A photo Lewis acid generator (PhLAG): controlled photorelease of B(C6F5)3.

A molecule that releases the strong organometallic Lewis acid B(C(6)F(5))(3) upon irradiation with 254 nm light has been developed. This photo Lewis acid generator (PhLAG) now enables the photocontrolled initiation of several reactions catalyzed by this important Lewis acid. Herein is described the synthesis of the triphenylsulfonium salt of a carbamato borate based on a carbazole function, its establishment as a PhLAG, and the application of the photorelease of B(C(6)F(5))(3) to the fabrication of thin films of a polysiloxane material.

Nanopatterning Si(111) surfaces as a selective surface-chemistry route.

Using wet-chemical self-assembly, we demonstrate that standard surface reactions can be markedly altered. Although HF etching of Si surfaces is known to produce H-terminated surfaces, we show that up to approximately 30% of a monolayer of stable Si-F bonds can be formed on atomically smooth Si(111) surfaces on HF reaction, when chemically isolated Si atoms are the target of the reaction. Similarly, approximately 30% Si-OH termination can be achieved by immersion of the partially covered F-Si(111) surface in water without oxidation of the underlying Si substrate. Such reactions are possible when H-terminated (111)-oriented Si surfaces are initially uniformly patterned with methoxy groups. These findings are contrary to the knowledge built over the past twenty years and highlight the importance of steric interactions at surfaces and the possibility to stabilize products at surfaces that cannot be obtained on chemically homogeneous surfaces.

Relaxivity of gadolinium complexes detected by atomic magnetometry.

Laser atomic magnetometry is a portable and low-cost yet highly sensitive method for low magnetic field detection. In this work, the atomic magnetometer was used in a remote-detection geometry to measure the relaxivity of aqueous gadolinium-diethylenetriamine pentaacetic acid Gd(DTPA) at the Earth's magnetic field (40 µT). The measured relaxivity of 9.7±2.0 s(-1) mM(-1) is consistent with field-cycling experiments measured at slightly higher magnetic fields, but no cryogens or strong and homogeneous magnetic field were required for this experiment. The field-independent sensitivity of 80 fT Hz(-1/2) allowed an in vitro detection limit of ∼10 µM Gd(DTPA) to be measured in aqueous buffer solution. The low detection limit and enhanced relaxivity of Gd-containing complexes at Earth's field motivate continued development of atomic magnetometry toward medical applications.

Theoretical investigation of the structure and coverage of the Si(111)-OCH3 surface.

The surface structure, strain energy, and charge profile of the methoxylated Si(111) surface, Si(111)-OCH3, has been studied using quantum mechanics, and the results are compared to those obtained previously for Si(111)-CH3 and Si(111)-C2H5. The calculations indicate that 100% coverage is feasible for Si(111)-OCH3 (similar to the methylated surface), as compared to only approximately 80% coverage for the ethylated surface. These differences can be understood in terms of nearest-neighbor steric and electrostatic interactions. Enthalpy and free energy calculations indicate that the formation of the Si(111)-OCH3 surface from Si(111)-H and methanol is favorable at 300 K. The calculations have also indicated the conditions under which stacking faults can emerge on Si(111)-OCH3, and such conditions are contrasted with the behavior of Si(111)-CH3 and Si(111)-CH2CH3 surfaces, for which stacking faults are calculated to be energetically feasible when etch pits with sufficiently long edges are present on the surface.

Infrared spectroscopic investigation of the reaction of hydrogen-terminated, (111)-oriented, silicon surfaces with liquid methanol.

Fourier transform infrared spectroscopy and first principles calculations have been used to investigate the reaction of atomically smooth, hydrogen-terminated Si(111) (H-Si) surfaces with anhydrous liquid methanol. After 10 min of reaction at room temperature, a sharp absorbance feature was apparent at approximately 1080 cm(-1) that was polarized normal to the surface plane. Previous reports have identified this mode as a Si-O-C stretch; however, the first principles calculations, presented in this work, indicate that this mode is a combination of an O-C stretch with a CH3 rock. At longer reaction times, the intensity of the Si-H stretching mode decreased, while peaks attributable to the O-C coupled stretch and the CH3 stretching modes, respectively, increased in intensity. Spectra of H-Si(111) surfaces that had reacted with CD3OD showed the appearance of Si-D signals polarized normal to the surface as well as the appearance of vibrations indicative of Si-OCD3 surface species. The data are consistent with two surface reactions occurring in parallel, involving (a) chemical attack of hydrogen-terminated Si(111) terraces by CH3OH, forming Si-OCH3 moieties having their Si-O bond oriented normal to the Si(111) surface and (b) transfer of the acidic hydrogen of the methanol to the silicon surface, either through a direct H-to-D exchange mechanism or through a mechanism involving chemical step-flow etching of Si-H step sites.

Transmission infrared spectroscopy of methyl- and ethyl-terminated silicon(111) surfaces.

Transmission infrared spectroscopy (TIRS) has been used to investigate the surface-bound species formed in the two-step chlorination/alkylation reaction of crystalline (111)-oriented Si surfaces. Spectra were obtained after hydrogen termination, chlorine termination, and reaction of the Cl-Si(111) surface with CH(3)MgX or C(2)H(5)MgX (X = Cl, Br) to form methyl (CH(3))- or ethyl (C(2)H(5))-terminated Si(111) surfaces, respectively. Freshly etched H-terminated Si(111) surfaces that were subsequently chlorinated by immersion in a saturated solution of PCl(5) in chlorobenzene were characterized by complete loss of the Si-H stretching and bending modes at 2083 and 627 cm(-1)(,) respectively, and the appearance of Si-Cl modes at 583 and 528 cm(-1). TIRS of the CH(3)-terminated Si(111) surface exhibited a peak at 1257 cm(-1) polarized perpendicular to the surface assigned to the C-H symmetrical bending, or "umbrella" motion, of the methyl group. A peak observed at 757 cm(-1) polarized parallel to the surface was assigned to the C-H rocking motion. Alkyl C-H stretch modes on both the CH(3)- and C(2)H(5)-terminated surfaces were observed near 2900 cm(-1). The C(2)H(5)-terminated Si(111) surface additionally exhibited broad bands at 2068 and 2080 cm(-1), respectively, polarized perpendicular to the surface, as well as peaks at 620 and 627 cm(-1), respectively, polarized parallel to the surface. These modes were assigned to the Si-H stretching and bending motions, respectively, resulting from H-termination of surface atoms that did not form Si-C bonds during the ethylation reaction.

Chemical and electrical passivation of single-crystal silicon(100) surfaces through a two-step chlorination/alkylation process.

Single-crystal Si(100) surfaces have been functionalized by using a two-step radical chlorination-Grignard (R = MgCl, R = CH3, C2H5, C4H9, C6H5, or CH2C6H5) alkylation method. After alkylation, no chlorine was detectable on the surface by X-ray photoelectron spectroscopy (XPS), and the C 1s region showed a silicon-induced peak shift indicative of a Si-C bond. The relative intensity of this peak decreased, as expected, as the steric bulk of the alkyl increased. Despite the lack of full alkyl termination of the atop sites of the Si(100) surface, functionalization significantly reduced the rate of surface oxidation in air compared to that of the H-terminated Si(100) surface, with alkylated surfaces forming less than half a monolayer of oxide after over one month of exposure to air. Studies of the charge-carrier lifetime with rf photoconductivity decay methods indicated a surface recombination velocity of <30 cm s(-1) for methylated surfaces, and <60 cm s(-1) for Si surfaces functionalized with the other alkyl groups evaluated. Soft X-ray photoelectron spectroscopic data indicated that the H-Si(100) surfaces were terminated by SiH, SiH2, and SiH3 species, whereas Cl-Si(100) surfaces were predominantly terminated by monochloro (SiCl and SiHCl) and dichloro (SiCl2 and SiHCl2) Si species. Methylation produced signals consistent with termination by Si-alkyl bonding arising from SiH(CH3)-, SiH2(CH3)-, and Si(CH3)2-type species.

High-resolution X-ray photoelectron spectroscopy of chlorine-terminated GaAs(111)A surfaces.

Oxide-terminated and Cl-terminated GaAs(111)A surfaces have been characterized in the As and Ga 3d regions by high-resolution, soft X-ray photoelectron spectroscopy. The Cl-terminated surface, formed by treatment with 6 M HCl(aq), showed no detectable As oxides or As(0) in the As 3d region. The Ga 3d spectrum of the Cl-terminated surface showed a broad, intense signal at 19.4 eV and a smaller signal at 21.7 eV. The Ga 3d peaks were fitted using three species, one representing bulk GaAs and the others representing two chemical species on the surface. The large peak was well-fitted by the bulk GaAs emission and by a second doublet, assigned to surface Ga atoms bonded to Cl, that was shifted by 0.34 eV from the bulk GaAs 3d emission. The smaller peak, shifted by 2.3 eV in binding energy relative to the bulk GaAs Ga 3d signal, is assigned to Ga(OH)3. The data confirm that wet chemical etching allows for the formation of well-defined, Cl-terminated GaAs(111)A surfaces free of detectable elemental As, that can provide a starting point for further functionalization of GaAs.

High-resolution soft X-ray photoelectron spectroscopic studies and scanning auger microscopy studies of the air oxidation of alkylated silicon(111) surfaces.

High-resolution soft X-ray photoelectron spectroscopy was used to investigate the oxidation of alkylated silicon(111) surfaces under ambient conditions. Silicon(111) surfaces were functionalized through a two-step route involving radical chlorination of the H-terminated surface followed by alkylation with alkylmagnesium halide reagents. After 24 h in air, surface species representing Si(+), Si(2+), Si(3+), and Si(4+) were detected on the Cl-terminated surface, with the highest oxidation state (Si(4+)) oxide signal appearing at +3.79 eV higher in energy than the bulk Si 2p(3/2) peak. The growth of silicon oxide was accompanied by a reduction in the surface-bound Cl signal. After 48 h of exposure to air, the Cl-terminated Si(111) surface exhibited 3.63 equivalent monoleyers (ML) of silicon oxides. In contrast, after exposure to air for 48 h, CH(3)-, C(2)H(5)-, or C(6)H(5)CH(2)-terminated Si surfaces displayed <0.4 ML of surface oxide, and in most cases only displayed approximately 0.20 ML of oxide. This oxide was principally composed of Si(+) and Si(3+) species with peaks centered at +0.8 and +3.2 eV above the bulk Si 2p(3/2) peak, respectively. The silicon 2p SXPS peaks that have previously been assigned to surface Si-C bonds did not change significantly, either in binding energy or in relative intensity, during such air exposure. Use of a high miscut-angle surface (7 degrees vs < or =0.5 degrees off of the (111) surface orientation) yielded no increase in the rate of oxidation nor change in binding energy of the resultant oxide that formed on the alkylated Si surfaces. Scanning Auger microscopy indicated that the alkylated surfaces formed oxide in isolated, inhomogeneous patches on the surface.

Surface etching, chemical modification and characterization of silicon nitride and silicon oxide--selective functionalization of Si3N4 and SiO2.

The ability to selectively chemically functionalize silicon nitride (Si3N4) or silicon dioxide (SiO2) surfaces after cleaning would open interesting technological applications. In order to achieve this goal, the chemical composition of surfaces needs to be carefully characterized so that target chemical reactions can proceed on only one surface at a time. While wet-chemically cleaned silicon dioxide surfaces have been shown to be terminated with surficial Si-OH sites, chemical composition of the HF-etched silicon nitride surfaces is more controversial. In this work, we removed the native oxide under various aqueous HF-etching conditions and studied the chemical nature of the resulting Si3N4 surfaces using infrared absorption spectroscopy (IRAS), x-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and contact angle measurements. We find that HF-etched silicon nitride surfaces are terminated by surficial Si-F and Si-OH bonds, with slightly subsurface Si-OH, Si-O-Si, and Si-NH2 groups. The concentration of surficial Si-F sites is not dependent on HF concentration, but the distribution of oxygen and Si-NH2 displays a weak dependence. The Si-OH groups of the etched nitride surface are shown to react in a similar manner to the Si-OH sites on SiO2, and therefore no selectivity was found. Chemical selectivity was, however, demonstrated by first reacting the -NH2 groups on the etched nitride surface with aldehyde molecules, which do not react with the Si-OH sites on a SiO2 surface, and then using trichloro-organosilanes for selective reaction only on the SiO2 surface (no reactivity on the aldehyde-terminated Si3N4 surface).

High-resolution X-ray photoelectron spectroscopic studies of alkylated silicon(111) surfaces.

Hydrogen-terminated, chlorine-terminated, and alkyl-terminated crystalline Si(111) surfaces have been characterized using high-resolution, soft X-ray photoelectron spectroscopy from a synchrotron radiation source. The H-terminated Si(111) surface displayed a Si 2p(3/2) peak at a binding energy 0.15 eV higher than the bulk Si 2p(3/2) peak. The integrated area of this shifted peak corresponded to one equivalent monolayer, consistent with the assignment of this peak to surficial Si-H moieties. Chlorinated Si surfaces prepared by exposure of H-terminated Si to PCl5 in chlorobenzene exhibited a Si 2p(3/2) peak at a binding energy of 0.83 eV above the bulk Si peak. This higher-binding-energy peak was assigned to Si-Cl species and had an integrated area corresponding to 0.99 of an equivalent monolayer on the Si(111) surface. Little dichloride and no trichloride Si 2p signals were detected on these surfaces. Silicon(111) surfaces alkylated with CnH(2n+1)- (n = 1 or 2) or C6H5CH2- groups were prepared by exposing the Cl-terminated Si surface to an alkylmagnesium halide reagent. Methyl-terminated Si(111) surfaces prepared in this fashion exhibited a Si 2p(3/2) signal at a binding energy of 0.34 eV above the bulk Si 2p(3/2) peak, with an area corresponding to 0.85 of a Si(111) monolayer. Ethyl- and C6H5CH2-terminated Si(111) surfaces showed no evidence of either residual Cl or oxidized Si and exhibited a Si 2p(3/2) peak approximately 0.20 eV higher in energy than the bulk Si 2p(3/2) peak. This feature had an integrated area of approximately 1 monolayer. This positively shifted Si 2p(3/2) peak is consistent with the presence of Si-C and Si-H surface functionalities on such surfaces. The SXPS data indicate that functionalization by the two-step chlorination/alkylation process proceeds cleanly to produce oxide-free Si surfaces terminated with the chosen alkyl group.

Flow in porous metallic materials: a magnetic resonance imaging study.

PURPOSE: To visualize flow dynamics of analytes inside porous metallic materials with laser-detected magnetic resonance imaging (MRI). MATERIALS AND METHODS: We examine the flow of nuclear-polarized water in a porous stainless steel cylinder. Laser-detected MRI utilizes a sensitive optical atomic magnetometer as the detector. Imaging was performed in a remote-detection mode: the encoding was conducted in the Earth's magnetic field, and detection is conducted downstream of the encoding location. Conventional MRI (7T) was also performed for comparison. RESULTS: Laser-detected MRI clearly showed MR images of water flowing through the sample, whereas conventional MRI provided no image. CONCLUSION: We demonstrated the viability of laser-detected MRI at low-field for studying porous metallic materials, extending MRI techniques to a new group of systems that is normally not accessible to conventional MRI.