Binay Singh

A new class of doped nanobulk high-figure-of-merit thermoelectrics by scalable bottom-up assembly

Obtaining thermoelectric materials with high figure of merit ZT is an exacting challenge because it requires the independent control of electrical conductivity, thermal conductivity and Seebeck coefficient, which are often unfavourably coupled. Recent works have devised strategies based on nanostructuring and alloying to address this challenge in thin films, and to obtain bulk p-type alloys with ZT>1. Here, we demonstrate a new class of both p- and n-type bulk nanomaterials with room-temperature ZT as high as 1.1 using a combination of sub-atomic-per-cent doping and nanostructuring. Our nanomaterials were fabricated by bottom-up assembly of sulphur-doped pnictogen chalcogenide nanoplates sculpted by a scalable microwave-stimulated wet-chemical method. Bulk nanomaterials from single-component assemblies or nanoplate mixtures of different materials exhibit 25-250% higher ZT than their non-nanostructured bulk counterparts and state-of-the-art alloys. Adapting our synthesis and assembly approach should enable nanobulk thermoelectrics with further increases in ZT for transforming thermoelectric refrigeration and power harvesting technologies.

Seebeck Tuning in Chalcogenide Nanoplate Assemblies by Nanoscale Heterostructuring

Chalcogenide nanostructures offer promise for obtaining nanomaterials with high electrical conductivity, low thermal conductivity, and high Seebeck coefficient. Here, we demonstrate a new approach of tuning the Seebeck coefficient of nanoplate assemblies of single-crystal pnictogen chalcogenides by heterostructuring the nanoplates with tellurium nanocrystals. We synthesized bismuth telluride and antimony telluride nanoplates decorated with tellurium nanorods and nanofins using a rapid, scalable, microwave-stimulated organic surfactant-directed technique. Heterostructuring permits two- to three-fold factorial tuning of the Seebeck coefficient, and yields a 40% higher value than the highest reported for bulk antimony telluride. Microscopy and spectroscopy analyses of the nanostructures suggest that Seebeck tunability arises from carrier-energy filtration effects at the Te-chalcogenide heterointerfaces. Our approach of heterostructuring nanoscale building blocks is attractive for realizing high figure-of-merit thermoelectric nanomaterials.

High Electrical Conductivity Antimony Selenide Nanocrystals and Assemblies

Antimony selenide is a promising thermoelectric material with a high Seebeck coefficient, but its figure of merit is limited by its low electrical conductivity. Here, we report a rapid and scalable (gram-a-minute) microwave synthesis of one-dimensional nanocrystals of sulfurized antimony selenide that exhibit 10(4)-10(10) times higher electrical conductivity than non-nanostructured bulk or thin film forms of this material. As the nanocrystal diameter increases, the nanowires transform into nanotubes through void formation and coalescence driven by axial rejection of sulfur incorporated into the nanowires from the surfactant used in our synthesis. Individual nanowires and nanotubes exhibit a charge carrier transport activation-energy of <60 meV arising from surface sulfur donor states. Nanocrystal assemblies also show high electrical conductivity, making the nanocrystals attractive building blocks to realize nanostructured thin film and bulk forms of this material for thermoelectric device applications.

Ultraviolet-oxidized mercaptan-terminated organosilane nanolayers as diffusion barriers at Cu-silica interfaces

We demonstrate the use of UV-exposed molecular nanolayers (MNLs) of 3-mercaptan-propyl-trimethoxysilane to inhibit copper-transport across Cu–SiO2 interfaces more efficiently than the pristine MNLs. Bias-thermal-annealing tests of Cu∕MNL∕SiO2∕Si(001)∕Al capacitors, with MNLs exposed to 254nm UV radiation, exhibit enhanced barrier properties to Cu diffusion, when compared with capacitors with MNLs not exposed to UV light. X-ray photoelectron spectroscopy reveals that UV exposure converts the mercaptan termini to sulfonates, which are more effective in inhibiting Cu diffusion. Our findings are of importance for tailoring the chemical and mechanical integrity of interfaces for use in applications such as nanodevice wiring and molecular electronics.

Ring-Opening-Induced Toughening of a Low-Permittivity Polymer−Metal Interface

Integrating low dielectric permittivity (low-k) polymers to metals is an exacting fundamental challenge because poor bonding between low-polarizability moieties and metals precludes good interfacial adhesion. Conventional adhesion-enhancing methods such as using intermediary layers are unsuitable for engineering polymer/metal interfaces for many applications because of the collateral increase in dielectric permittivity. Here, we demonstrate a completely new approach without surface treatments or intermediary layers to obtain an excellent interfacial fracture toughness of >13 J/m(2) in a model system comprising copper and a cross-linked polycarbosilane with k approximately 2.7 obtained by curing a cyclolinear polycarbosilane in air. Our results suggest that interfacial oxygen catalyzed molecular ring-opening and anchoring of the opened ring moieties of the polymer to copper is the main toughening mechanism. This novel approach of realizing adherent low-k polymer/metal structures without intermediary layers by activating metal-anchoring polymer moieties at the interface could be adapted for applications such as device wiring and packaging, and laminates and composites.

Metal-dielectric interface toughening by molecular nanolayer decomposition

Recent work has shown that copper–silica interfaces can be toughened several fold by combining interface functionalization with an organosilane molecular nanolayer (MNL) and thermal annealing. In order to understand the role of annealing-induced MNL instabilities on interface toughness, we studied the effects of interface chemical changes on the fracture toughness of copper–silica interfaces tailored with organosilane or organogermane MNLs. Our results indicate that MNL decomposition into its inorganic constituents and consequent intermixing can provide an interface toughening mechanism. Organogermane–tailored interfaces exhibit higher toughness values due to Ge-diffusion induced copper silicate formation, not observed at organosilane tailored interfaces. These findings show that organic nanolayer decomposition at a buried interface could be exploited to tailor interfacial properties through appropriate choice of MNL chemistry and processing treatments.

Hydrophobic fluoroalkylsilane nanolayers for inhibiting copper diffusion into silica

Molecular nanolayers (MNLs) are attractive for suppressing chemical transport and mixing at copper-silica interfaces. Here, we demonstrate that hydrophobic fluoroalkyl moieties enhance the effectiveness of organosilane MNLs to inhibit copper diffusion. Bias thermal annealing of copper-MNL-silica capacitors with MNLs having different fluoroalkyl contents, combined with electron spectroscopy and contact angle measurements, show that the enhanced barrier properties are due to diminished water uptake and curtailed copper ionization. Our results suggest that controlling interface moisture content using hydrophobic moieties can complement copper ion immobilization by hydrophilic groups in MNL barriers.

Pore orientation and silylation effects on mesoporous silica film properties

Low dielectric permittivity mesoporous silica (MPS) films with high mechanical and chemical stability are attractive for electrically isolating multilevel wiring in future nanodevices. Here, we show that pore structure is a crucial determinant of chemically induced leakage currents in pristine and silylated MPS films and strongly influences film stiffness and hardness in silylated MPS films. Films with three-dimensional pore networks exhibit superior mechanical properties than films with cylindrical pores oriented exclusively parallel to the surface. The latter, however, exhibit a fourfold higher resilience to copper diffusion. These differences are attributed to the pore structure and its influence on silylation-induced bond-breaking and passivation.

Stabilization of mesoporous silica films using multiple organosilanes

Mesoporous silica (MPS) thin films are attractive for electrically isolating Cu wiring in nanodevices. While porosity is conducive for realizing low-dielectric permittivity k necessary for low signal propagation delays, it renders the MPS susceptible to moisture uptake and metal diffusion. Here, we show that passivating MPS with more than one organosilane with different molecular termini provides several fold greater protection against such instabilities than improvements observed by functionalizing MPS with either type of organosilane individually. MPS films functionalized with bis[3-(triethoxysilyl)propyl] tetrasulfide (BTPTS) and trimethylchlorosilane (TMCS) exhibit at least three orders of magnitude greater time to dielectric breakdown. Bias thermal annealing and infrared spectroscopy measurements indicate that the increased stability is due to Cu blocking by the tetrasulfide groups in BTPTS and decreased moisture uptake is caused by hydrophobic passivation with TMCS. These findings are germane for re...

Effects of silylation on fracture and mechanical properties of mesoporous silica films interfaced with copper

Mesoporous silica (MPS) films are attractive for isolating Cu wiring in nanodevices but are susceptible to pore wall collapse and water and metal uptake. Pore-sealing and chemical passivation with molecular surfactants are potential solutions that could address these challenges. Here, we show that silylated MPS films capped with a Cu overlayer fracture near the Cu/MPS interface at a distance that correlates with the Cu penetration depth into MPS. Pristine MPS films fracture farther from the MPS/Cu interface than silylated MPS, where silylation-induced pore passivation hinders Cu penetration. Silylation also lowers the tensile stress and the fracture toughness of MPS films, but the relative extent of the decreases in these properties decreases the overall driving force for cracking. Such effects of molecular passivation on metal penetration, film stress, and fracture toughness and pathways are important for engineering stable porous dielectrics for nanodevice wiring structures.