L. Burstein

A Study of Highly Oriented Pyrolytic Graphite as a Model for the Graphite Anode in Li‐Ion Batteries

The mechanisms of oxidation of the basal plane and of the cross-sectional face of highly oriented pyrolytic graphite (HOPG) and the formation of a solid electrolyte interphase (SEI) on HOPG samples that were cycled in ethylene carbonate:diethyl carbonate (EC:DEC 1:2) solutions containing 1 M LiAsF{sub 6} were studied. X-ray photoelectron spectroscopy, energy dispersive spectrometry, and scanning electron microscope techniques were used for the analysis of the surface layer formed on the basal plane and cross section of HOPG. The analysis indicates that the oxidation mechanisms of the basal plane and the cross section are entirely different. The SEI formed in the LiAsF{sub 6} solution is thinner on the basal plane than on the cross section and its composition is different. The SEI formed on the cross section is rich in inorganic compounds whereas the SEI formed on the basal plane is rich in organic compounds. Thus it can be concluded that on the basal plane, the greatest contribution to SEI formation is solvent reduction (EC and DEC), whereas on the cross-sectional face, it is electrolyte salt (LiAsF{sub 6}) reduction.

XPS analysis of the SEI formed on carbonaceous materials

Two carbonaceous materials were produced by chemical vapour deposition of ethylene and by pyrolysis of dehydrated sucrose. Electrochemical cells assembled from these materials and metallic lithium were cycled between 0.00 and 2.00 V vs. Li/Li+ in ethylene carbonate/diethylcarbonate electrolytes containing LiPF6 or LiAsF6. The solid electrolyte interphase (SEI) formed on the carbons was characterised by X-ray photoelectron spectroscopy (XPS). We suggest that the carbon matrix has a more marked effect on the composition and thickness of the SEI than does the nature of the electrolyte. The SEI formed on graphite-like soft carbon in both electrolytes proved to be carbonate-free, its inorganic part consisting almost exclusively of LiF, while the SEI formed on hard (non-graphitizable) carbon was found to be considerably thicker and contained, in addition, phosphorus and arsenic compounds. In the bulk SEI, polymer structures (i.e., solvent-polymerisation products) were abundant in all cases, while carbonates were found only on hard carbon in the presence of LiAsF6.

Biorecognition Layer Engineering: Overcoming Screening Limitations of Nanowire-Based FET Devices

Detection of biological species is of great importance to numerous areas of medical and life sciences from the diagnosis of diseases to the discovery of new drugs. Essential to the detection mechanism is the transduction of a signal associated with the specific recognition of biomolecules of interest. Nanowire-based electrical devices have been demonstrated as a powerful sensing platform for the highly sensitive detection of a wide-range of biological and chemical species. Yet, detecting biomolecules in complex biosamples of high ionic strength (>100 mM) is severely hampered by ionic screening effects. As a consequence, most of existing nanowire sensors operate under low ionic strength conditions, requiring ex situ biosample manipulation steps, that is, desalting processes. Here, we demonstrate an effective approach for the direct detection of biomolecules in untreated serum, based on the fragmentation of antibody-capturing units. Size-reduced antibody fragments permit the biorecognition event to occur in closer proximity to the nanowire surface, falling within the charge-sensitive Debye screening length. Furthermore, we explored the effect of antibody surface coverage on the resulting detection sensitivity limit under the high ionic strength conditions tested and found that lower antibody surface densities, in contrary to high antibody surface coverage, leads to devices of greater sensitivities. Thus, the direct and sensitive detection of proteins in untreated serum and blood samples was effectively performed down to the sub-pM concentration range without the requirement of biosamples manipulation.

Characterization of modified NG7 graphite as an improved anode for lithium-ion batteries

Abstract Mild oxidation (burnoff) has been found to improve the performance of NG7 (natural graphite) in Li/LixC6 cells. The reversible capacity of the graphite (QR) increased (up to 405 mAh/g for the 0–2 V range at 4–11% burnoff), its irreversible capacity (QIR) decreased with burnoff and the degradation rate of the LixC6 electrode was much lower. The increase in capacity at 4–11% burnoff of NG7 was found to be associated with the formation of less than 1% void volume. On further burning beyond 11% weight loss, the density decreases faster, down to 1.92 g/cm3 at 34% burnoff. This is associated with some decrease in QR. X-ray photoelectron spectrocopy studies showed that the surface oxygen content of NG7 has a broad minimum at 4–22% burnoff. The highest oxygen peak shifted monotonically with burnoff time, rising from 531.05 eV for pristine NG7 to 534.0 eV for 34% burnoff sample. The X-ray photoemission spectra may be assigned to hydroxyl surface groups in the case of the pristine sample and to surface acid groups in the case of burnt samples. From the powder X-ray diffraction and dq/dv results, it was found that a more ordered structure appears on cycling. Performance improvement was attributed to the formation of a solid electrolyte interface chemically bonded to the surface carboxylic groups at the zigzag and armchair faces, better wetting by the electrolyte and to accommodation of extra lithium at the zigzag, armchair and other edge sites and nanovoids.

Surface states and surface oxide in GaN layers

Surface photovoltage spectroscopy, photoluminescence, Auger electron spectroscopy and x-ray photoelectron spectroscopy were used to correlate the chemical changes induced by HCl etching of GaN surface to changes in the yellow luminescence related states, through their manifestation in surface photovoltage. The results show a correlation between a removal of the gallium oxide from the surface and a reduction of the yellow luminescence related transition in the surface photovoltage spectra. Based on this observation, it is suggested that the well known yellow luminescence is emitted from surface states associated with the gallium oxide that decorates the free surface and possibly also the substrate interface and internal grain boundaries.

Interface of p‐type Hg1−xCdxTe passivated with native sulfides

The process for forming native sulfide films on p‐type Hg1−xCdxTe with x≂0.2–0.3 by anodic sulfidization is described and studied. The analytical, optical, and electrical properties of the resulting interface are reported. The results of Auger electron spectroscopy analysis indicate that homogeneous CdS films with an abrupt interfacial transition are formed on Hg1−xCdxTe with x=0.215–0.290. The measured dispersion of the index of refraction and the dielectric constant of the native sulfide films are consistent with the reported data for bulk CdS. The interface between Hg1−xCdxTe and its native sulfide in combination with deposited ZnS has excellent electrical properties. A low fixed surface state density of the order of Nss ≂5×1010 cm−2, a low concentration of fast surface state density of the order of 5×109 cm−2 eV−1, and a small amount of trapping effects in the sulfide are observed. The MIS capacitors exhibit thermal stability up to 95 °C anneal in vacuum. The main features of the interface of p‐HgCdTe...

Tin Alloy-Graphite Composite Anode for Lithium-Ion Batteries

A composite anode material was prepared that contains nanosize (< 100 nm) particles of tin alloy Sn 65 Sb 18 Cu 17 and Sn 62 Sb 21 Cu 17 . The alloys were electroplated at high current densities (above i L ) from aqueous solutions, directly onto the copper current collector, and were coated by a polyvinylidene fluoride-graphite matrix at a ratio of alloy:graphite matrix 70:30 and 80:20 w/w, respectively. The processes involved in electrode production by this method are inexpensive, simple, and fast. Over 40 (100% depth of discharge) cycles were demonstrated, in half-cell, and over 30 were demonstrated with a LiCoO 2 battery containing 1 M LiPF 6 ethylene carbonate-diethyl carbonate electrolyte. The faradaic efficiency (Q De-ins /Q Ins ) is less than 100%. Lithium is fully deinserted from the host matrix only when the anode is cycled at low current densities. The kinetics of lithium insertion to and deinsertion from the composite anode material, slow gradually as the cycle number increases. X-ray diffraction patterns of the anode material show that the alloy becomes amorphous during cycling, while the graphite does not. X-ray photoelectron-spectroscopy measurements reveal that the solid electrolyte interphase consists of mainly LiF, small amounts of Li 2 O, and possibly, polymeric substances. The electrochemical behavior of the alloy changes with cycle number, while that of the graphite does not. The fall of the deinsertion capacity of the graphite from the first cycle to the 34th by more than 50% proves that the active material in the anode suffers from particle-to-particle break off.

Band diagram of the polycrystalline CdS/Cu(In,Ga)Se2 heterojunction

Contact potential difference measurements in the dark and under illumination are used to derive the conduction band offset (ΔEc) in a solar cell quality junction formed by chemical bath deposition of CdS on a polycrystalline thin film of Cu(In,Ga)Se2. Our experimental measurements and the estimates made for dipole contributions show that the junction is of type II, i.e., without a spike in the conduction band ( ΔEc=80 meV±100 meV). This is consistent with the high performance of the actual solar cell. However, it differs from most previous results on junctions based on single crystals and/or vacuum deposited CdS, which indicated the existence of a conduction band spike.

Electrochemical processes of nucleation and growth of calcium phosphate on titanium supported by real‐time quartz crystal microbalance measurements and X‐ray photoelectron spectroscopy analysis

Real-time, in situ electrochemical quartz crystal microbalance (EQCM) measurements are conducted to better understand the electrocrystallization of calcium phosphates (CaP) on CP-Ti. X-ray photoelectron spectroscopy is used to identify the exact phase deposited, so that reliable estimation of the electrochemical processes involved is made. Analysis of the integrated intensity of the oxygen shake-up peaks, in combination with the determination of Ca/P and O/Ca atomic ratios, enables to determine unambiguously that the octacalcium phosphate (OCP) is formed. Its role as a precursor to hydroxyapatite (HAp) is discussed. After an incubation period, the process by which OCP is formed follows a Faradaic behavior. The incubation time may be related to the need for local increase of pH before precipitation from solution can occur. The standard enthalpy of activation is approximately 40 kJ/mol, which excludes diffusion-controlled processes from being rate determining. The OCP deposit has thickness approximately 0.61 microm, apparent density approximately 0.95 g/cm3, 63.6% porosity, and deposition rate of 23.5 ng/(cm2 s) or 15 nm/min. The low-equivalent weight value of 20.5 g/equiv, and the associated remarkably high number of electrons transferred in the reaction n approximately 24, indicates that most of the current is consumed either by electrolysis of water or by a complex set of parasitic reactions. The low-solubility product allows precipitation of CaP even at relatively low concentrations of calcium and phosphate/hydrogen phosphate ions. It is shown that HAp most likely forms via transformation of precursor phases, such as OCP, rather than directly.

Anodic sulfide films on Hg1−xCdxTe

A novel anodic sulfidization process for forming native sulfide films on Hg1−xCdxTe is described. In the new process native sulfide films rather than native oxides terminate the lattice and passivate the surface. The results of Auger electron spectroscopy analysis indicate that native homogeneous CdS films are formed with an abrupt interfacial transition. The measured capacitance‐voltage characteristics of metal‐insulator‐semiconductor devices indicate that the films have a low negative fixed surface charge density of the order of –1×1011 e cm−2 and a relatively low concentration of fast surface charge density. The native sulfide films leave the surface of p‐type Hg1−xCdxTe practically at flat band and in this respect are superior to native oxide films which invert the surface of p‐type material. The new surface passivation is in particular suitable for photovoltaic diodes implemented on p‐type Hg1−xCdxTe.