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 LiClO₄, 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⁺ ratio of 5:1, programming tests for which the backgate is held at −<i>V</i>_BG shift the Dirac point by ∼15 V, corresponding to a sheet carrier density on the order of 10¹² cm^–2. This charge carrier density agrees with the packing density of monolayer CoCrPc on graphene that would be expected with one Li⁺ for every five crown ethers (at the maximum possible Li⁺ concentration, 10¹³ cm^–2 is predicted). The crown ethers provide two stable states for the Li⁺: 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 10⁶ 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) Fe₂O₃ 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 LiClO₄ filled with 1 wt % γ-phase core (maghemite) and α-phase shell (hematite) Fe₂O₃ 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)₆/LiClO₄. 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)₆/LiClO₄. The results demonstrate that field-effect alignment of magnetic nanorods increases the crystal fraction and homogeneity of PEO/LiClO₄, 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 LiClO₄, a solid polymer electrolyte with a glass transition temperature (<i>T</i>_g) of 80 °C, is used to electrostatically gate graphene field-effect transistors. The ions in PVA:LiClO₄ are drifted into place by field-effect at <i>T</i> > <i>T</i>_g, 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>_g, 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 LiClO₄. Hall measurements confirm that large sheet carrier densities can be achieved with PVA:LiClO₄ at top gate programming voltages of ±2 V (−6.3 ± 0.03 × 10¹³ cm^–2 for electrons and 1.6 ± 0.3 × 10¹⁴ cm^–2 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:LiClO₄ 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

Influence of Fe₂O₃ 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 LiClO₄ filled with 1–20 wt % spherical Fe₂O₃ nanoparticles and 0.5–10 wt % Fe₂O₃ 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)₆:LiClO₄ 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⁺) 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⁺ 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_<i>z</i> 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⁺ (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 Al₂O₃ on WSe₂ Functionalized by Titanyl Phthalocyanine

To deposit an ultrathin dielectric onto WSe₂, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy as a seed layer for atomic layer deposition (ALD) of Al₂O₃ on WSe₂. TiOPc molecules are arranged in a flat monolayer with 4-fold symmetry as measured by scanning tunneling microscopy. ALD pulses of trimethyl aluminum and H₂O nucleate on the TiOPc, resulting in a uniform deposition of Al₂O₃, 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/μm² 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