New fabrication technique for highly sensitive qPlus sensor with well-defined spring constant

A new technique for the fabrication of highly sensitive qPlus sensor for atomic force microscopy (AFM) is described. Focused ion beam was used to cut then weld onto a bare quartz tuning fork a sharp micro-tip from an electrochemically etched tungsten wire. The resulting qPlus sensor exhibits high resonance frequency and quality factor allowing increased force gradient sensitivity. Its spring constant can be determined precisely which allows accurate quantitative AFM measurements. The sensor is shown to be very stable and could undergo usual UHV tip cleaning including e-beam and field evaporation as well as in-situ STM tip treatment. Preliminary results with STM and AFM atomic resolution imaging at $4.5\,K$ of the silicon $Si(111)-7\times 7$ surface are presented.Comment: 5 pages, 3 figure

Determination of localized visibility in off-axis electron holography

Off-axis electron holography is a wavefront-split interference method for the transmission electron microscope that allows the phase shift and amplitude of the electron wavefront to be separated and quantitatively measured. An additional, third component of the holographic signal is the coherence of the electron wavefront. Historically, wavefront coherence has been evaluated by measurement of the holographic fringe visibility (or contrast) based on the minimum and maximum intensity values. We present a method based on statistical moments is presented that allows allow the visibility to be measured in a deterministic and reproducible fashion suitable for quantitative analysis. We also present an algorithm, based on the Fourier-ratio method, which allows the visibility to be resolved in two-dimensions, which we term the local visibility. The local visibility may be used to evaluate the loss of coherence due to electron scattering within a specimen, or as an aid in image analysis and segmentation. The relationship between amplitude and visibility may be used to evaluate the composition and mass thickness of a specimen by means of a 2-D histogram. Results for a selection of elements (C, Al, Si, Ti, Cr, Cu, Ge, and Au) are provided. All presented visibility metrics are biased at low-dose conditions by the presence of shot-noise, for which we provide methods for empirical normalization to achieve linear response.Peer reviewed: YesNRC publication: Ye

Silicon nanowire lithium-ion battery anodes with ALD deposited TiN coatings demonstrate a major improvement in cycling performance

We demonstrate that nanometer-scale TiN coatings deposited by atomic layer deposition (ALD), and to a lesser extent by magnetron sputtering, will significantly improve the electrochemical cycling performance of silicon nanowire lithium-ion battery (LIB) anodes. A 5 nm thick ALD coating resulted in optimum cycling capacity retention (55% vs. 30% for the bare nanowire baseline, after 100 cycles) and coulombic efficiency (98% vs. 95%, at 50 cycles), also more than doubling the high rate capacity retention (e.g. 740 mA h g-\ub9 vs. 330 mA h g-\ub9, at 5 C). We employed a variety of advanced analytical techniques such as electron energy loss spectroscopy (EELS), focused ion beam analysis (FIB) and X-ray photoelectron spectroscopy (XPS) to elucidate the origin of these effects. The conformal 5 nm TiN remains sufficiently intact to limit the growth of the solid electrolyte interphase (SEI), which in turn both improves the overall coulombic efficiency and reduces the life-ending delamination of the nanowire assemblies from the underlying current collector. Our findings should provide a broadly applicable coating design methodology that will improve the performance of any nanostructured LIB anodes where SEI growth is detrimental. \ua9 2013 The Royal Society of Chemistry.Peer reviewed: YesNRC publication: Ye

ALD TiO2 coated silicon nanowires for lithium ion battery anodes with enhanced cycling stability and coulombic efficiency

We demonstrate that silicon nanowire (SiNW) Li-ion battery anodes that are conformally coated with TiO2 using atomic layer deposition (ALD) show a remarkable performance improvement. The coulombic efficiency is increased to 3c99%, among the highest ever reported for SiNWs, as compared to 95% for the baseline uncoated samples. The capacity retention after 100 cycles for the nanocomposite is twice as high as that of the baseline at 0.1 C (60% vs. 30%), and more than three times higher at 5 C (34% vs. 10%). We also demonstrate that the microstructure of the coatings is critically important for achieving this effect. Titanium dioxide coatings with an as-deposited anatase structure are nowhere near as effective as amorphous ones, the latter proving much more resistant to delamination from the SiNW core. We use TEM to demonstrate that upon lithiation the amorphous coating develops a highly dispersed nanostructure comprised of crystalline LiTiO2 and a secondary amorphous phase. Electron energy loss spectroscopy (EELS) combined with bulk and surface analytical techniques are employed to highlight the passivating effect of TiO2, which results in significantly fewer cycling-induced electrolyte decomposition products as compared to the bare nanowires. \ua9 2013 The Owner Societies.Peer reviewed: YesNRC publication: Ye

Si nanotubes ALD coated with TiO2, TiN or Al2O 3 as high performance lithium ion battery anodes

Silicon based hollow nanostructures are receiving significant scientific attention as potential high energy density anodes for lithium ion batteries. However their cycling performance still requires further improvement. Here we explore the use of atomic layer deposition (ALD) of TiO2, TiN and Al2O3 on the inner, the outer, or both surfaces of hollow Si nanotubes (SiNTs) for improving their cycling performance. We demonstrate that all three materials enhance the cycling performance, with optimum performance being achieved for SiNTs conformally coated on both sides with 1.5 nm of Li active TiO2. Substantial improvements are achieved in the cycling capacity retention (1700 mA h g-1vs. 1287 mA h g-1 for the uncoated baseline, after 200 cycles at 0.2 C), steady-state coulombic efficiency ( 3c100% vs. 97-98%), and high rate capability (capacity retention of 50% vs. 20%, going from 0.2 C to 5 C). TEM and other analytical techniques are employed to provide new insight into the lithiation cycling-induced failure mechanisms that turn out to be intimately linked to the microstructure and the location of these layers.Peer reviewed: YesNRC publication: Ye

Thiophene mitigates high temperature fouling of metal surfaces in oil refining

Inorganically driven fouling of metal heat-transfer surfaces employed in crude oil refining operations is not well understood. The object of this study is twofold: First, we systematically elucidate the time-dependent mechanism of the interrelated carbonaceous and sulfidic build up that occurs at high temperatures on a metal surface (540 \ub0C metal temperature, 250\ub0C oil bath temperature). Second, we demonstrate that additions of 0.5, 1.3 and 5.7 vol% thiophene (C4H4S) cause a 2 7, 10 7, and 20 7 reduction in the fouling factor after a 1400 min exposure. Analytical techniques including TEM, SEM-EDX, FIB, Auger electron spectroscopy and XRD were employed to detail the fouling phenomenology for a heated stainless steel wire immersed in atmospheric bottoms fraction crude oil, exposed for 1-1400 min. A key microstructural observation is the transformation of the wire's as-received near-surface textured austenitic grain structure into a micron scale (e.g. 3c10 \u3bcm at 1400 min) highly porous inner-sulfide/chromium oxide bilayer composite. Additionally, we observe significant localized sulfidic attack into the bulk of the metal. During testing, an iron sulfide (pyrrhotite Fe(1 - x )S) corrosion product forms almost instantaneously at the metal surface, followed by coke formation around its periphery at longer times. This temporal sequence, combined with the observation that the thicker regions of the foulant are clearly associated with detached plumes of the sulfide, leads us to argue that the sulfide is essential for promoting organic fouling. This is brought about by the sulfide's action as a potent dehydrogenation catalyst that drives the transformation of pitch to coke. We hypothesize that the tremendous fouling inhibition effect of the thiophene originates from its adsorption onto the sulfide surfaces, thereby blocking the dehydrogenation reactions.Peer reviewed: YesNRC publication: Ye

Anodes for Sodium Ion Batteries Based on Tin–Germanium–Antimony Alloys

Here we provide the first report on several compositions of ternary Sn–Ge–Sb thin film alloys for application as sodium ion battery (aka NIB, NaB or SIB) anodes, employing Sn50Ge50, Sb50Ge50, and pure Sn, Ge, Sb as baselines. Sn33Ge33Sb33, Sn50Ge25Sb25, Sn60Ge20Sb20, and Sn50Ge50 all demonstrate promising electrochemical behavior, with Sn50Ge25Sb25 being the best overall. This alloy has an initial reversible specific capacity of 833 mAhg^–1 (at 85 mAg^–1) and 662 mAhg^–1 after 50 charge–discharge cycles. Sn50Ge25Sb25 also shows excellent rate capability, displaying a stable capacity of 381 mAhg^–1 at a current density of 8500 mAg^–1 (∼10C). A survey of published literature indicates that 833 mAhg^–1 is among the highest reversible capacities reported for a Sn-based NIB anode, while 381 mAhg^–1 represents the optimum fast charge value. HRTEM shows that Sn50Ge25Sb25 is a composite of 10–15 nm Sn and Sn-alloyed Ge nanocrystallites that are densely dispersed within an amorphous matrix. Comparing the microstructures of alloys where the capacity significantly exceeds the rule of mixtures prediction to those where it does not leads us to hypothesize that this new phenomenon originates from the Ge(Sn) that is able to sodiate beyond the 1:1 Na:Ge ratio reported for the pure element. Combined TOF-SIMS, EELS TEM, and FIB analysis demonstrates substantial Na segregation within the film near the current collector interface that is present as early as the second discharge, followed by cycling-induced delamination from the current collector