Browsing by Subject "Electrolyte gating"

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    Charge transport in single crystal organic semiconductors
    (2013-10) Xie, Wei
    Organic electronics have engendered substantial interest in printable, flexible and large-area applications thanks to their low fabrication cost per unit area, chemical versatility and solution processability. Nevertheless, fundamental understanding of device physics and charge transport in organic semiconductors lag somewhat behind, partially due to ubiquitous defects and impurities in technologically useful organic thin films, formed either by vacuum deposition or solution process. In this context, single-crystalline organic semiconductors, or organic single crystals, have therefore provided the ideal system for transport studies. Organic single crystals are characterized by their high chemical purity and outstanding structural perfection, leading to significantly improved electrical properties compared with their thin-film counterparts. Importantly, the surfaces of the crystals are molecularly flat, an ideal condition for building field-effect transistors (FETs). Progress in organic single crystal FETs (SC-FETs) is tremendous during the past decade. Large mobilities ~ 1 - 10 cm2V-1s-1 have been achieved in several crystals, allowing a wide range of electrical, optical, mechanical, structural, and theoretical studies. Several challenges still remain, however, which are the motivation of this thesis. The first challenge is to delineate the crystal structure/electrical property relationship for development of high-performance organic semiconductors. This thesis demonstrates a full spectrum of studies spanning from chemical synthesis, single crystal structure determination, quantum-chemical calculation, SC-OFET fabrication, electrical measurement, photoelectron spectroscopy characterization and extensive device optimization in a series of new rubrene derivatives, motivated by the fact that rubrene is a benchmark semiconductor with record hole mobility ~ 20 cm2V-1s-1. With successful preservation of beneficial π-stacking structures, these rubrene derivatives form high-quality single crystals and exhibit large ambipolar mobilities. Nevertheless, a gap remains between the theory-predicted properties and this preliminary result, which itself is another fundamental challenge. This is further addressed by appropriate device optimization, and in particular, contact engineering approach to improve the charge injection efficiencies. The outcome is not only the achievement of new record ambipolar mobilities in one of the derivatives, namely, 4.8 cm2V-1s-1 for holes and 4.2 cm2V-1s-1 for electrons, but also provides a comprehensive and rational pathway towards the realization of high-performance organic semiconductors. Efforts to achieve high mobility in other organic single crystals are also presented. The second challenge is tuning the transition of electronic ground states, i.e., semiconducting, metallic and superconducting, in organic single crystals. Despite an active research area since four decades ago, we aim to employ the electrostatic approach instead of chemical doping for reversible and systematic control of charge densities within the same crystal. The key material in this study is the high-capacitance electrolyte, such as ionic liquids (ILs), whose specific capacitance reaches ~ μF/cm2, thus allowing accumulation of charge carrier above 1013 cm-2 when novel transport phenomena, such as insulator-metal transition and superconductivity, are likely to occur. This thesis addresses the electrical characterization, device physics and transport physics in electrolyte-gated single crystals, in the device architecture known as the electrical double layer transistor (EDLT). A detailed characterization scheme is first demonstrated for accurate determination of several key parameters, e.g., carrier mobility and charge density, in organic EDLTs. Further studies, combining both experiments and theories, are devoted to understanding the unusual charge density dependent channel conductivity and gate-to-channel capacitance behaviors. In addition, Hall effect and temperature-dependent measurements are employed for more in-depth understandings of the transport mechanism in these unconventional devices at the extreme charge densities. Inspiringly, a truly metallic state is within reach of this type of device structure. Overall, this thesis demonstrates high mobility, high charge density and high performance organic single crystal transistors, with versatile fabrication techniques, comprehensive electrical and structural characterizations, well-developed theories and models and advanced transport measurements.
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    Data for "Scattering mechanisms and mobility enhancement in epitaxial BaSnO3thin films probed via electrolyte gating"
    (2020-08-05) Wang, Helin; Prakash, Abhinav; Reich, Konstantin; Ganguly, Koustav; Leighton, Chris; leighton@umn.edu; Leighton, Chris; Leighton Electronic and Magnetism Lab
    Data includes temperature-dependent electronic transport (sheet resistance, electron density, and mobility) of ion-gel-gated BaSnO3 thin films of various thicknesses and growth methods. The mobility vs electron density experimental data and the fitting results (fits) are also provided.
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    Electrochemical control of oxygen stoichiometry and materials properties in ion-gel-gated cobaltite thin films
    (2023-12) Postiglione, William
    Wide-ranging control of materials properties using applied voltages represents a longstanding goal in physics and technology, particularly for low-power applications. To this end, substantial interest has developed around electric-double-layer transistors (EDLTs) based on functional materials. More recently, electrochemical EDLTs, where ions such as O2-, H+, Li+, etc., are driven into / out of a channel material via voltage, have proven capable of offering unique benefits (including non-volatility) for a variety of novel applications. Cobaltites, such as SrCoO3-δ (SCO) have recently emerged as an archetypal example of electrochemical control of materials properties in electrolyte-gate devices. This is accomplished by voltage-driven redox cycling between two distinct phases: fully oxygenated perovskite (P) (δ ≈ 0) and oxygen-vacancy-ordered brownmillerite (BM) (δ = 0.5). To date, SCO has received the most attention in this regard, despite significant issues with air stability in the P phase, and few alternatives have been considered. Additionally, critical issues of voltage hysteresis and fundamental limits on reversibility and cycling endurance remain unaddressed.To address this, using EDLTs based on epitaxial La1-xSrxCoO3-δ (LSCO) thin films, we first investigate the electrochemical reduction that is known to occur at positive gate voltages (Vg) in such systems, establishing that the P → BM transformation occurs in LSCO over a wide doping range. Importantly, both the P and BM phase of x = 0.5 LSCO are robustly air stable, and the electrochemical reduction behavior was found to be voltage-tunable with both doping and strain. We then leverage this voltage-tuned P → BM transformation to demonstrate large property modulations in electronic transport, magnetism, thermal transport, and optical properties, achieving similar or greater ranges of control than in SCO. Next, to explore the reversibility of the transformation, we performed detailed analysis of Vg hysteresis loops, revealing a wealth of new mechanistic findings, including asymmetric transformations due to differing oxygen diffusivities in the P vs. the BM phase, non-monotonic transformation rates due to the first-order nature of the P-BM transformation, and limits on reversibility due to first-cycle structural degradation. Additionally, using minor hysteresis loops, we demonstrate the first rational design of an optimal Vg cycle, leading to state-of-the-art cycling of electronic and magnetic properties, encompassing >105 transport ON/OFF ratios at room temperature, reversible and non-volatile metal-insulator-metal and ferromagnet-nonferromagnet-ferromagnet cycling, all at ultrathin 10-unit-cell thickness. Finally, to further investigate the magnetic properties of the BM nonferromagnet “OFF” state, we performed neutron diffraction experiments, finding the first direct evidence of antiferromagnetic order in BM-SCO films and identifying weak ferromagnetism in x = 0.5 BM-LSCO. These findings thus significantly advance the understanding of voltage-induced P ↔ BM transformations in cobaltite films and pave the way for future work establishing the ultimate cycling frequency and endurance in such electrolyte-gated devices.
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    Ion gel gating of perovskite cobaltite thin films: Understanding mechanisms and control of magnetism
    (2018-05) Walter, Jeff
    Recently, electrolyte gating techniques using ionic liquids and gels have proven highly effective in tuning large carrier densities at material surfaces. These electrolytes enable electric double layer transistor operation, the large capacitances (10’s of µF/cm2) generating electron/hole densities up to 1015 cm-2, i.e., significant fractions of an electron per unit cell in most materials. Uncertainties remain, however, including the true doping mechanism (i.e., electrostatic vs. electrochemical), the challenge of in operando characterization, and the need to understand the full potential and universality. In regard to universality, superconductivity and insulator-metal transitions have been extensively studied with electrolyte gating, but this promising technique has been less applied to controlling magnetism. Electrical control of magnetism is a long-standing goal in physics and technology, electrolyte gating techniques providing a promising route to realization. Employing electric double layer transistors based on ultrathin epitaxial La1-xSrxCoO3 as a model system, our findings first address the true doping mechanism, clarifying charge carrier vs. oxygen defect creation. Transport measurements reveal dramatic asymmetry with respect to bias polarity. Negative gate biases lead to reversible behavior (i.e., predominantly electrostatic operation) up to some threshold, whereas positive bias immediately induces irreversibility. Experiments in inert/O2 atmospheres directly implicate oxygen vacancies in this irreversibility, supported by atomic force microscopy and X-ray photoelectron spectroscopy. We then demonstrate the use of synchrotron hard X-ray diffraction and polarized neutron reflectometry as in operando probes to further investigate the gating mechanism. An asymmetric gate bias response is confirmed to derive from electrostatic hole accumulation at negative bias vs. oxygen vacancy formation at positive bias. The latter is detected via a large gate-induced lattice expansion (up to 1 %), complementary bulk measurements and density functional calculations enabling quantification of the bias-dependent oxygen vacancy density. Remarkably, the gate-induced oxygen vacancies proliferate through the entire thickness of 30-40-unit-cell-thick films, quantitatively accounting for changes in the magnetization depth profile and demonstrating electrochemical control of magnetism. This is interpreted in a simple picture where electrostatic vs. electrochemical response is dictated by the low formation enthalpy and high diffusivity of oxygen vacancies in La1-xSrxCoO3. These results, therefore, directly elucidate the issue of electrostatic vs. redox-based response in electrolyte-gated oxides, also demonstrating powerful new approaches to their in operando investigation. Control of ferromagnetism is then demonstrated in electrostatic mode by working at negative bias. Guided by theory, we demonstrate reversible electrical control of Curie temperature over a 150 K window. This is achieved via gate-induced cluster percolation, leading to optimized control of ferromagnetism, directly verified by magnetoresistance, anomalous Hall effect, and PNR measurements.