Robert Y. Wang

Enhanced Thermopower in PbSe Nanocrystal Quantum Dot Superlattices

We examine the effect of strong three-dimensional quantum confinement on the thermopower and electrical conductivity of PbSe nanocrystal superlattices. We show that for comparable carrier concentrations PbSe nanocrystal superlattices exhibit a substantial thermopower enhancement of several hundred microvolts per Kelvin relative to bulk PbSe. We also find that thermopower increases monotonically as the nanocrystal size decreases due to changes in carrier concentration. Lastly, we demonstrate that thermopower of PbSe nanocrystal solids can be tailored by charge-transfer doping.

Nanostructuring expands thermal limits

Scientists and engineers can exploit nanostructures to manipulate thermal transport in solids. This is possible because the dominant heat carriers in nonmetals – crystal vibrations (or phonons) – have characteristic lengths in the nanometer range. We review research where this approach is used and propose future research directions. For instance, concepts such as phonon filtering, correlated scattering, and waveguiding could expand the extremes of thermal transport in both the insulating and conducting limits. This will have major implications on energy conservation and conversion, information technology, and thermal management systems.

Room temperature thermal conductance of alkanedithiol self-assembled monolayers

Solid-solid junctions with an interfacial self-assembled monolayer (SAM) are a class of interfaces with very low thermal conductance. Au–SAM–GaAs junctions were made using alkanedithiol SAMs and fabricated by nanotransfer printing. Measurements of thermal conductance using the 3ω technique were very robust and no thermal conductance dependence on alkane chain length was observed. The thermal conductances using octanedithiol, nonanedithiol, and decanedithiol SAMs at room temperature are 27.6±2.9, 28.2±1.8, and 25.6±2.4MWm−2K−1, respectively.

Electronic and optical switching of solution-phase deposited SnSe2 phase change memory material

We report the use of chalcogenidometallate clusters as a solution-processable precursor to SnSe2 for phase change memory applications. This precursor is spin-coated onto substrates and then thermally decomposed into a crystalline SnSe2 film. Laser testing of our SnSe2 films indicate very fast recrystallization times of 20 ns. We also fabricate simple planar SnSe2 electronic switching devices that demonstrate switching between ON and OFF resistance states with resistance ratios varying from 7−76. The simple cell design resulted in poor cycling endurance. To demonstrate the precursor’s applicability to advanced via-geometry memory devices, we use the precursor to create void-free SnSe2 structures inside nanowells of ∼25 nm in diameter and ∼40 nm in depth.

Ionic and Electronic Transport in Ag2S Nanocrystal–GeS2 Matrix Composites with Size-Controlled Ag2S Nanocrystals

The Ag-Ge-S system is among the best-performing active materials for a new class of high-performance electronic memory known as programmable metallization cells.[1,2] Memory switching in a programmable metallization cell relies on the electrochemical formation and dissolution of an electronically conducting filament within a solid electrolyte. In the Ag-Ge-S system, this electrochemical process is facilitated by the transport of both Ag+ ions and electrons. Consequently, it is important to understand how the ionic conductivity and electronic conductivity relate to the structure of the Ag-Ge-S system. However, this understanding is hampered by the ill-defined morphology of Ag-Ge-S, which consists of an amorphous Ge-rich matrix with embedded Ag-rich nanoinclusions of random size, shape, and interparticle spacing.[1,2] We circumvent these morphological ambiguities by controlling these structural variables with our recently developed nanocomposite formation technique.[3] We create Ag2S nanocrystal–GeS2 matrix composites and demonstrate that their ionic and electronic properties can be systematically controlled by varying the diameter of the Ag2S nanocrystals. We also observe an ionic conductivity enhancement relative to pure Ag2S and (GeS2)0.5(Ag2S)0.5 glass. Additionally, the thermal phase transition of Ag2S into its superionic phase exhibits differences in thermal hysteresis and transition temperature when comparing the composites to pure Ag2S. The combined structural control and chemical composition flexibility of our nanocomposite formation technique will allow the careful study of structure-property relationships in many important nanocomposite systems. The flexibility of the nanocomposite fabrication is enabled by a modular formation process in which nanocrystals and chalcogenidometallate (ChaM) clusters are independently synthesized. After assembling the nanocrystals into a thin film, the ChaM clusters are intercalated into the interstitial space between nanocrystals. The ChaM clusters are then converted into an inorganic matrix

Metal Matrix–Metal Nanoparticle Composites with Tunable Melting Temperature and High Thermal Conductivity for Phase-Change Thermal Storage

Phase-change materials (PCMs) are of broad interest for thermal storage and management applications. For energy-dense storage with fast thermal charging/discharging rates, a PCM should have a suitable melting temperature, large enthalpy of fusion, and high thermal conductivity. To simultaneously accomplish these traits, we custom design nanocomposites consisting of phase-change Bi nanoparticles embedded in an Ag matrix. We precisely control nanoparticle size, shape, and volume fraction in the composite by separating the nanoparticle synthesis and nanocomposite formation steps. We demonstrate a 50-100% thermal energy density improvement relative to common organic PCMs with equivalent volume fraction. We also tune the melting temperature from 236-252 °C by varying nanoparticle diameter from 8.1-14.9 nm. Importantly, the silver matrix successfully prevents nanoparticle coalescence, and no melting changes are observed during 100 melt-freeze cycles. The nanocomposites Ag matrix also leads to very high thermal conductivities. For example, the thermal conductivity of a composite with a 10% volume fraction of 13 nm Bi nanoparticles is 128 ± 23 W/m-K, which is several orders of magnitude higher than typical thermal storage materials. We complement these measurements with calculations using a modified effective medium approximation for nanoscale thermal transport. These calculations predict that the thermal conductivity of composites with 13 nm Bi nanoparticles varies from 142 to 47 W/m-K as the nanoparticle volume fraction changes from 10 to 35%. Larger nanoparticle diameters and/or smaller nanoparticle volume fractions lead to larger thermal conductivities.

Synthesis and Size-Dependent Crystallization of Colloidal Germanium Telluride

Colloidal nanocrystals have long been used to study the dependence of phase stability and transitions on size. Both structural phase stability and phase transitions change dramatically in the nanometre size regime where the surface plays a significant role in determining the overall energetics of the system. We investigate the solid-solid phase transformation of crystallization in amorphous GeTenanoparticles. We report a colloidal synthetic route to amorphous GeTenanoparticles. Using in situ X-ray diffraction while heating, we observe the crystallization of the nanoparticles and find a dramatic increase of the crystallization temperature of over 150 C above the bulk value. Using size-selected nanoparticle films, we show that the crystallization temperature depends strongly on the particle size. In addition, we measure the electrical resistance of nanoparticle films and observe over 5 orders of magnitude lower resistance for the crystalline film compared to the amorphous film. Finally, we discuss the implications of the size-dependence of crystallization in the context of both understanding the behavior of phase stability in the nanosize regime and applications to phase change memory devices.

Elevated expression of circulating miR876-5p is a specific response to severe EV71 infections

Human enterovirus 71 (EV71) is a major causative agent of hand, foot, and, mouth disease, accounting for more than 65% of recent outbreaks. Following enteroviral infection, the host responses are crucial indicators for the development of a diagnosis regarding the clinical severity of EV71 infections. In this study, we implemented NanoString nCounter technology to characterize the responses of serum microRNA (miRNA) profiles to various EV71 infection diseases. Upon EV71 infection, 44 miRNAs were observed in patients with EV71 infections, with at least a 2-fold elevation and 133 miRNAs with a 2-fold reduction compared with the same miRNAs in healthy controls. Further detailed work with miR876-5p, a 9.5-fold change of upregulated miR-876-5p expression was observed in cases with severe EV71 symptoms, revealed that in vitro and in vivo knockdown of miR876-5p reduced viral RNA in cultured cells, and attenuated the severity of symptoms in EV71-infected mice. Altogether, we demonstrated that the elevated expression of circulating miR876-5p is a specific response to severe EV71 infections.

Phase change nanocomposites with tunable melting temperature and thermal energy storage density

Size-dependent melting decouples melting temperature from chemical composition and provides a new design variable for phase change material applications. To demonstrate this potential, we create nanocomposites that exhibit stable and tunable melting temperatures through numerous melt-freeze cycles. These composites consist of a monodisperse ensemble of Bi nanoparticles (NPs) embedded in a polyimide (PI) resin matrix. The Bi NPs operate as the phase change component whereas the PI resin matrix prevents nanoparticle coalescence during melt-freeze cycles. We tune melting temperature and enthalpy of fusion in these composites by varying the NP diameter. Adjusting the NP volume fraction also controls the composites thermal energy storage density. Hence it is possible to leverage size effects to tune phase change temperature and energy density in phase change materials.

Size-Dependent Melting Behavior of Colloidal In, Sn, and Bi Nanocrystals

Colloidal nanocrystals are a technologically important class of nanostructures whose phase change properties have been largely unexplored. Here we report on the melting behavior of In, Sn, and Bi nanocrystals dispersed in a polymer matrix. This polymer matrix prevents the nanocrystals from coalescing with one another and enables previously unaccessed observations on the melting behavior of colloidal nanocrystals. We measure the melting temperature, melting enthalpy, and melting entropy of colloidal nanocrystals with diameters of approximately 10 to 20 nm. All of these properties decrease as nanocrystal size decreases, although the depression rate for melting temperature is comparatively slower than that of melting enthalpy and melting entropy. We also observe an elevated melting temperature during the initial melt-freeze cycle that we attribute to surface stabilization from the organic ligands on the nanocrystal surface. Broad endothermic melting valleys and very large supercoolings in our calorimetry data suggest that colloidal nanocrystals exhibit a significant amount of surface pre-melting and low heterogeneous nucleation probabilities during freezing.