Search

Nanocrystallization has been an important approach for reducing thermal conductivity in thermoelectric materials due to limits on phonon mean-free path imposed by the characteristic structural size. We report on thermal conductivity as low as 0.3 Wm−1K−1 of nanocrystalline silicon thin films prepared by plasma-enhanced chemical-vapor deposition as grain size is reduced to 2.8 nm by controlling hydrogen dilution of silane gas during growth. A multilayered film composed by alternating growth conditions, with layer thicknesses of 3.6 nm, is measured to have a thermal conductivity 30% and 15% lower than its two constituents. Our quantitative analysis attributes the strong reduction of thermal conductivity with decreasing grain size to the magnifying effect of porosity which occurs concomitantly due to increased mass density fluctuations. Our results demonstrate that ultrasmall grain sizes, multilayering, and porosity, all at a similar nanometer-size scale, may be a promising way to engineer thermoelectric materials. Thermoelectric materials convert heat into electricity and their performance is determined by their figure of merit ZT, which is generally too small in many materials for practical applications. Here, the authors demonstrate that a reduction in grain size for nanocrystalline Si can reduce thermal conductivity and potentially be used as a method to engineer greater ZT in Si for thermoelectric applications.

Achieving both high energy and power density, two traditionally opposed battery metrics, requires moving away from the 200+ years-old 2D layered battery configuration to a three-dimensional all solid-state battery (3D SSB) configuration. The enhanced power and energy benefits bestowed by a 3D SSB design have been demonstrated in the Li-ion 3D SSB literature.1 However, due to limitations in thermal and electronic conductivity of Li-Ion active materials, 3D Li-ion SSBs are limited to microscale dimensions. A potential means around this limitation is to switch to conductive metal electrodes such as Zn and Ag, for which 3D architectures have been demonstrated.2 In such a case, the remaining roadblock to a macroscale 3D SSB is the submicron-thick solid-state electrolyte, which requires a non-line-of-sight deposition method for incorporation into the complex 3D base electrode. Initiated chemical vapor deposition (iCVD) is a non–line-of-sight method that is ideal for generating conformal polymer coatings on complex 3D architectures. We focus on polymers amenable to iCVD protocols that can be modified post-deposition to introduce anion conduction pathways that can facilitate Ag and Zn redox. Submicron-thick coatings of poly-dimethylaminomethylstyrene (pDMAMS) that are pinhole-free and electronically insulating on both 2D planar and complex 3D electrode architectures are prepared via iCVD. The pDMAMS coatings are rendered anion-conducting through a vapor-phase methylation process and subsequent ion exchange to yield desirable anion species (e.g., OH–, Br–, or HCO3 –). X-ray photoelectron spectroscopy, ATR-IR spectroscopy, and solid-state magic-angle spinning NMR spectroscopy confirm the structure of the pDMAMS film before and after quaternization, while atomic force microscopy and cyclic voltammetry with redox probes confirm conformality and absence of pinholes in the submicron film. With the use of cyclic voltammetry and AC electrochemical impedance spectroscopy, the electronic and ionic conductivities of the polymer films are measured before and after quaternization. Molecular dynamics simulations of pDMAMS as a function of anion and H2O content in the film are used to generate computational results (e.g., glass-transition temperature Tg, anion conductivity) for comparison to experimental results. 1. Long, J.W.; Dunn, B.; Rolison, D.R.; White, H.S. Chem. Rev. 2004, 104, 10, 4463–4492. 2. Parker, J.F.; Chervin, C.N.; Nelson, E.S.; Rolison, D.R.; Long, J.W. Energy Environ Sci. 2014, 7, 1117-1124.

We report thermoelectric measurements over a temperature range of 80 K to 300 K of heavily boron-doped nanocrystalline silicon films prepared by hot-wire and plasma-enhanced chemical-vapor depositions. The nanocrystalline silicon films were doped by either gaseous deposition precursors or post-deposition ion implantation, resulting in boron concentrations ranging from 1–2×1020 cm−3 to 3 × 1021 cm−3. Reasonable values of the Seebeck coefficient and electrical conductivity were obtained at 300 K, comparable to many other research work. We also report thermal conductivity measurements on these films before doping, which we use to estimate their prospective thermoelectric efficiency. These measurements show values as low as 0.76 W/mK at 300 K which depend highly upon the grain sizes of the nc-Si films. We find that post-deposition doping by ion-implantation is more effective at enhancing the power factor than gaseous doping, and the power factor is only weakly dependent upon doping concentration for the films doped by ion implantation. We conclude that improvements of the thermoelectric efficiency of nc-Si films may depend more on a reduction of their thermal conductivity than doping optimization. The small grain sizes and the low thermal conductivity of the undoped nc-Si films accomplished in this work are therefore encouraging developments.

We have studied the electrical resistivity and the temperature coefficient of resistance (TCR) of titanium nitride (TiNx) thin films deposited by radio-frequency (RF) reactive magnetron sputtering from a high purity titanium target in a nitrogen-argon gas mixture environment with high nitrogen-to-argon ratio (20:1). The electrical resistivity and TCR are measured from room temperature to 500 °C for films deposited with substrate temperatures of 25 °C, 350 °C and 600 °C. After deposition, some films are annealed at 600 °C for four hours either with or without breaking the vacuum. The structural stability of the films was examined by measuring electrical resistivity from room temperature to 500 °C repeatedly up to four cycles. Selected films were further characterized by Rutherford backscattering, X-Ray diffraction, and Raman spectroscopy. Our results show that RF sputtered TiNx films with good electrical resistivity and temperature-stable TCR for high-temperature applications of conductive diffusion barrier and temperature sensing can be produced. We conclude that high substrate temperature, high annealing temperature, and annealing without breaking the vacuum yield optimal structural stability and electrical resistivity. High mass density is also important to prevent oxidation and degradation of electrical performance.

The investigation of two-level-state (TLS) loss in dielectric materials and interfaces remains at the forefront of materials research in superconducting quantum circuits. We demonstrate a method of TLS loss extraction of a thin film dielectric by using a lumped element resonator fabricated from a superconductor-dielectric-superconductor trilayer. We extract the dielectric loss by formulating a circuit model for a lumped element resonator with TLS loss and then fitting to this model using measurements from a set of three resonator designs: a coplanar waveguide resonator, a lumped element resonator with an interdigitated capacitor, and a lumped element resonator with a parallel plate capacitor that includes the dielectric thin film of interest. Unlike the commonly used single measurement technique, this method allows the accurate measurement of materials with TLS loss lower than 10 − 6. We demonstrate this method by extracting a TLS loss of 1.00 × 10 − 3 for sputtered Al 2 O 3 using a set of samples fabricated from an Al / Al 2 O 3 / Al trilayer. We compare this method with the single measurement technique and observe a difference of 11% in extracted loss of the trilayer.

It has previously been shown that amorphous silicon (a-Si) thin films can be produced free of tunneling two-level systems (TLS) by e-beam evaporation onto substrates held at elevated temperatures, and there appears to be a strong anticorrelation between the atomic density of these films and the number density of TLS. We have prepared a-Si films with higher atomic density using magnetron sputtering at substrate temperatures comparable to those used in the e-beam studies. We compare the atomic densities measured using Rutherford backscattering and the shear moduli, the speeds of sound, and the densities of TLS calculated using internal friction measurements at cryogenic temperatures of sputtered a-Si films to those of the e-beam films. Our results show that despite their higher atomic densities, sputtered a-Si films prepared at elevated substrate temperatures have lower speeds of sound and higher densities of TLS, which we attribute to the different film growth mechanism from that of e-beam evaporation. We conclude that a collaborative improvement of both local structure and network connectivity, determined by atomic density and speed of sound, respectively, to approach their crystalline values is required to eliminate atomic tunneling states.

We have measured the thermal conductivity of amorphous and nanocrystalline silicon films with varying crystalline content from 85 K to room temperature. The films were prepared by the hot-wire chemical-vapor deposition, where the crystalline volume fraction is determined by the hydrogen (H2) dilution ratio to the processing silane gas (SiH4), R = H2/SiH4. We varied R from 1 to 10, where the films transform from amorphous for R

Large low field magnetoresistance (LFMR) of about 28% is observed in La0.67Sr0.33MnO3 nanowires with 80 nm in diameter at T=300 K. A gradual decrease in the LFMR has been found with increase in wire diameter. The LFMR drops to zero for wires above 280 nm in diameter. The nanowires are grown by means of electrospinning process and exhibit distorted orthorhombic crystal structure. The large LFMR is considered as a grain boundary effect as observed in several perovskite systems. The large LFMR observed in these manganites with reduced dimensions may be useful for room temperature device applications.

Boron-incorporated Mn5SiC nanowires were grown using chemical vapor deposition method. The nanowire cluster exhibits magnetic hysteresis loops at room temperature and the strength of the magnetic behavior depends on the concentration of the boron incorporation. Mn5SiC nanowire-based devices exhibit spin dependent transport properties which shows significant changes with boron content. Large magnetoresistance is observed in lightly boron-incorporated nanowire devices and it decreases with increase in boron content.

We report $3\ensuremath{\omega}$ thermal conductivity measurements of amorphous and nanocrystalline silicon thin films from 85 to 300 K prepared by hot-wire chemical-vapor deposition, where the crystallinity of the films is controlled by the hydrogen dilution during growth. The thermal conductivity of the amorphous silicon film is in agreement with several previous reports of amorphous silicon prepared by a variety of deposition techniques. The thermal conductivity of the as-grown nanocrystalline silicon film is 70% higher and increases 35% more after an anneal at $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. They all have similarly weak temperature dependence. Structural analysis shows that the as-grown nanocrystalline silicon is approximately $60%$ crystalline, nanograins and grain boundaries included. The nanograins, averaging 9.1 nm in diameter in the as-grown film, are embedded in an amorphous matrix. The grain size increases to 9.7 nm upon annealing, accompanied by the disappearance of the amorphous phase. We extend the models of grain boundary scattering of phonons with two different non-Debye dispersion relations to explain our result of nanocrystalline silicon, confirming the strong grain size dependence of heat transport for nanocrystalline materials. However, the similarity in thermal conductivity between amorphous and nanocrystalline silicon suggests the heat transport mechanisms in both structures may not be as dissimilar as we currently understand.