In-situ resonant band engineering of solution-processed semiconductors generates high performance n-type thermoelectric nano-inks.

Thermoelectric devices possess enormous potential to reshape the global energy landscape by converting waste heat into electricity, yet their commercial implementation has been limited by their high cost to output power ratio. No single "champion" thermoelectric material exists due to a broad range of material-dependent thermal and electrical property optimization challenges. While the advent of nanostructuring provided a general design paradigm for reducing material thermal conductivities, there exists no analogous strategy for homogeneous, precise doping of materials. Here, we demonstrate a nanoscale interface-engineering approach that harnesses the large chemically accessible surface areas of nanomaterials to yield massive, finely-controlled, and stable changes in the Seebeck coefficient, switching a poor nonconventional p-type thermoelectric material, tellurium, into a robust n-type material exhibiting stable properties over months of testing. These remodeled, n-type nanowires display extremely high power factors (~500 µW m-1K-2) that are orders of magnitude higher than their bulk p-type counterparts

A Mechanistic Analysis of Phase Evolution and Hydrogen Storage Behavior in Nanocrystalline Mg(BH4)2 within Reduced Graphene Oxide.

Magnesium borohydride (Mg(BH4)2, abbreviated here MBH) has received tremendous attention as a promising onboard hydrogen storage medium due to its excellent gravimetric and volumetric hydrogen storage capacities. While the polymorphs of MBH-alpha (α), beta (β), and gamma (γ)-have distinct properties, their synthetic homogeneity can be difficult to control, mainly due to their structural complexity and similar thermodynamic properties. Here, we describe an effective approach for obtaining pure polymorphic phases of MBH nanomaterials within a reduced graphene oxide support (abbreviated MBHg) under mild conditions (60-190 °C under mild vacuum, 2 Torr), starting from two distinct samples initially dried under Ar and vacuum. Specifically, we selectively synthesize the thermodynamically stable α phase and metastable β phase from the γ-phase within the temperature range of 150-180 °C. The relevant underlying phase evolution mechanism is elucidated by theoretical thermodynamics and kinetic nucleation modeling. The resulting MBHg composites exhibit structural stability, resistance to oxidation, and partially reversible formation of diverse [BH4]- species during de- and rehydrogenation processes, rendering them intriguing candidates for further optimization toward hydrogen storage applications

Carrier Transport in Hybrid Organic-Inorganic Thermoelectric Materials

Thermoelectric devices have the unique ability to interconvert heat and electricity directly. Soft thermoelectric materials, including conjugated polymers and organic-inorganic hybrids, now demonstrate figures of merit approaching those of inorganic materials. These breakthroughs in materials development enable the design of thermoelectric devices that exhibit appropriate efficiencies for commercial use, while simultaneously leveraging the unique processing and mechanical advantages of soft materials. Such technology opens the door to a suite of new thermoelectric applications, including power generation for biomedical implants and the Internet of Things, or wearable heating and cooling devices. However, in order to realize deployment of such technologies, there is a fundamental need for deeper understanding of the complex transport physics underlying thermoelectric transport in soft materials.The central focus of this dissertation is investigating the fundamental physical phenomena critical to carrier transport in hybrid organic-inorganic thermoelectric material. Due to the complex nature of this class of multiphase material, there remains a problematic lack of consensus in the field regarding transport in hybrid materials. The mechanisms of carrier transport, key physics responsible for high thermoelectric performance, and even how to model transport in these materials are all subjects of debate within the field. Here, I describe the design, synthesis, and characterization of a prototypical PEDOT:PSS-Te hybrid nanomaterial with the goal of performing careful study of the carrier physics and relevant molecular-scale phenomena in this material. A novel technique for patterning alloy nanophases is demonstrated, resulting in well-controlled PEDOT:PSS-Te-Cu1.75Te heterowires. The Te-Cu1.75Te energetics are well aligned to leverage the carrier filtering effects proposed in literature. Using a full suite of experimental, theoretical, and modeling tools, we reveal the key physics responsible for dictating carrier transport and thermoelectric properties in this material, testing each of the major hypothesis in the field. Contrary to popular belief in the field, it is revealed that energy filtering does not play a major role in the carrier transport and high thermoelectric performance of these materials; rather, organic structural effects at the hard-soft interface and interfacial charge transport emerge as the key phenomena underlying transport.In a complementary study, I describe a platform approach for the synthesis of new solution-based, air stable n-type soft thermoelectrics. Using this approach, a composite perylene diimide-Te nanowire thermoelectric ink is prepared, demonstrating up to 20-fold enhancement over the individual components. The performance of these materials is competitive with the best-in-class for fully solution-processed, air stable n-type thermoelectric inks. We find experimental evidence linking reorganization of the perylene diimide molecules on the Te surface to enhanced electrical conductivity in the composite, further emphasizing the importance of structural effects in the organic phase to the overall thermoelectric properties of hybrid materials. Finally, leveraging the best materials from among the work in this dissertation, we demonstrate power generation in an all-ink flexible thermoelectric module with an innovative folded geometry.The findings in this dissertation provide critical insight into the physics underlying carrier transport and high thermoelectric performance in hybrid organic-inorganic nanomaterials. This work highlights the importance of developing hybrid design strategies capable of leveraging molecular-level effects at the hard-soft interface. In furthering the field’s fundamental understanding of this material class, we drive progress towards the realization of flexible thermoelectric modules compatible with applications such as implantable medical devices, wearable technologies, and the Internet of Things

Polymer morphology and interfacial charge transfer dominate over energy-dependent scattering in organic-inorganic thermoelectrics.

Hybrid (organic-inorganic) materials have emerged as a promising class of thermoelectric materials, achieving power factors (S2σ) exceeding those of either constituent. The mechanism of this enhancement is still under debate, and pinpointing the underlying physics has proven difficult. In this work, we combine transport measurements with theoretical simulations and first principles calculations on a prototypical PEDOT:PSS-Te(Cux) nanowire hybrid material system to understand the effect of templating and charge redistribution on the thermoelectric performance. Further, we apply the recently developed Kang-Snyder charge transport model to show that scattering of holes in the hybrid system, defined by the energy-dependent scattering parameter, remains the same as in the host polymer matrix; performance is instead dictated by polymer morphology manifested in an energy-independent transport coefficient. We build upon this language to explain thermoelectric behavior in a variety of PEDOT and P3HT based hybrids acting as a guide for future work in multiphase materials

Bottom-up meets top-down: Tailored raspberry-like Fe3O4-Pt nanocrystal superlattices

Supported catalysts are widely used in industry and can be optimized by tuning the composition, chemical structure, and interface of the nanoparticle catalyst and oxide support. Here we firstly combine a bottom up colloidal synthesis method with a top down atomic layer deposition (ALD) process to achieve a raspberry-like Pt-decorated Fe3O4 (Fe3O4-Pt) nanoparticle superlattices. This nanocomposite ensures the precision of the catalyst/support interface, improving the catalytic efficiency of the Fe3O4-Pt nanocomposite system. The morphology of the hybrid nanocomposites resulting from different cycles of ALD was monitored by scanning transmission electron microscopy, giving insight into the nucleation and growth mechanism of the ALD process. X-ray photoelectron spectroscopy studies confirm the anticipated electron transfer from Fe3O4 to Pt through the nanocomposite interface. Photocurrent measurement further suggests that Fe3O4 superlattices with controlled decoration of Pt have substantial promise for energy-efficient photoelectrocatalytic oxygen evolution reaction. This work opens a new avenue for designing supported catalyst architectures via precisely controlled decoration of single component superlattices with noble metals

Bottom-up meets top-down:Tailored raspberry-like Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e-Pt nanocrystal superlattices

\u3cp\u3eSupported catalysts are widely used in industry and can be optimized by tuning the composition, chemical structure, and interface of the nanoparticle catalyst and oxide support. Here we firstly combine a bottom up colloidal synthesis method with a top down atomic layer deposition (ALD) process to achieve a raspberry-like Pt-decorated Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e (Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e-Pt) nanoparticle superlattices. This nanocomposite ensures the precision of the catalyst/support interface, improving the catalytic efficiency of the Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e-Pt nanocomposite system. The morphology of the hybrid nanocomposites resulting from different cycles of ALD was monitored by scanning transmission electron microscopy, giving insight into the nucleation and growth mechanism of the ALD process. X-ray photoelectron spectroscopy studies confirm the anticipated electron transfer from Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e to Pt through the nanocomposite interface. Photocurrent measurement further suggests that Fe\u3csub\u3e3\u3c/sub\u3eO\u3csub\u3e4\u3c/sub\u3e superlattices with controlled decoration of Pt have substantial promise for energy-efficient photoelectrocatalytic oxygen evolution reaction. This work opens a new avenue for designing supported catalyst architectures via precisely controlled decoration of single component superlattices with noble metals.\u3c/p\u3

Exchange Bias in a Layered Metal–Organic Topological Spin Glass

The discovery of conductive and magnetic two-dimensional (2D) materials is critical for the development of next generation spintronics devices. Coordination chemistry in particular represents a highly versatile, though underutilized, route toward the synthesis of such materials with designer lattices. Here, we report the synthesis of a conductive, layered 2D metal–organic kagome lattice, Mn3(C6S6), using mild solution-phase chemistry. Strong geometric spin frustration in this system mediates spin freezing at low temperatures, which results in glassy magnetic behavior consistent with a geometrically frustrated (topological) spin glass. Notably, the material exhibits a large exchange bias of 1625 Oe, providing the first example of exchange bias in a coordination solid or a topological spin glass. More generally, these results demonstrate the potential utility of geometrically frustrated lattices in the design of new nanoscale spintronic materials.</p

Exchange Bias in a Layered Metal-Organic Topological Spin Glass.

The discovery of conductive and magnetic two-dimensional (2D) materials is critical for the development of next generation spintronics devices. Coordination chemistry in particular represents a highly versatile, though underutilized, route toward the synthesis of such materials with designer lattices. Here, we report the synthesis of a conductive, layered 2D metal-organic kagome lattice, Mn3(C6S6), using mild solution-phase chemistry. Strong geometric spin frustration in this system mediates spin freezing at low temperatures, which results in glassy magnetic dynamics consistent with a rare geometrically frustrated (topological) spin glass. Notably, we show that this geometric frustration engenders a large, tunable exchange bias of 1625 Oe in Mn3(C6S6), providing the first example of exchange bias in a coordination solid or a topological spin glass. Exchange bias is a critical component in a number of spintronics applications, but it is difficult to rationally tune, as it typically arises due to structural disorder. This work outlines a new strategy for engineering exchange bias systems using single-phase, crystalline lattices. More generally, these results demonstrate the potential utility of geometric frustration in the design of new nanoscale spintronic materials

Exchange Bias in a Layered Metal-Organic Topological Spin Glass.

The discovery of conductive and magnetic two-dimensional (2D) materials is critical for the development of next generation spintronics devices. Coordination chemistry in particular represents a highly versatile, though underutilized, route toward the synthesis of such materials with designer lattices. Here, we report the synthesis of a conductive, layered 2D metal-organic kagome lattice, Mn3(C6S6), using mild solution-phase chemistry. Strong geometric spin frustration in this system mediates spin freezing at low temperatures, which results in glassy magnetic dynamics consistent with a rare geometrically frustrated (topological) spin glass. Notably, we show that this geometric frustration engenders a large, tunable exchange bias of 1625 Oe in Mn3(C6S6), providing the first example of exchange bias in a coordination solid or a topological spin glass. Exchange bias is a critical component in a number of spintronics applications, but it is difficult to rationally tune, as it typically arises due to structural disorder. This work outlines a new strategy for engineering exchange bias systems using single-phase, crystalline lattices. More generally, these results demonstrate the potential utility of geometric frustration in the design of new nanoscale spintronic materials