltages of ±2 V (−6.3 ±
0.03 × 10<sup>13</sup> cm<sup>–2</sup> for electrons and
1.6 ± 0.3 × 10<sup>14</sup> cm<sup>–2</sup> for holes).
Transient drain current measurements show that at least 75% of the
EDL is retained after more than 4 h at room temperature. Unlike PEO-based
electrolytes, PVA:LiClO<sub>4</sub> is compatible with the chemicals
used in standard photolithographic processes enabling the direct deposition
of patterned, metal contacts on the surface of the electrolyte. A
thermal instability in the electrolyte is detected by both <i>I</i>–<i>V</i> measurements and differential
scanning calorimetry, and FTIR measurements suggest that thermally
catalyzed cross-linking may be driving phase separation between the
polymer and the salt. Nevertheless, this work highlights how the relationship
between polymer and ion mobility can be exploited to tune the state
retention time and the charge carrier density of a 2D crystal transistor
Addressable Direct-Write Nanoscale Filament Formation and Dissolution by Nanoparticle-Mediated Bipolar Electrochemistry
Nanoscale
conductive filaments, usually associated with resistive
memory or memristor technology, may also be used for chemical sensing
and nanophotonic applications; however, realistic implementation of
the technology requires precise knowledge of the conditions that control
the formation and dissolution of filaments. Here we describe and characterize
an addressable direct-write nanoelectrochemical approach to achieve
repeatable formation/dissolution of Ag filaments across a ∼100
nm poly(ethylene oxide) (PEO) film containing either Ag<sup>+</sup> alone or Ag<sup>+</sup> together with 50 nm Ag-nanoparticles acting
as bipolar electrodes. Using a conductive AFM tip, formation occurs
when the PEO film is subjected to a forward bias, and dissolution
occurs under reverse bias. Formation–dissolution kinetics were
studied for three film compositions: Ag|PEO-Ag<sup>+</sup>, Ag|poly(ethylene
glycol) monolayer-PEO-Ag<sup>+</sup>, and Ag|poly(ethylene glycol)
monolayer-PEO-Ag<sup>+</sup>/Ag-nanoparticle. Statistical analysis
shows that the distribution of formation times exhibits Gaussian behavior,
and the fastest average initial formation time occurs for the Ag|PEO-Ag<sup>+</sup> system. In contrast, formation in the presence of Ag nanoparticles
likely proceeds by a noncontact bipolar electrochemical mechanism,
exhibiting the slowest initial filament formation. Dissolution times
are log-normal for all three systems, and repeated reformation of
filaments from previously formed structures is characterized by rapid
regrowth. The direct-write bipolar electrochemical deposition/dissolution
strategy developed here presents an approach to reconfigurable, noncontact <i>in situ</i> wiring of nanoparticle arraysthereby enabling
applications where actively controlled connectivity of nanoparticle
arrays is used to manipulate nanoelectronic and nanophotonic behavior.
The system further allows for facile manipulation of experimental
conditions while simultaneously characterizing surface conditions
and filament formation/dissolution kinetics
Influence of Fe<sub>2</sub>O<sub>3</sub> Nanofiller Shape on the Conductivity and Thermal Properties of Solid Polymer Electrolytes: Nanorods versus Nanospheres
The influence of nanofiller shape on the ionic conductivity
and
thermal properties of solid polymer electrolytes is investigated.
Electrolytes of polyethylene oxide (PEO) and LiClO<sub>4</sub> filled
with 1–20 wt % spherical Fe<sub>2</sub>O<sub>3</sub> nanoparticles
and 0.5–10 wt % Fe<sub>2</sub>O<sub>3</sub> nanorods are measured
at an ether oxygen to Li ratio of 10:1. Nanorods improve the ionic
conductivity to a similar extent as spherical nanoparticles, except
at concentrations 10–20 times lower. The maximum conductivity
improvement occurs at a spherical metal oxide nanoparticle loading
of 10 wt %; however, an equivalent nanorod loading decreases the conductivity
below that of the unfilled electrolyte. This result suggests that
the long-range morphology of the two nanocomposites differs widely,
where high concentrations of nanorods will inhibit instead of enhance
Li transport. The shape of the nanofiller also affects the crystallization
rate and resulting crystal structure. Differential scanning calorimetry
measurements show that samples containing nanorods crystallize faster
than those containing spherical nanoparticles, and nanorods favor
formation of the (PEO)<sub>6</sub>:LiClO<sub>4</sub> crystal phase.
Previous studies have shown that this channel-like structure is more
conductive than the amorphous phase. If nanorods could be used to
induce the formation and alignment of this conductive structure normal
to the electrode surface, perhaps ionic conductivity could be further
enhanced in nanofilled solid polymer electrolytes where the nanoscale
structure is precisely controlled
First-Principles Study of Crown Ether and Crown Ether-Li Complex Interactions with Graphene
Adsorption of molecules on graphene
is a promising route to achieve
novel functionalizations, which can lead to new devices. Density functional
theory is used to calculate stabilities, electronic structures, charge
transfer, and work function for a crown-4 ether (CE) molecule and
a CE–Li (or CE–Li<sup>+</sup>) complex adsorbed on graphene.
For a single CE on graphene, the adsorption distance is large with
small adsorption energies, regardless of the relative lateral location
of the CE. Because CE interacts weakly with graphene, the charge transfer
between the CE and graphene is negligibly small. When Li and Li<sup>+</sup> are incorporated, the adsorption energies significantly increase.
Simultaneously, an <i>n</i>-type doping of graphene is introduced
by a considerable amount of charge transfer in CE–Li adsorbed
system. In all of the investigated systems, the linear dispersion
of the p<sub><i>z</i></sub> band in graphene at the Dirac
point is well-preserved; however, the work function of graphene is
effectively modulated in the range of 3.69 to 5.09 eV due to the charge
transfer and the charge redistribution by the adsorption of CE–Li
and CE–Li<sup>+</sup> (or CE), respectively. These results
provide graphene doping and work function modulation without compromising
graphene’s intrinsic electronic property for device applications
using CE-based complexes
Growth Mode Transition from Monolayer by Monolayer to Bilayer by Bilayer in Molecularly Flat Titanyl Phthalocyanine Film
To
avoid defects associated with inhomogeneous crystallites and
uneven morphology that degrade organic device performance, the deposition
of ultraflat and homogeneous crystalline organic active layers is
required. The growth mode transition of organic semiconducting titanyl
phthalocyanine (TiOPc) molecule from monolayer-by-monolayer to bilayer-by-bilayer
can be observed on highly ordered pyrolytic graphic (HOPG), while
maintaining large and molecularly flat domains. The first monolayer
of TiOPc lies flat on HOPG with a ∼98% face-up orientation.
However, as the thickness of the TiOPc increases to over 15 monolayers
(ML), the growth mode transitions to bilayer-by-bilayer with the repeated
stacking of bilayers (BL), each of which has face-to-face pairs. Density
functional theory calculations reveal that the increasing of thickness
induces weakening of the substrate effect on the deposited TiOPc layers,
resulting in the growth mode transition to BL-by-BL. The asymmetric
stacking provides the driving force to maintain nearly constant surface
order during growth, allowing precise, subnanometer thickness control
and large domain growth
Loading and Distribution of a Model Small Molecule Drug in Poly(<i>N</i>‑isopropylacrylamide) Brushes: a Neutron Reflectometry and AFM Study
The
structure of a hydrated poly(<i>N</i>-isopropylacrylamide)
brush loaded with 5 vol % Isoniazid is studied as a function of temperature
using neutron reflectometry (NR) and atomic force microscopy (AFM).
NR measurements show that Isoniazid increases the thickness of the
brush before, during and after
the polymer collapse, and it is retained inside the brush at all measured
temperatures. The Isoniazid concentration in the expanded brush is
∼14% higher than in the bulk solution, and the concentration
nearly doubles in the collapsed polymer, suggesting stronger binding
between Isoniazid and the polymer compared to water, even at temperatures
below the lower critical solution temperature (LCST) where the polymer
is hydrophilic. Typically, additives that bind strongly to the polymer
backbone and increase the hydrophilicity of the polymer will delay
the onset of the LCST, which is suggested by AFM and NR measurements.
The extent of small-molecule loading and distribution throughout a
thermo-responsive polymer brush, such as pNIPAAm, will have important
consequences for applications such as drug delivery and gating
Solution-Cast Monolayers of Cobalt Crown Ether Phthalocyanine on Highly Ordered Pyrolytic Graphite
15-Crown-5-ether-substituted cobalt(II)
phthalocyanine (CoCrPc)
is an atomically thin and flat-laying, electrically insulating molecule
that can solvate ions; these properties are desirable for nanoelectronic
devices. A simple, solution-phase deposition method is demonstrated
to produce a monolayer of CoCrPc on highly ordered pyrolytic graphite
(HOPG). A uniform and continuous CoCrPc layer is obtained on freshly
cleaved HOPG by solution drop casting, followed by thermal annealing
under ambient pressure in Ar in the temperature range of 150–210
°C. While the quality of the monolayer is independent of annealing
time, the composition of the annealing atmosphere is critical; exposure
to ambient air degrades the quality of the monolayer over the time
scale of minutes. Using ultrahigh vacuum scanning tunneling microscopy,
a highly ordered and flat CoCrPc layer with hexagonal symmetry and
average spacing of 4.09 ± 0.2 nm is observed. The band gap of
the CoCrPc, measured by scanning tunneling spectroscopy, is 1.34 ±
0.07 eV. The ability to prepare uniform, ordered, and conformal monolayers
of CoCrPc molecules on HOPG represents the first step toward using
these materials to seed dielectric growth on 2D crystals and provide
a 2D electrolyte for the electrostatic gating of semiconductors at
the ultimate limit of scaling
Atomic Layer Deposition of Al<sub>2</sub>O<sub>3</sub> on WSe<sub>2</sub> Functionalized by Titanyl Phthalocyanine
To
deposit an ultrathin dielectric onto WSe<sub>2</sub>, monolayer
titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy
as a seed layer for atomic layer deposition (ALD) of Al<sub>2</sub>O<sub>3</sub> on WSe<sub>2</sub>. TiOPc molecules are arranged in
a flat monolayer with 4-fold symmetry as measured by scanning tunneling
microscopy. ALD pulses of trimethyl aluminum and H<sub>2</sub>O nucleate
on the TiOPc, resulting in a uniform deposition of Al<sub>2</sub>O<sub>3</sub>, as confirmed by atomic force microscopy and cross-sectional
transmission electron microscopy. The field-effect transistors (FETs)
formed using this process have a leakage current of 0.046 pA/μm<sup>2</sup> at 1 V gate bias with 3.0 nm equivalent oxide thickness,
which is a lower leakage current than prior reports. The n-branch
of the FET yielded a subthreshold swing of 80 mV/decade