Scanning tunneling microscopy of ethylated Si(111) surfaces prepared by a chlorination/alkylation process

Scanning tunneling microscopy (STM) and computational modeling have been used to study the structure of ethyl-terminated Si(111) surfaces. The ethyl-terminated surface was prepared by treating the H-terminated Si(111) surface with PCl5 to form a Cl-terminated Si(111) surface with subsequent exposure to C(2)H(5)MgCl in tetrahydrofuran to produce an alkylated Si(111) surface. The STM data at 77 K revealed local, close-packed, and relatively ordered regions with a nearest-neighbor spacing of 0.38 nm as well as disordered regions. The average spot density corresponded to approximately 85% of the density of Si atop sites on an unreconstructed Si(111) surface. Molecular dynamics simulations of a Si(111) surface randomly populated with ethyl groups to a total coverage of approximately 80% confirmed that the ethyl-terminated Si(111) surface, in theory, can assume reasonable packing arrangements to accommodate such a high surface coverage, which could be produced by an exoergic surface functionalization route such as the two-step chlorination/alkylation process. Hence, it is possible to consistently interpret the STM data within a model suggested by recent X-ray photoelectron spectroscopic data and infrared absorption data, which indicate that the two-step halogenation/alkylation method can provide a relatively high coverage of ethyl groups on Si(111) surfaces.     

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.     

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.     

Production of highly homogeneous Si(100) surfaces by H2O etching: surface morphology and the role of strain

The etching of Si(100) surfaces in ultrapure water was studied with a combination of infrared spectroscopy (FTIR) and scanning tunneling microscopy (STM). While the FTIR results show that the initially rough H/Si(100) surface becomes highly homogeneous during etching, a phenomenon generally associated with surface smoothing, STM images reveal that the homogeneity is associated with the formation of well-defined etch hillocks. After many hours of etching, the resulting H-terminated surface is composed of stripes of atomically flat Si(100) terminated by etch hillocks bounded by {111}- and {110}-oriented microfacets. Polarization analysis of the Si-H stretching modes provides strong evidence for uniform dihydride-termination of the flat regions, with the narrow (approximately 25 A) width of these stripes allowing for relaxation of steric strain between neighboring dihydrides. The unusual hill-and-valley etch morphology is attributed to the effects of steric strain on the reactivity of sites on the etched surface.     

Chemical and electrical passivation of silicon (111) surfaces through functionalization with sterically hindered alkyl groups

Crystalline Si(111) surfaces have been alkylated in a two-step chlorination/alkylation process using sterically bulky alkyl groups such as (CH3)2CH- (iso-propyl), (CH3)3C- (tert-butyl), and C6H5- (phenyl) moieties. X-ray photoelectron spectroscopic (XPS) data in the C 1s region of such surfaces exhibited a low energy emission at 283.9 binding eV, consistent with carbon bonded to Si. The C 1s XPS data indicated that the alkyls were present at lower coverages than methyl groups on CH(3)-terminated Si(111) surfaces. Despite the lower alkyl group coverage, no Cl was detected after alkylation. Functionalization with the bulky alkyl groups effectively inhibited the oxidation of Si(111) surfaces in air and produced low (<100 cm s(-1)) surface recombination velocities. Transmission infrared spectroscopy indicated that the surfaces were partially H-terminated after the functionalization reaction. Application of a reducing potential, -2.5 V vs Ag+/Ag, to Cl-terminated Si(111) electrodes in tetrahydrofuran resulted in the complete elimination of Cl, as measured by XPS. The data are consistent with a mechanism in which the reaction of alkyl Grignard reagents with the Cl-terminated Si(111) surfaces involves electron transfer from the Grignard reagent to the Si, loss of chloride to solution, and subsequent reaction between the resultant silicon radical and alkyl radical to form a silicon-carbon bond. Sites sterically hindered by neighboring alkyl groups abstract a H atom to produce Si-H bonds on the surface.     

Low-temperature STM images of methyl-terminated Si(111) surfaces

Low-temperature scanning tunneling microscopy (STM) has been used to image CH(3)-terminated Si(111) surfaces that were prepared through a chlorination/alkylation procedure. The STM data revealed a well-ordered structure commensurate with the atop sites of an unreconstructed 1 x 1 overlayer on the silicon (111) surface. Images collected at 4.7 K revealed bright spots, separated by 0.18 +/- 0.01 nm, which are assigned to adjacent H atoms on the same methyl group. The C-H bonds in each methyl group were observed to be rotated by 7 +/- 3 degrees away from the center of an adjacent methyl group and toward an underlying Si atom. Hence, the predominant interaction that determines the surface structure arises from repulsions between hydrogen atoms on neighboring methyl groups, and secondary interactions unique to the surface are also evident.     

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.     

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.     

Quantum mechanics calculations of the thermodynamically controlled coverage and structure of alkyl monolayers on Si(111) surfaces

The heat of formation, Delta E, for silicon (111) surfaces terminated with increasing densities of the alkyl groups CH3- (methyl), C2H5- (ethyl), (CH3)2CH- (isopropyl), (CH3)3C- (tert-butyl), CH3(CH2)5- (hexyl), CH3(CH2)7- (octyl), and C6H5- (phenyl) was calculated using quantum mechanics (QM) methods, with unalkylated sites being H-terminated. The free energy, Delta G, for the formation of both Si-C and Si-H bonds from Si-Cl model compounds was also calculated using QM, with four separate Si-H formation mechanisms proposed, to give overall Delta G(S) values for the formation of alkylated Si(111) surfaces through a two step chlorination/alkylation method. The data are in good agreement with measurements of the packing densities for alkylated surfaces formed through this technique, for Si-H free energies of formation, Delta G(H), corresponding to a reaction mechanism including the elimination of two H atoms and the formation of a C=C double bond in either unreacted alkyl Grignard groups or tetrahydrofuran solvent.     

HREELS, STM, and STS study of CH(3)-terminated Si(111)-(1x1) surface

An ideally (1x1)-CH(3)(methyl)-terminated Si(111) surface was composed by Grignard reaction of photochlorinated Si(111) and the surface structure was for the first time confirmed by Auger electron spectroscopy, low energy electron diffraction, high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). HREELS revealed the vibration modes associated to the CH(3)-group as well as the C-Si bond. STM discerned an adlattice with (1x1) periodicity on Si(111) composed of protrusions with internal features, covering all surface terraces. The surface structure was confirmed to be stable at temperatures below 600 K. STS showed that an occupied-state band exists at gap voltage of -1.57 eV, generated by the surface CH(3) adlattice. This CH(3):Si(111)-(1x1) adlayer with high stability and unique electronic property is prospective for applications such as nanoscale lithography and advanced electrochemistry.     

Covalent attachment of acetylene and methylacetylene functionality to Si(111) surfaces: scaffolds for organic surface functionalization while retaining Si-C passivation of Si(111) surface sites

Si(111) surfaces have been functionalized with Si-CC-R species, where R = H or -CH3, using a two-step reaction sequence involving chlorination of H-Si(111) followed by treatment with Na-CC-H or CH3-CC-Na reagents. The resulting surfaces showed no detectable oxidation as evidenced by X-ray photoelectron spectroscopic (XPS) data in the Si 2p region, electrochemical measurements of Si-H oxidation, or infrared spectroscopy. The Si-CC-R-terminated surfaces exhibited a characteristic CC stretch in the infrared at 2179 cm-1, which was strongly polarized perpendicular to the Si(111) surface plane. XPS measurements in the C 1s region showed a low binding energy peak indicative of Si-C bonding, with a coverage that was, within experimental error, identical to that of the CH3-terminated Si(111) surface, which has been shown to fully terminate the Si atop sites on an unreconstructed Si(111) surface. The Si-CC-H-terminated surfaces were further functionalized by exposure to n-C4H9Li followed by exposure to para Br-C6H5-CF3, allowing for introduction of para -C6H5CF3 groups while maintaining the desirable chemical and electrical properties that accompany complete Si-C termination of the atop sites on the Si(111) surface.     

Formation of organic monolayers on silicon via gas-phase photochemical reactions

A new method for the formation of molecular monolayers on silicon surfaces utilizing gas-phase photochemical reactions is reported. Hydrogen-terminated Si(111) surfaces were exposed to various gas-phase molecules (hexene, benzaldehyde, and allylamine) and irradiated with ultraviolet light from a mercury lamp. The surfaces were studied with in situ Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and scanning tunneling microscopy. The generation of gas-phase radicals was found to be the initiator for organic monolayer formation via the abstraction of hydrogen from the H/Si(111) surface. Monolayer growth can occur through either a radical chain reaction mechanism or through direct radical attachment to the silicon dangling bonds.     

Control of the stability, electron-transfer kinetics, and pH-dependent energetics of Si/H2O interfaces through methyl termination of Si(111) surfaces

Methyl-terminated, n-type, (111)-oriented Si surfaces were prepared via a two-step chlorination-alkylation method. This surface modification passivated the Si surface toward electrochemical oxidation and thereby allowed measurements of interfacial electron-transfer processes in contact with aqueous solutions. The resulting semiconductor/liquid junctions exhibited interfacial kinetics behavior in accord with the ideal model of a semiconductor/liquid junction. In contrast to the behavior of H-terminated Si(111) surfaces, current density vs. potential measurements of CH(3)-terminated Si(111) surfaces in contact with an electron acceptor having a pH-independent redox potential (methyl viologen(2+/+)) were used to verify that the band edges of the modified Si electrode were fixed with respect to changes in solution pH. The results provide strong evidence that the energetics of chemically modified Si interfaces can be fixed with respect to pH and show that the band-edge energies of Si can be tuned independently of pH-derived variations in the electrochemical potential of the solution redox species.     

Chlorination-methylation of the hydrogen-terminated silicon(111) surface can induce a stacking fault in the presence of etch pits

Recently, we reported STM images of the methylated Si(111) surface [prepared through chlorination-alkylation of the Si(111)-H surface] taken at 4.7 K, indicating that the torsion angle of the methyl group with respect to the subsurface silicon layer is phi = 23 +/- 3 degrees . Repulsions between H atoms in adjacent methyl groups are minimized at 30 degrees , while repulsions between H atoms and second layer Si atoms are minimized at 60 degrees . The experimental result of 23 degrees is surprising because it suggests a tendency of the methyl group toward the eclipsed configuration (0 degrees ) rather than staggered (60 degrees ). In contrast, extensive fully periodic quantum mechanical Density Functional Theory studies of this surface give an equilibrium torsion angle of 37.5 degrees , indicating a tendency toward the staggered configuration. This discrepancy can be resolved by showing that the CH3 on the step edges and etch pits interacts repulsively with the CH3 on the surface terraces unless a stacking fault is introduced between the first and second silicon layers of the Si(111)-CH3 surface terraces. We propose that this could occur during the chlorination-alkylation of the Si(111)-H surface. This stacking fault model predicted phi = 22.5 degrees measured with respect to the bulk (corresponding to phi = 37.5 degrees with respect to the second layer Si atoms). This model can be tested by measuring the orientation of the CH3 within the etch pits, which should have phi = 37.5 degrees , or by making a surface without etch pits, which should have phi = 37.5 degrees .     

Chemical force titrations of functionalized Si(111) surfaces

Chemical force titrations-plots of the adhesive force between an atomic force microscope tip and sample as a function of pH-were acquired on alkyl monolayer-derivatized Si(111) surfaces. Gold-coated AFM tips modified with thioalkanoic acid self-assembled monolayers (SAM) were employed. Alkyl monolayer-derivatized Si(111) surfaces terminated with methyl, carboxyl, and amine groups were produced via hydrosilylation reactions between 1-alkene reagents and H-terminated silicon. The functionalized surfaces were characterized using standard surface science techniques (AFM, FTIR, and XPS). Titration of the methyl-terminated surface using the modified (carboxyl-terminated) atomic force microscope tip resulted in a small pH-independent hydrophobic interaction. Titration of the amine-terminated surface using the same tip resulted in the determination of a surface pKa of 5.8 for the amine from the pH value from the maximum in the force titration curve. A pK(1/2) of 4.3 was determined for the carboxyl-terminated Si(111) in a similar way. These results will be discussed in relation to the modified Si(111) surface chemistry and organic layer structure, as well as with respect to existing results on Au surfaces modified with SAMs bearing the same functional groups.     

UV-induced immobilization of tethered zirconocenes on H-terminated silicon surfaces

A tethered ethylenebis(indenyl) zirconocene was covalently immobilized on H-terminated Si(111) surfaces using UV-mediated alkene hydrosilylation, thus making possible the development of structured catalytic surfaces with highly controlled properties.     

Azidation of silicon(111) surfaces

A two-step chlorination/azidation process was reported to prepare azide-modified silicon(111) surfaces. XPS and IR analyses show the covalent bonding of azide with silicon. In combination with scanning tunneling microscopy and spectroscopy analyses, different kinetic rates, azide coverages, and surface-area distributions were derived depending on the azidation solvent.     

Understanding the contrast mechanism in scanning tunneling microscopy (STM) images of an intermixed tetraphenylporphyrin layer on Ag(111)

The appearance of tetraphenylporphyrins in scanning tunneling micrographs depends strongly on the applied bias voltage. Here, we report the observation and identification of certain features in scanning tunneling microscopy (STM) images of intermixed layers of tetraphenylporphyrin (2HTPP) and cobalt-tetraphenylporphyrin (CoTPP) on Ag(111). A significant fraction of an ordered monolayer of commercially available CoTPP appears as "pits" at negative bias voltages around -1 V. The obvious possibility that these pits are missing molecules within the ordered layer could be ruled out by imaging the molecules at reduced bias voltages, at which the contrast of the pits fades, and at positive bias voltages around +1 V, at which the image contrast is inverted. With the investigation of the electronic structure, in particular the density of states (DOS) close to the Fermi level, of CoTPP and 2HTPP layers by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS), the contrast mechanism was clarified. The correlation of the bias dependent contrast with the UPS data enabled us to interpret the "pits" as individual 2HTPP molecules. Additional evidence could be provided by imaging layers of different mixtures of 2HTPP and CoTPP and by high-resolution STM imaging of the features in CoTPP.     

Molecular simulation of alkyl monolayers on the Si(111)surface

The structure of twelve-carbon monolayers on the H-terminated Si(111) surface is investigated by molecular simulation method. The best substitution percent on Si(111) surface obtained via molecular mechanics calculation is equal to 50%, and the (8×8) simulated cell can be used to depict the structure of alkyl monolayer on Si surface. After two-dimensional cell containing alkyl chains and four-layer Si(111) crystal at the substitution 50% is constructed, the densely packed and well-ordered monolayer on Si(111) surface can be shown through energy minimization in the suitable-size simulation cell. These simulation results are in good agreement with the experiments. These conclusions show that molecular simulation can provide otherwise inaccessible mesoscopic information at the molecular level, and can be considered as an adjunct to experiments.     

Molecular simulation of alkyl monolayers on the Si(111) surface YUAN

The preparation of monolayers on silicon surface is of growing interest for potential applica-tions in biosensor or semiconductor technology[1—5]. The alkyl modified Si(111) surfaces[6—10] can be obtained using the thermal, catalyzed, or photochemical reaction of hydrogen-terminated sili-con with alkenes, Grignard reagents, and so on. At the same time, the monolayer properties on Si(111) surface have been studied by a variety of experimental methods[8—10] such as X-ray photo-electron spect…