Modification of hydroxyl-terminated self-assembled monolayer (HO-SAM) surfaces by collision of low-energy (15 eV) hyperthermal Si(CH3)3+ ions is shown to lead to Si-O bond formation and terminal trimethylsilyl ether formation. Modification was verified by in situ mass spectrometry using chemical sputtering with CF3+ ions (70 eV), ex situ secondary ion mass spectrometric analysis (12 kV Ga+ primary ion beam), and through X-ray photoelectron spectroscopy by monitoring Si (2s). The nature of the surface modification was further established by analysis of synthetic SAM surfaces made up of mixtures of the trimethylsilyl-11-mercapto-1-undecane ether and various proportions of the hydroxyl-terminated mercaptan (11-mercapto-1-undecanol). These mixed surfaces, as well as the spectroscopic data, indicate that ca. 30% of the hydroxyl chains are covalently modified at saturation coverage. Analogous surface transformations are achieved using Si(CH3)2F+ and Si(CH3)2C6H5+.
Silylium cations, SiCl3 + and Si(CH3)3 +, undergo dissociative ion/surface reactions in the course of low-energy (20−90 eV) collisions with hydroxyl-terminated (HO−SAM), hydrocarbon (H−SAM), and fluorocarbon (F−SAM) self-assembled monolayer surfaces. Formation of the substitution product, SiCl2F+, upon collision of SiCl3 + with the F−SAM surface is the result of a transhalogenation reaction. In an analogous fashion, one observes substitution of a chlorine in the SiCl3 + projectile ion by either an OH group from the HO−SAM surface or a CH3 group from the H−SAM surface to form the scattered reaction products, SiCl2OH+ and SiCl2CH3 +, respectively. The concomitant transfer of a Cl atom from the projectile ion into the surface is indicated by the sputtered ion, CH2Cl+. The scattered product SiCl(OH)2 + involves disubstitution, and reaction with more than one chain at the surface. These and related reactions involve the activation of C−O, C−F, C−C, C−H, and O−H bonds at the appropriate surface, and they occur after, or in concert with, surface-induced dissociation of the polyatomic projectile. Surface effects on the dissociation of projectile ions are studied using the Si(C2H5)4 •+ ion, and threshold values for translational to internal energy (T ⇒ V) conversion for this ion are measured as 13%, 13%, and 20% for the H−SAM, HO−SAM, and F−SAM surfaces, respectively. At higher collision energies, (>40 eV), the HO−SAM surface demonstrated greater internal energy conversion efficiency than the H−SAM surface. The process of neutralization and the accompanying release of chemically sputtered ions also served to distinguish the three surfaces. Decreased neutralization at the F−SAM surface is associated with increased amounts of dissociatively and reactively scattered product ions. Thermodynamic estimates regarding charge exchange between the surface and the projectile ion are consistent with the relative amounts of chemically sputtered products observed for each of the surfaces.
Specific covalent chemical modification at the outermost atomic layers of fluorinated self-assembled monolayers (F-SAMs) on gold is achieved by bombardment with low-energy polyatomic ions (`100 eV). The projectile ion CH 2 Br 2 Á (m/z 172), mass and energy selected using a hybrid ion/surface scattering mass spectrometer and scattered from the F-SAM surface, CF 3 (CF 2 ) 7 (CH 2 ) 2 -S-Au, undergoes ion/surface reactions evident from the nature of the scattered ions, CH 2 F (m/z 33), CHBrF (m/z 111), and CF 2 Br (m/z 129). The chemical transformation of the reactive F-SAM surface was independently monitored by in situ chemical sputtering with the projectile Xe Á . Representative species sputtered from the modified surface include CF 2 Br , an indicator of terminal CF 3 to CF 2 Br conversion. X-ray photoelectron spectroscopy (XPS) was used to confirm the presence of organic bromine at the surface; Br ( 3 P 3/2 ) and Br ( 3 P 1/2 ) peaks were present at binding energies of 182 and 190 eV, respectively. XPS analysis also revealed increased surface modification at higher collision energies in these reactive ion bombardment experiments, as exemplified by the increased hydrocarbon/fluorocarbon peak ratio in the C(1s) region and incorporation of oxygen in the surface seen in the observation of an O(1s) peak. , 986-993 (1999) FLUOROCARBON MONOLAYERS MODIFIED BY LOW-ENERGY ION BEAMS 987 Figure 4. Partial X-ray photoelectron spectra of fluorocarbon monolayer surfaces, illustrating the Br(3p) region: (a) unmodified surface and (b)-(d) modified by the projectile CH 2 Br 2 Á using the collision energies shown.
Using a multi-sector ion-surface scattering mass spectrometer, reagent ions of the general form SiR(3) (+) were mass and energy selected and then made to collide with a hydroxy-terminated self-assembled monolayer (HO-SAM) surface at energies of approximately 15 eV. These ion-surface interactions result in covalent transformation of the terminal hydroxy groups at the surface into the corresponding silyl ethers due to Si--O bond formation. The modified surface was characterized in situ by chemical sputtering, a low-energy ion-surface scattering experiment. These data indicate that the ion-surface reactions have high yields (i.e. surface reactants converted to products). Surface reactions with Si(OCH(3))(3) (+), followed by chemical sputtering using CF(3) (+), yielded the reagent ion, Si(OCH(3))(3) (+), and several of its fragments. Other sputtered ions, namely SiH(OCH(3))(2)OH(2) (+) and SiH(2)(OCH(3))OH(2) (+), contain the newly formed Si--O bond and provide direct evidence for the covalent modification reaction. Chemical sputtering of modified surfaces, performed using CF(3) (+), was evaluated over a range of collision energies. The results showed that the energy transferred to the sputtered ions, as measured by their extent of fragmentation in the scattered ion mass spectra, was essentially independent of the collision energy of the projectile, thus pointing to the occurrence of reactive sputtering.A set of silyl cations, including SiBr(3) (+), Si(C(2)H(3))(3) (+) and Si(CH(3))(2)F(+), were similarly used to modify the HO-SAM surface at low collision energies. A reaction mechanism consisting of direct electrophilic attack by the cationic projectiles is supported by evidence of increased reactivity for these reagent ions with increases in the calculated positive charge at the electron-deficient silicon atom of each of these cations. In a sequential set of reactions, 12 eV deuterated trimethylsilyl cations, Si(CD(3))(3) (+), were used first as the reagent ions to modify covalently a HO-SAM surface. Subsequently, 70 eV SiCl(3) (+) ions were used to modify the surface further. In addition to yielding sputtered ions of the modified surface, SiCl(3) (+) reacted with both modified and unmodified groups on the surface, giving rise not only to such scattered product ions as SiCl(2)OH(+) and SiCl(2)H(+), but also to SiCl(2)CD(3) (+) and SiCl(2)D(+). This result demonstrates that selective, multi-step reactions can be performed at a surface through low-energy ionic collisions. Such processes are potentially useful for the construction of novel surfaces from a monolayer substrate and for chemical patterning of surfaces with functional groups.
With the use of a multisector ion/surface scattering mass spectrometer, the benzoyl cation, C6H5CO+, is mass- and energy-selected and then made to collide at hyperthermal energies (i.e., <100 eV) with an HO-terminated self-assembled monolayer (HO−SAM) surface. Fourier transform infrared external reflectance spectroscopy (FTIR-ERS) indicates that this ion/surface collision results in C−O bond formation at the surface, producing the terminal benzoate. The covalent modification of the hydroxyl surface is further substantiated by subsequent ion/surface scattering experiments, in which 70-eV CF3 + ions are used as projectiles. Chemically sputtered ions resulting from the collision of the CF3 + ion with the ion-modified surface include the reagent ion, C6H5CO+, and its fragments, C6H5 + and C4H3 +. The same chemically sputtered ions are observed when a C6H5CO2-terminated self-assembled monolayer surface is similarly analyzed using CF3 +. Using collisions of CF3 + to analyze a series of mixed SAM surfaces (prepared from varying amounts of the HO-terminated disulfide and the C6H5CO2-terminated disulfide), calibration of the chemically sputtered products reveals that a 2-h modification with the benzoyl cation results in ca. 15% reaction yield (i.e., 15% of the surface groups are converted to products). The reaction efficiency (i.e., fraction of gas-phase cations converted to surface-bound products) is roughly estimated as 75%, the first ion/surface reaction efficiency to be reported. Analogous surface transformations are achieved using CH3CO+ and C6H5CH2 +.
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