John E. Crowell

Desorption/ionization on silicon (DIOS): A diverse mass spectrometry platform for protein characterization

Since the advent of matrix-assisted laser desorption/ionization and electrospray ionization, mass spectrometry has played an increasingly important role in protein functional characterization, identification, and structural analysis. Expanding this role, desorption/ionization on silicon (DIOS) is a new approach that allows for the analysis of proteins and related small molecules. Despite the absence of matrix, DIOS-MS yields little or no fragmentation and is relatively tolerant of moderate amounts of contaminants commonly found in biological samples. Here, functional assays were performed on an esterase, a glycosidase, a lipase, as well as exo- and endoproteases by using enzyme-specific substrates. Enzyme activity also was monitored in the presence of inhibitors, successfully demonstrating the ability of DIOS to be used as an inhibitor screen. Because DIOS is a matrix-free desorption technique, it also can be used as a platform for multiple analyses to be performed on the same protein. This unique advantage was demonstrated with acetylcholine esterase for qualitative and quantitative characterization and also by its subsequent identification directly from the DIOS platform.

Chemical methods of thin film deposition: Chemical vapor deposition, atomic layer deposition, and related technologies

This article provides a brief overview of thin film growth utilizing the reactive processes of chemical vapor deposition (CVD) and atomic layer deposition (ALD). In CVD, thin films are deposited upon the chemical reaction of vapor phase precursors with a solid surface. ALD is a surface reaction-controlled variant of CVD in which the chemical precursors are introduced in a sequential, pulsed manner, resulting in the growth of a self-limited monolayer (or less) film for each pulse step. The aim of both methodologies is the controlled growth of thin films with desired and reproducible properties. Emphasis is given to the latest advancements and future directions, and the processes and precursors commonly used to achieve controlled deposition.

A mass spectrometry plate reader: monitoring enzyme activity and inhibition with a Desorption/Ionization on Silicon (DIOS) platform.

A surface‐based laser desorption/ionization mass spectrometry assay that makes use of Desorption/Ionization on Silicon Mass Spectrometry (DIOS‐MS) has been developed to monitor enzyme activity and enzyme inhibition. DIOS‐MS has been used to characterize inhibitors from a library and then to monitor their activity against selected enzyme targets, including proteases, glycotransferase, and acetylcholinesterase. An automated DIOS‐MS system was also used as a high‐throughput screen for the activity of novel enzymes and enzyme inhibitors. On two different commercially available instruments, a sampling rate of up to 38 inhibitors per minute was accomplished, with thousands of inhibitors being monitored. The ease of applying mass spectrometry toward developing enzyme assays and the speed of surface‐based assays such as DIOS for monitoring inhibitor effectiveness and enzyme activity makes it attractive for a broad range of screening applications.

Model studies of dielectric thin film growth: Chemical vapor deposition of SiO2

The reactive adsorption and decomposition of tetraethoxysilane is compared on Si(100) and SiO2 surfaces. The adsorption and decomposition behavior is compared to that observed for ethanol adsorption on both these surfaces. Tetraethoxysilane and ethanol both decompose to produce ethylene and hydrogen on Si(100). Ethylene desorption is also observed from decomposition of these molecules on SiO2. Ethanol adsorption on SiO2 is shown to model the chemistry of tetraethoxysilane on this surface. However, ethanol is significantly more reactive than tetraethoxysilane on the clean Si(100) surface. The decomposition is shown to be reaction limited, and distinct from that of adsorbed ethylene. The adsorbed species produced from adsorption of tetraethoxysilane on SiO2 at 450 K is shown to be a mixture of di‐ and triethoxysiloxanes.

Investigation on the growth rate enhancement by Ge during SiGe alloy deposition by chemical vapor deposition

The desorption of deuterium from clean and Ge‐covered Si(100) surfaces has been studied using temperature‐programmed desorption. The Ge/Si(100) surfaces were prepared using the dissociative adsorption of digermane on Si(100). The presence of Ge on Si(100) dramatically lowers the deuterium (or hydrogen) desorption temperature. The desorption maxima shift to lower temperature with increasing Ge coverage until a new low‐temperature desorption state becomes dominant. The lowering of the deuterium desorption energies due to the presence of Ge on the Si(100) surface is used to explain the acceleration in growth rate observed during GexSi1−x alloy growth by chemical vapor deposition upon the introduction of germane to the Si source gas.

Mechanistic studies of dielectric thin film growth by low pressure chemical vapor deposition: The reaction of tetraethoxysilane with SiO2 surfaces

The adsorption and reaction of tetraethoxysilane (TEOS) with hydroxylated SiO2 has been studied over the range of 100–450 K using transmission infrared spectroscopy. At 100 K, TEOS [Si(OC2H5)4] condenses on SiO2. Upon warming in vacuum, some of the condensed phase TEOS desorbs molecularly while a significant portion of the layer enters into a physisorption state. The physisorption state maximizes near 250 K, with strictly molecular desorption occurring upon warming to higher temperatures. When exposure occurs at 450 K, Si(OC2H5)4 reacts to form adsorbed siloxanes, thought to be a mixture of (SiO)2Si(OC2H5)2 and (SiO)Si(OC2H5)3. The adsorbed di‐ and triethoxysiloxanes decompose completely on heating in vacuum to 900 K. The chemistry of TEOS on SiO2 has been modeled using ethanol adsorption. Exposure of SiO2 to ethanol at 450 K leads to the formation of an adsorbed ethoxide species. Ethanol is shown to spectroscopically and chemically model the surface bound siloxanes produced upon reaction of Si(OC2H5)4 wi...

The effect of germanium on the desorption of hydrogen from Si(100)

Abstract The desorption of deuterium from Ge covered Si(100) surfaces has been studied using temperature programmed desorption. The presence of germanium on the surface generates a new low temperature state through which deuterium may desorb. The desorption maxima for all surface deuteride species are observed to shift to lower temperatures with increasing germanium coverage until the low temperature state becomes dominant at high surface hydride coverages and moderate to high germanium coverages. The general prominence of the low temperature feature is explained in terms of combined contributions from germanium monodeuteride and silicon dideuteride desorption. A single high temperature desorption feature is observed at low D atom exposures for θGe ⩽ 0.20 within this range, the β1 desorption state shifts uniformly to lower temperatures with increasing Ge coverage. The shift in the β1 desorption maxima from 805 K for Si(100) to 740 K for a 10.0% Ge Si (100) surface translates into a lowering of the desorption energy from 57.9 to 53.7 kcal mol . The lowered desorption energies for both silicon mono- and dihydrides are explained in terms of a lowered activation barrier for recombinative desorption; the presence of germanium lowers this barrier through electronic interactions with Si atoms.

The photo-induced reaction of digermane with the Si(100)(2 × 1):D surface

Abstract Photo-induced surface reactions brought about by photolysis of adsorbed digermane on the Si(100)(2 × 1):D surface have been investigated under ultrahigh vacuum conditions using Auger electron spectroscopy and temperature programmed desorption. On the monodeuterated Si(100)(2 × 1):D surface, no spontaneous thermal reaction of digermane is observed, due to the termination of the dangling bonds with D atoms. Molecular Ge2H6, weakly adsorbed on Si(100)(2 × 1):D at 120 K, dissociates upon UV irradiation, leading to the incorporation of Ge atoms and the adsorption of H atoms on the surface. The appearance of a new low temperature α desorption state for hydrogen (deuterium) desorption near 580 K is further evidence for Ge deposition and the photo-induced decomposition of physi-adsorbed Ge2H6. The observation of α-HD and α-D2 desorption indicates that the photoreaction intermediate inserts into the surface Si-D bond to produce a Ge-D bond. Studies of the thermal reaction of Ge2H6 with partially deuterated (ΘD

The adsorption of hydrogen, digermane, and disilane on Ge(111): a multiple internal reflection infrared spectroscopy study

Abstract The adsorption of hydrogen, digennane, and disilane on the Ge(111) surface has been investigated using multiple internal reflection infrared Spectroscopy (MIRIRS). Adsorption of atomic hydrogen on Ge(111) leads to the production of GeH3, GeH2, and GeH at low temperatures (⩽, 150K), and strictly a monohydride at higher temperatures (⩾ 400K). Atomic H occupies primarily a single GeH adsorption site at 500K, whereas a range of monohydride adsorption sites exists at lower temperatures. At temperatures below 300K, GeH production dominates H atom exposure, with minor production of the higher hydrides, while dissociative adsorption of Ge2H6 favors formation of GeH2 and GeH3. Similar behavior is observed for disilane adsorption, with formation of SiH3 and SiH2 below 300K and SiH above this temperature. Molecular adsorption of Ge2H6and Si2H6 occurs for adsorption below 150K. Molecular and dissociative adsorption are competitive processes at 110–140K. Decomposition of all surface hydrides occurs by 600K.

The adsorption and thermal decomposition of digermane on Ge(111)

We have used multiple internal reflection infrared spectroscopy to investigate the interaction of digermane with Ge(111) at temperatures between 104–600 K. Digermane predominantly adsorbs molecularly on the surface below 120 K, displaying a vibrational spectrum similar to that of condensed digermane. At temperatures between 120–150 K, digermane dissociates via Ge–Ge bond scission to form adsorbed GeH3. Chemisorbed germyl GeH3 has a distinct symmetric deformation vibration at ∼772 cm−1, compared to a value of 721 cm−1 for molecularly adsorbed Ge2H6. At 200 K, Ge2H6 adsorption produces surface GeH3, GeH2, and GeH species with stretching vibrations at 2063, 2023, and 1968 cm−1, respectively. The surface GeH2 species is also identified by a characteristic scissor mode at ∼830 cm−1. Adsorption at 300 and 400 K produces only GeH2 and GeH, with a much lower concentration of GeH2 at 400 K. The surface GeH2 and GeH species are also generated by the successive decomposition of GeH3 upon heating. All surface hydroge...