Anders J. Barlow

Accurate argon cluster-ion sputter yields: Measured yields and effect of the sputter threshold in practical depth-profiling by x-ray photoelectron spectroscopy and secondary ion mass spectrometry

Argon Gas Cluster-Ion Beam sources are likely to become widely used on x-ray photoelectron spectroscopy and secondary ion mass spectrometry instruments in the next few years. At typical energies used for sputter depth profiling the average argon atom in the cluster has a kinetic energy comparable with the sputter threshold, meaning that for the first time in practical surface analysis a quantitative model of sputter yields near threshold is needed. We develop a simple equation based on a very simple model. Though greatly simplified it is likely to have realistic limiting behaviour and can be made useful for estimating sputter yields by fitting its three parameters to experimental data. We measure argon cluster-ion sputter yield using a quartz crystal microbalance close to the sputter threshold, for silicon dioxide, poly(methyl methacrylate), and polystyrene and (along with data for gold from the existing literature) perform least-squares fits of our new sputter yield equation to this data. The equation performs well, with smaller residuals than for earlier empirical models, but more importantly it is very easy to use in the design and quantification of sputter depth-profiling experiments.

Observed damage during Argon gas cluster depth profiles of compound semiconductors

Argon Gas Cluster Ion Beam (GCIB) sources have become very popular in XPS and SIMS in recent years, due to the minimal chemical damage they introduce in the depth-profiling of polymer and other organic materials. These GCIB sources are therefore particularly useful for depth-profiling polymer and organic materials, but also (though more slowly) the surfaces of inorganic materials such as semiconductors, due to the lower roughness expected in cluster ion sputtering compared to that introduced by monatomic ions. We have examined experimentally a set of five compound semiconductors, cadmium telluride (CdTe), gallium arsenide (GaAs), gallium phosphide (GaP), indium arsenide (InAs), and zinc selenide (ZnSe) and a high-κ dielectric material, hafnium oxide (HfO), in their response to argon cluster profiling. An experimentally determined HfO etch rate of 0.025 nm/min (3.95 × 10−2 amu/atom in ion) for 6 keV Ar gas clusters is used in the depth scale conversion for the profiles of the semiconductor materials. The a...

X-ray enhanced sputter rates in argon cluster ion sputter-depth profiling of polymersa)

The authors have observed for the first time that x-ray exposure of certain polymers of “degrading” type can greatly enhance the sputter rate of these polymers by gas cluster ion beam (GCIB) profiling. They have observed craters of similar dimensions to the x-ray spot well within the perimeter of sputter craters, indicating that x-rays can assist GCIB sputtering very significantly. This can be a major source of the loss of depth-resolution in sputter depth profiles of polymers. The authors have measured experimentally sputter craters in 14 different polymers by white-light interferometry. The results show that x-ray exposure can introduce much more topography than might previously have been expected, through both thermal and direct x-ray degradation and cross-linking. Within the region exposed to x-rays, the response of the polymer surface depends on its chemistry, with degrading (also known as type II) polymers being susceptible to large increases in sputter rate in some cases. For example, this leads to...

Visible wavelength surface-enhanced Raman spectroscopy from In-InP nanopillars for biomolecule detection

Visible wavelength surface-enhanced Raman spectroscopy (SERS) has been observed from bovine serum albumin (BSA) using In-InP nanopillars synthesised by Ar gas cluster ion beam sputtering of InP wafers. InP provides a high local refractive index for plasmonic In structures, which increases the wavelength of the In surface plasmon resonance. The Raman scattering signal was determined to be up to 285 times higher for BSA deposited onto In-InP nanopillars when compared with Si wafer substrates. These substrates demonstrate the label-free detection of biomolecules by visible wavelength SERS, without the use of noble metal particles.

Plasma Fluorination of Highly Ordered Pyrolytic Graphite and Single Walled Carbon Nanotube Surfaces

A simple room temperature fluorination process for highly ordered pyrolytic graphite (HOPG) and single walled carbon nanotube (SWCNT) surfaces is presented and discussed. Graphite and SWCNT surfaces are functionalised using radio frequency SF6 plasma. Samples were characterised before and after treatment using XPS, EDX, SEM, and STM techniques. It is found that functionalisation of SWCNT surfaces occurs rapidly indicating an efficient fluorination process with minimal damage to the carbon surfaces, and furthermore, without presence of sulphur-containing functional groups

Stability of reference masses: VII. Cleaning methods in air and vacuum applied to a platinum mass standard similar to the international and national kilogram prototypes

Mercury contamination and the build-up of carbonaceous contamination are two contributing factors to the instability observed in kilogram prototype masses. The kilogram prototypes that lie at the core of the dissemination of the SI base unit were manufactured in the late 19th century, and have polished surfaces. In papers IV and V of this series we developed a method for cleaning noble metal mass standards in air to remove carbonaceous contamination. At the core of this ?UVOPS? protocol is the application of UV light and ozone gas generated in situ in air. The precise nature of the carbonaceous contamination that builds up on such surfaces is difficult to mimic demonstrably or quickly on new test surfaces, yet data from such tests are needed to provide the final confidence to allow UVOPS to be applied to a real 19th century kilogram prototype. Therefore, in the present work we have applied the UVOPS method to clean a platinum avoirdupois pound mass standard, ?RS2?, manufactured in the mid-19th century. This is thought to have been polished in a similar manner to the kilogram prototypes. To our knowledge this platinum surface has not previously been cleaned by any method. We used x-ray photoelectron spectroscopy to identify organic contamination, and weighing to quantify the mass lost at each application of the UVOPS procedure. The UVOPS procedure is shown to be very effective.It is likely that the redefinition of the kilogram will require mass comparisons in vacuum in the years to come. Therefore, in addition to UVOPS a cleaning method for use in vacuum will also be needed. We introduce and evaluate gas cluster ion-beam (GCIB) treatment as a potential method for cleaning reference masses in vacuum. Again, application of this GCIB cleaning to a real artefact, RS2, allows us to make a realistic evaluation of its performance. While it has some attractive features, we cannot recommend it for cleaning mass standards in its present form.

New insights into the electrochemistry of magnesium molybdate hierarchical architectures for high performance sodium devices

Magnesium molybdate (MgMoO4), which possesses synergistic features combining both hierarchical plate-like nanomaterials and porous architectures, has been successfully synthesized through a facile combustion synthesis at a low temperature. The hierarchical architecture is characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), scanning transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analyses. The as-obtained MgMoO4 nanoplates showed a porous structure with a pore-size distribution ranging from 50 to 70 nm. This porosity provides an electron transport pathway and enhanced surface reaction kinetics. The binding energies measured for Mg 2p, Mo 3d, 3p and O 1s are consistent with the literature, and with the metal ions being present as M(ii) and M(vi) states, respectively. This indicates that the oxidation states of the metal cations are as expected. The electrochemical behaviour of MgMoO4 was investigated using aqueous (NaOH) and non-aqueous solvents (NaClO4 in EC : DMC : FEC) for supercapacitor and battery applications. The sodium-ion capacitor involves ion absorption and insertion into the MgMoO4 electrodes resulting in superior power and energy densities. However, the cycling stability was found to be stable only for an aqueous system. The formation of a solid electrolyte surface layer restricted the reversible capacity of the MgMoO4 in the sodium-battery. Nevertheless, it does offer some promise as an anode material for storing energy with high rate performance and excellent capacity retention. Detailed comparative analyses of various electrolytes in storage devices such as hybrid sodium-ion capacitors and sodium-ion batteries are vital for the integration of hierarchical structured materials into practical applications. The reaction mechanisms are postulated.

Demonstration of chemistry at a point through restructuring and catalytic activation at anchored nanoparticles

Metal nanoparticles prepared by exsolution at the surface of perovskite oxides have been recently shown to enable new dimensions in catalysis and energy conversion and storage technologies owing to their socketed, well-anchored structure. Here we show that contrary to general belief, exsolved particles do not necessarily re-dissolve back into the underlying perovskite upon oxidation. Instead, they may remain pinned to their initial locations, allowing one to subject them to further chemical transformations to alter their composition, structure and functionality dramatically, while preserving their initial spatial arrangement. We refer to this concept as chemistry at a point and illustrate it by tracking individual nanoparticles throughout various chemical transformations. We demonstrate its remarkable practical utility by preparing a nanostructured earth abundant metal catalyst which rivals platinum on a weight basis over hundreds of hours of operation. Our concept enables the design of compositionally diverse confined oxide particles with superior stability and catalytic reactivity.Metal nanoparticles prepared by exsolution at the surface of perovskite oxides are key species in catalysis and energy fields. Here, the authors develop a chemistry at a point concept by tracking individual nanoparticles with excellent activity and stability throughout various chemical transformations.

An ionic liquid based sodium metal-hybrid supercapacitor-battery

There is growing interest in developing sodium based energy storage devices as alternatives to Li for large-scale energy storage. We report a highly stable hybrid system comprising a sodium metal anode, a highly porous N/S co-doped mesoporous carbon cathode and the non-flammable ionic liquid electrolyte N-propyl-N-methyl pyrrolidinium bis(fluorosulfonyl) imide (C3mpyrFSI). This hybrid device operates at 100% coulombic efficiency and shows almost complete capacity retention over 3000 cycles. Owing to the reversible cathode reactions between Na+ and N/S functionalities on the carbon surface, a very high capacity of 716 mA h g−1 (at a rate of C/16) was achieved between 3.8 V and 0.005 V vs. Na+/Na. An optimised device could provide energy density as high as 263 W h kg−1. At high rate, the devices achieved power density of 1463 W kg−1. These metrics increase to 270 W h kg−1 and 3822 W kg−1 at 50 °C, highlighting the excellent combination of a supercapacitor-type cathode with a sodium metal anode and an ionic liquid electrolyte for large-scale energy storage.

The plasmonic properties of argon cluster-bombarded InP surfaces

Gas cluster ion beam sputtering has been used to study the self-organising behaviour of In metallic nanoparticles produced by preferential sputtering of phosphorus atoms in InP. Discrete plasmonic In nanoparticles are observed at the earliest stages of surface modification. The surfaces have been investigated in situ by reflection electron energy loss spectroscopy, Auger electron spectroscopy, and photoluminescence spectroscopy. By altering the excitation intensity, we observe alterations of the photoluminescence spectrum that are attributed to photoconductive-coupling between In nanoparticles. The devices presented are suitable for visible wavelength surface enhanced Raman spectroscopy and, potentially, offer a route to active all-optical switches.