High Vacuum Treatment Research Articles

Effect of Environmental Contaminants on the Interfacial Properties of Two-Dimensional Materials

ConspectusThe surface of 2D materials can spontaneously adsorb and react with molecules in the environment during their processing and storage. This effect, while having a significant impact on many properties of 2D materials, is not always recognized and accounted for in the research involving them. This Account summarizes our recent work in understanding how the ambient environment impacts the properties of 2D materials and its mitigation strategies. We highlight graphene and hydrocarbons in our discussion and complement it with selected studies involving other 2D materials as well as water and oxygen.When graphene and graphite are exposed to air and water, their surfaces adsorb the residue hydrocarbons, typically at part-per-trillion to part-per-billion levels, in the environment. The adsorption of hydrocarbons reduces the surface energy of graphene and graphite and creates a barrier between them and the electrolyte. As a result, the wettability and electrochemical properties of graphene and graphite can be significantly altered by mere exposure to the ambient environment. These changes can be very significant yet highly variable depending on the local environment: several hours of air exposure can increase the water contact angle of graphene by up to 40° and reduce the double-layer capacitance of graphite by up to 50%! The high hydrophobicity and poor electrochemical performance of pristine graphitic carbons, once believed to be intrinsic properties of these materials, are largely due to unintentional surface contamination. The same type of hydrocarbon adsorption was reported for many other 2D materials, such as MoS2, hexagonal BN, and mica. In the case of mica, which is highly ionic in nature, the adsorption of hydrocarbons disrupts its interaction with ionic liquid and alters the self-assembly structure of ionic liquid at the mica surface. Similarly, water also impacts the surface properties of graphene in several ways. Water vapor can compete with hydrocarbons for adsorption onto the surface of graphene, thus reducing the rate of hydrocarbon contamination. Water can intercalate between graphene and some of its supporting substrate, altering their interactions. Finally, water enhances the doping of 2D materials by O2 by promoting an electrochemical doping mechanism involving the O2/H2O redox couple.Reducing and reversing the surface contamination of 2D materials can greatly enhance material and device performances. While completely stopping the contamination is still challenging, a high-humidity environment is shown to reduce the rate of contamination, as mentioned above. For samples already contaminated by airborne hydrocarbons, their surface properties can be partially restored by treatment in high-vacuum, high-temperature, or mildly oxidative environments.

Nickelocene-Filled Purely Metallic Single-Walled Carbon Nanotubes: Sorting and Tuning the Electronic Properties.

We conducted the filling of single-walled carbon nanotubes (SWCNTs) with nickelocene molecules and separation of the filled SWCNTs by conductivity type by density-gradient ultracentrifugation. We tailored the electronic properties of nickelocene-filled purely metallic SWCNTs by thermal treatment in high vacuum. Our results demonstrated that annealing at low temperatures (360–600 °C) leads to n-doping of SWCNTs, whereas annealing at high temperatures (680–1200 °C) results in p-doping of SWCNTs. We found a correlation between the chemical state of the incorporated substances at different annealing temperatures and its influence on the electronic properties of SWCNTs.

Study of the kinetics of gas evolution of a polymer composite under the influence of vacuum and VUV

The paper presents data on the effect of high vacuum and vacuum ultraviolet (VUV) on a polymer composite based on polyimide track membranes. Polyimide track membranes were filled with POSS structures. For comparison, we also used pure polyimide films without filler. Exposure in high vacuum and surface treatment of the developed VUV composite were carried out in the imitation chamber of near-Earth space, located in the Space Materials Science Laboratory of BSTU named after V.G. Shukhov (Russia, Belgorod). The high resistance of the composite to high vacuum and VUV has been established. The total weight loss of the composite by 24 hours was 0.26 wt. % (after holding in vacuum) and 0.28 wt. % (after VUV).

Thermal extraction of volatiles from the lunar regolith simulant NU-LHT-2M: Preparations for in-situ analyses on the Moon

The present work describes the end-to-end demonstration of enriching the lunar highland regolith simulant NU-LHT-2M with loosely adsorbed water, releasing this and other volatile compounds by thermal treatment in high vacuum, and identifying the released volatile species through mass spectrometry. This demonstration was performed to characterize how different sample conditions will affect the in-situ measurements performed by the ProSPA gas analysis instrument that is to operate near the lunar south pole on board the Russian Luna-27 lander. A laboratory breadboard was set up that allows testing of variable parameter combinations, such as different initial water contents, particle sizes, quantities, and bulk densities of the sample, as well as different heating rates. Three distinct temperature-dependent phases of outgassing were identified. Between −50 °C and 300 °C loosely adsorbed volatiles, mainly water in a mass fraction of around 0.1%–0.2%, were released from the samples. Above that the samples showed mineral decomposition which led to the release of trapped water, carbon dioxide, and hydrogen sulfide. It was shown that the gas pressure produced by outgassing of the volatile species in a continuously pumped system is noticeably higher if the sample is larger, contains smaller particles, or if a higher heating rate is applied.

Influence of atmosphere during vacuum heat treatment of stainless steels AISI 304 and 446

Specimens of austenitic AISI 304 and ferritic 446 (contains 1.7 wt.% Al) underwent heat treatments up to 1200 °C in ultrahigh vacuum (UHV) in an analysis chamber with a base pressure of 10-10 mbar and high vacuum (HV) in an industrial vacuum brazing furnace with a base pressure of 10-6 mbar. The effect of the two oxygen partial pressure ranges on surface and bulk composition was determined by XPS, AES and EDX, imaging was provided by SEM.For AISI 304, we observe similar behavior for both atmospheres and the removal of the surface oxide layer. For AISI 446, we observe a strong dependency on the atmosphere. Heat treatment in high vacuum yielded a thick aluminum oxide layer on the surface that inhibited wetting with copper silver eutectic. Heat treatments in ultrahigh vacuum did not lead to aluminum oxide formation but rather carbide formation of foreign elements such as titanium was observed on the surface.

Low Boiling Point Solvent Additives for Improved Photooxidative Stability in Organic Photovoltaics

AbstractThe efficiency of organic photovoltaic (OPV) cells is rapidly increasing. However, many of the highest performance OPV devices are also the most unstable, including the two widely used benzodithiophene‐co‐thieno[3,4‐b]thiophene polymers, PTB7 and PTB7‐Th. Here it is shown that both the choice of pendant side chain, and solvents used for film deposition, have an important effect on active layer stability. Specifically, polymers with alkylthienyl side chains are found to be less stable than polymers with alkoxy side chains. Secondly, high boiling point solvent additives such as 1,8‐diiodooctane are found to dramatically accelerate photooxidation unless they are removed fully during device processing, which may be attributed to increased oxygen diffusion in films containing residual solvent. Replacing these higher boiling point solvent additives with lower boiling additives can greatly enhance the polymer stability without requiring high vacuum treatment, making these systems more scalable and compatible with roll‐to‐roll printing. Based on this finding, the non‐halogenated solvent system o‐xylene with 2% N‐methylpyrrolidone is explored as a route to achieving higher active layer stability, while maintaining the favorable, intermixed morphology achieved with higher boiling point, halogenated solvent additives. This result offers a promising way forward in the development of scalable, stable, and high‐efficiency organic solar cells.

Surface hardening of biocompatible ultrafine-grained niobium zirconium alloy by two-stage oxidation treatment

The present study reports on an optimized surface hardening process for biocompatible ultrafine-grained (UFG) niobium 2.3 wt% zirconium (NbZr) alloy, a promising candidate implant material. The as-received material of conventional grain size (CG) was processed using multipass equal channel angular processing at room temperature to obtain an UFG microstructure featuring high strength and ductility. Subsequent surface hardening was performed by a heat treatment leading to internal oxidation. Using a thermogravimetric system, the influence of temperatures, time, and partial pressure of oxygen ( $$ p_{{\text{O}}_2} $$ ) on the oxidation kinetics were investigated. Metallographic and microscopic analyses and hardness measurements were employed to evaluate maximum hardness, penetration-depth and scale formation under various conditions. Heat treatment at 620 °C for 6 h at a $$ p_{{\text{O}}_2} $$ of 0.2 hPa led to sufficiently rapid oxidation kinetics yielding a relatively high depth of penetration without formation of loose Nb2O5 on the surface, which was observed at higher $$ p_{{\text{O}}_2} $$ . As compared to CG material, improved hardness profiles were reached using the same heat treatment parameters, since the UFG structure significantly changes diffusion conditions and therefore oxidation kinetics. After a second heat treatment in high vacuum the high maximum hardness of 820 HV0.01 in the UFG material was reduced effectively and a less steep hardness gradient was achieved, both contributing to a less brittle behavior under mechanical loading. High-cycle fatigue tests performed on surface-hardened UFG NbZr samples showed a substantial improvement of fatigue life in tests conducted near the endurance limit. Especially when high fatigue and wear resistance are key issues for a given application, the internal oxidation process offers an effective way to further improve the properties of UFG NbZr.

Double-Pattern Textured ZnO:Ga Thin Films Fabricated by an APPJ and an DC Sputtering

Transparent conductive oxides (TCO) are essential for superstrate solar cells due to its outstanding optical and electrical properties. 1 In ordertoimprovetheefficiencyofsolarcells,itisimportanttofabricate high-quality TCO substrates. Recently, zinc oxide (ZnO) based TCO have attracted significant attention because of high abundance, low cost and non-toxicity. Moreover, ZnO films doped with group III elements are more stable than tin-doped indium oxides (ITO) and fluorine-doped tin oxides (FTO) in the hydrogen and silan (SiH4) plasma discharge. 2,3 Hence, they are very suitable for the process of microcrystalline silicon (μc-Si) and amorphous silicon (a-Si) solar cells. For silicon based thin film solar cells, textured ZnO electrodes are necessary to increase light absorption in μc-Si absorbers and to reduce the degradation in a-Si absorbers. This is because that textured ZnO films can scatter the incident light efficiently and enhance the light trapping effect. To date, many efforts have been made to achieve superior textured ZnO films. Post wet etching by diluted acids for aluminum-doped zinc oxides (AZO) and gallium-doped zinc oxides (GZO) is the well-known method. 4,5 However, this process creates a lot of waste in ZnO and is not fit for large-scale production. Recently, pyramid-like textured zinc oxides doped with boron (BZO) have been successfully fabricated by a low pressure chemical vapor deposition (LPCVD) or a metal organic chemical vapor deposition (MOCVD) without any post treatments. 6,7 Nevertheless, disadvantages of these techniques are expensive equipments and the usage of toxic precursors. Here, we developed a novel, safe, and low-cost technique to produce textured GZO films. A bi-layer structure of organosilicon/ GZO was conducted to fabricate double-pattern textured GZO films on glass substrates. This bi-layer structure was achieved by an atmospheric pressure plasma jet (APPJ) and a dc magnetron sputtering. The surface morphology, optical characteristics and conductivity of double-pattern textured GZO films can be effectively controlled by the haze of organosilicon underlayers. After a post-annealing treatment in high vacuum, double-pattern textured GZO films can reveal the excellent optical transmittance and electrical conductivity.

The effect of different processing methods on phenolic acid content and antioxidant activity of red beet

Red beet is a rich source of phenolic acids and has high antioxidant capacity. Effect of and different processing methods (microwave, vacuum, boiling, roasting and germination) on phenolic acid (HPLC) content and antioxidant activity of red beet were studied. The measured antioxidant activity included free radical scavenging activities against DPPH and ABTS. Among the phenolic acids from red beet extracts, 4-hydroxy benzoic acid was the major constituent followed by cinnamic acid, vanillic, chlorogenic, trans ferulic acid and caffeic acid. The concentrations of the phenolic acid differ depending on treatments. The present study exhibited that there was increase in phenolic acid concentrations with high vacuum treatments, roasting for 5min and also with germination. Different phenolic acids seem to be more stable with vacuum treatment. The antioxidant activity has gradually increased to 1.5 to 3 fold during different treatments when compared to control. With high vacuum treatment there was up to 13% increase with DPPH assay and 11% increase with ABTS assay.

An Examination of Kernite (Na2B4O6(OH)2·3H2O) Using X-Ray and Electron Spectroscopies: Quantitative Microanalysis of a Hydrated Low-Z Mineral

Mineral borates, the primary industrial source of boron, are found in a large variety of compositions. One such source, kernite (Na2B4O6(OH)2·3H2O), offers an array of challenges for traditional electron-probe microanalysis (EPMA)-it is hygroscopic, an electrical insulator, composed entirely of light elements, and sensitive to both low pressures and the electron beam. However, the approximate stoichiometric composition of kernite can be analyzed with careful preparation, proper selection of reference materials, and attention to the details of quantification procedures, including correction for the time dependency of the sodium X-ray signal. Moreover, a reasonable estimation of the mineral's water content can also be made by comparing the measured oxygen to the calculated stoichiometric oxygen content. X-ray diffraction, variable-pressure electron imaging, and visual inspection elucidate the structural consequences of high vacuum treatment of kernite, while Auger electron spectroscopy and X-ray photoelectron spectroscopy confirm electron beam-driven migration of sodium and oxygen out of the near-surface region (sampling depth ≈ 2 nm). These surface effects are insufficiently large to significantly affect the EPMA results (sampling depth ≈ 400 nm at 5 keV).

Wettability and surface composition of partly and fully regenerated cellulose thin films from trimethylsilyl cellulose

The wettability and surface free energy (SFE) of partly and fully regenerated cellulose model surfaces from spin coated trimethylsilyl cellulose were determined by static contact angle (SCA) measurements. In order to gain detailed insight into the desilylation reaction of the surfaces the results from SCA measurements were compared with data from other surface analytical methods, namely thickness measurements, X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance infrared spectroscopy (ATR-IR). Additionally, the influence of ultra high vacuum treatment (UHV) during XPS measurements on the water wettability and surface morphology of regenerated cellulose thin films was investigated. The wetting of polar and non-polar liquids increased with prolonged regeneration time, which is reflected in the higher SFE values and polarities of the films. After UHV treatment the water SCA of partly regenerated films decreases, whereas fully regenerated cellulose shows a higher water SCA. Therefore it is assumed that volatile desilylation products tend to adsorb on partly regenerated films, which strongly influences their wettability.

Synthesis and consolidation of titanium diboride

Abstract Titanium diboride (TiB2) is one of the candidate materials for high temperature structural applications and also for control rod elements in high temperature nuclear reactors. This paper presents the experimental data on the synthesis, consolidation to dense pellets and property evaluation of TiB2 carried out in laboratory scale. TiB2 powder was prepared by the reduction of titanium oxide with boron carbide and carbon, purified by high temperature – high vacuum treatment and milled to obtain micron sized particles. The powders were characterized by XRD, chemical analysis and laser particle size analyzer. The reduction process was optimized for the production of titanium boride of ∼1 μm size median dia with oxygen, carbon and nitrogen contents close to 0.5% each. Studies on densification of TiB2 powders by hot pressing were conducted. Pellets with a high density of 97.56% ρth were prepared by hot pressing at 1800 °C. Hardness of this sample was measured as 26 GPa, fracture toughness as 5.3 MPa m1/2 and thermal conductivity as 59.44 W/m/K at 500 °C.

Process Improvement and Reliability Characteristics of Spin-On Poly-3-hexylthiophene Thin-Film Transistor

Poly-3-hexylthiophene (P3HT) based thin-film transistors were fabricated. Various spin-coating conditions including spin speeds, curing temperatures, solvents, and weight percentages of P3HT have been examined. It was found that the P3HT of 0.3 wt % dissolved in chloroform contributes to the lowest surface roughness and the highest ON/OFF current ratio over four orders of magnitude. Besides, the anomalous gate leakage current was eliminated successfully by growing a thick field oxide layer in the source/drain regions. Through or high-vacuum treatments, it was confirmed that the threshold voltage and mobility of organic devices degrades rapidly when the P3HT-based organic thin-film transistors were exposed to the atmosphere within 8 h. Moreover, the threshold voltage increases under a positive gate bias stress while the decreases after a negative gate bias stress. The mechanism was attributed to the polarization effect of P3HT film. © 2004 The Electrochemical Society. All rights reserved.

H2, CO and high vacuum regeneration of ozone poisoned pseudo-Schottky Pd–InP based gas sensor

This article deals with pseudo-Schottky diode Pd–InP gas sensor. Catalytic activity of such palladium layer permits to measure very low concentrations of two powerful oxidizing gases, nitrogen dioxide (NO2) and ozone (O3), constituting main urban atmospheric pollution. After many O3 submissions, degradation of gas sensor characteristics (response time, recovery time and sensitivity) appears, probably due to ozone oxidation of palladium metallization. Different treatments (H2 reduction, CO reduction and high vacuum) were then tested to avoid degradation of sensor parameters. Initial response and recovery times of this sensor can be recovered by H2 reduction and high vacuum treatment of metallization layer after exposure to ozone. Moreover, results emphasize that these treatments improve sensor sensitivity. CO reduction will permit to have reproducible measurements of low NO2 and O3 concentrations.

Reconstitution of a Porin from Mycobacterium smegmatis at HOPG covered with hydrophobic host layers

The unique stability and self-assembling properties of porin MspA, a channel protein isolated from Mycobacterium smegmatis, were established using three very different host systems: (1)A very simple and straightforward nanostructuring process was achieved by depositing buffer droplets containing MspA onto highly ordered pyrolytic graphite (HOPG) from the gas phase, followed by thermal curing and high vacuum treatment. Three lateral nanostructures were obtained depending on the protein mass deposited, the temperature during deposition and the subsequent curing process. We report here a detailed statistical analysis of the MspA pore sizes on HOPG. (2)The hydrophobic, water-soluble copolymer poly(N-isopropylacrylamide)–poly(acrylic acid) at HOPG was used as the host system on HOPG for the successful reconstitution of MspA from the liquid phase. The existence of hydrophilic nanopores, which formed exclusively when MspA was present, was detected at temperatures above the lower critical solution temperature of the copolymer. (3)The outer membrane of Mycobacterium tuberculosis at HOPG was employed as the hydrophobic host system for the channel porin from Mycobacterium smegmatis (MspA). Evidence for the reconstitution of MspA into the related membrane of M. tuberculosis was obtained. The nanostructures were analysed using high-resolution transmission electron microscopy. The program IMAGE (NIH, USA) permitted a quantitative insight into the lateral nanostructures at HOPG. Copyright © 2004 John Wiley & Sons, Ltd.

Nanostructuring by Deposition of Protein Channels Formed on Carbon Surfaces

The unique stability and self-assembling properties of MspA, a channel protein from Mycobacterium smegmatis, were exploited for a simple and straightforward nanostructuring process. Deposition of buffer droplets containing MspA onto highly ordered pyrolitic graphite (HOPG), followed by thermal curing and high vacuum treatment, yielded three distinct lateral nanostructures, depending upon the deposition conditions. The determining experimental parameters were the protein mass deposited and the temperature during and after deposition. The nanostructures were analyzed using high-resolution transmission (TEM) and raster electron microscopy (REM). The program IMAGE (NIH/USA) yielded a quantitative insight into the geometries and the ordering principles of three lateral nanostructures engineered. Whereas phase I consisted of individual proteins, deposited on HOPG, phase II was found to be a nanostructured supramolecular assembly of protein monomers. Phase III, however, featured arrays of distinct nanochannels, ...

Nanostructuring of Carbon Surfaces by Deposition of a Channel-Forming Protein and Subsequent Polymerization of Methyl Methacrylate Prepolymers

A novel procedure for nanostructuring on carbon surfaces by depositing buffer droplets containing a unique protein (MspA porin from Mycobacterium smegmatis) and a PMMA prepolymer, followed by thermal curing and high vacuum treatment, was developed. The formation of protein/PMMA nanostructures on carbon surfaces depended on the amount of protein/prepolymer deposited and the temperature during and after the deposition process. The nanostructures were analyzed quantitativly using computer-supported high-resolution transmission electron micrography (TEM).

Direct evidence for the incorporation of univalent indium into high-silica zeolite, H-ZSM-5, by thermal auto-reductive solid-state ion exchange.

In mechanical mixtures of H-ZSM-5 and In2O3 thermal auto-reductive solid-state ion exchange (AR-SSIE) was found to proceed upon treatment in high vacuum at 840 K resulting in the incorporation of In+ ions into, and in an increase of the thermal stability of, the zeolitic component.

Comparison of the redox properties of iron incorporated in different amounts into MCM-41

Mesoporous Fe-MCM-41 samples with different framework iron contents were tested as catalysts in CO oxidation and characterized by TPR,Mossbauer and FTIR spectroscopies after heat treatment in high vacuum and reductive or oxidative atmospheres. In samples with Si/Fe ratiosof 20 and 12 all of the iron was found to be reducible with H2, but only a part of Fe(III) species could be reduced to Fe(II) in materialswith lower iron content. In neither of the materials could Fe(III) be fully converted with CO to the divalent oxidation state. A significant part ofiron can be reversibly reduced and reoxidized in successive redox cycles. Large Fe-O clusters could not be detected.

High Resolution Surface Structure of Porous Silicon

The work demonstrates that the surface atom layers of variously-prepared samples of porous silicon can, after high-temperature heat treatment in high vacuum, yield low energy electron diffraction patterns and also atomic resolution in scanning tunneling microscopy. The (100) surfaces show some patches of 2 × 1 reconstruction but no clear reconstructions are visible on (111) surfaces. We also show that although evaporated metal contacts give poor reproducibility of current–voltage characteristics, the latter can be made stable and reproducible if the surfaces are first coated with a conducting polymer