Surface-oxidized Silicon Substrates Research Articles

Transfer Printing of Freestanding Nanoassemblies: A Route to Membrane Resonators with Adjustable Prestress.

Freestanding nanoassemblies represent a new class of functional materials with highly responsive optical, electrical, and mechanical properties. Hence, they are well-suited for applications in advanced sensor devices. Here, it is shown that transfer printing enables the well-controlled fabrication of freestanding membranes from layered nanoassemblies: Using a polydimethylsiloxane (PDMS) stamp, thin films (thickness: ∼45 to ∼51 nm) of 1,6-hexanedithiol cross-linked gold nanoparticles (diameter: ∼3.9 ± 0.8 nm) were transferred onto surface-oxidized silicon substrates featuring square microcavities with edge lengths of ∼78 μm. After adjusting the contact pressure to 1.8 bar, intact membranes were printed in yields of ∼70%. The prestress of printed membranes was determined by measuring their resonance frequencies under electrostatic actuation. In general, the prestress values were in the ∼10 MPa range with standard deviations below 10% for parallel printed resonators. The deviations in average prestress between resonators printed onto different substrates were 21% or less. By increasing the temperature during the final transfer step from 5 to 48 °C, it was possible to tune the average prestress from ∼14 to ∼28 MPa. This effect was attributed to the pronounced thermal expansion of the PDMS stamp. Finally, by transfer printing layered films of graphene oxide/silk fibroin (GO/SF), it is shown that the approach can be adapted for the fabrication of freestanding membranes from very different nanomaterials.

Influence of Nucleation Conditions on the Structure of Zinc Oxide Films

The temperature conditions of nucleation of ZnO films on surface-oxidized silicon substrates during magnetron sputtering of a cermet ZnO–Zn target in an Ar atmosphere and chemical vapor deposition in a hydrogen flow upon Zn and H2O vapor interaction have been investigated. It is shown that the rate of zinc desorption from a growing surface increases significantly and the film growth is suppressed at a substrate temperature above 450°C. It is revealed that two-step deposition with preliminary low-temperature sublayer formation is necessary for ZnO film formation at high temperatures.

Photothermal laser fabrication of micro- and nanostructured chemical templates for directed protein immobilization.

Photothermal patterning of poly(ethylene glycol) terminated organic monolayers on surface-oxidized silicon substrates is carried out using a microfocused beam of a CW laser operated at a wavelength of 532 nm. Trichlorosilane and trimethoxysilane precursors are used for coating. Monolayers from trimethoxysilane precursors show negligible unspecific protein adsorption in the background, i.e., provide platforms of superior protein repellency. Laser patterning results in decomposition of the monolayers and yields chemical templates for directed immobilization of proteins at predefined positions. Characterization is carried out via complementary analytical methods including fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. Appropriate labeling techniques (fluorescent markers and gold clusters) and substrates (native and thermally oxidized silicon substrates) are chosen in order to facilitate identification of protein adsorption and ensure high sensitivity and selectivity. Variation of the laser parameters at a 1/e(2) spot diameter of 2.8 μm allows for fabrication of protein binding domains with diameters on the micrometer and nanometer length scale. Minimum domain sizes are about 300 nm. In addition to unspecific protein adsorption on as-patterned monolayers, biotin-streptavidin coupling chemistry is exploited for specific protein binding. This approach represents a novel facile laser-based means for fabrication of protein micro- and nanopatterns. The routine is readily applicable to femtosecond laser processing of glass substrates for the fabrication of transparent templates.

Nonlinear femtosecond laser processing of alkylsiloxane monolayers on surface-oxidized silicon substrates

Femtosecond laser patterning of octadecylsiloxane monolayers on surface-oxidized silicon substrates via single-pulse processing at λ=800nm, τ<30fs, and ambient conditions has been investigated. Depending on the laser pulse fluence, local irradiation results in circular spots of distinct size and morphology. At high fluences, a particular rich complexity of distinct surface morphologies is observed including hole, rim, and ripple formation, and a faint boundary area where monolayer decomposition sets in. At low fluences, subwavelength patterning of the organic monolayer is feasible. In particular, at a 1∕e laser spot diameter of 1.3μm, surface spots with a width down to 300nm are fabricated. Selective processing of the organic monolayer, though, is restricted to a very narrow range of fluences between 1.1 and 1.2J∕cm2. A significantly larger parameter range for selective processing is anticipated in the case of functional monolayers that incorporate aromatic groups. Promising perspectives in femtosecond laser processing of organic monolayers are discussed.

Temporal stability of photothermally fabricated micropatterns in supported phospholipid multilayers

The temporal stability of photothermally fabricated micropatterns in multilayered films of 1,2-dioleoyl-sn-glycero-3-phosphate on surface-oxidized silicon substrates is investigated. Multilayered films are fabricated via spin coating of diluted phospholipid solutions. A focused beam of an Ar+ laser at λ=514nm and a 1∕e2 spot diameter of 2.5μm is used to locally remove the coating and fabricate micropatterns. Subsequently, the temporal stability of such patterns at ambient conditions in air and in water is examined using optical microscopy. Generally, these patterns are stable on a time scale of several hours to days, both in air and in water, and hence provide sufficient time for most follow-on experiments. The spin-coated phospholipid films, though, are intrinsically unstable in water. In particular, some hours after immersion in water, dewetting of the film starts to set in. The implications of these results for potential applications are discussed.

Photothermally induced bromination and decomposition of alkylsiloxane monolayers on surface-oxidized silicon substrates

Photothermal laser processing of alkylsiloxane monolayers in gaseous bromine is investigated. Surface-oxidized silicon samples are coated with octadecylsiloxane monolayers and locally irradiated with a focused beam of an Ar+-laser at λ=514 nm and a 1/e2 spot diameter of 3 μm. For characterization, atomic force microscopy, scanning electron microscopy, and optical microscopy in conjunction with labeling techniques and condensation experiments are used. At low laser powers, monolayer bromination in micron-sized areas is observed. Additionally, at high laser powers, decomposition of the monolayer takes place at the center of the brominated areas. Prospects and limitations of this procedure in fabrication of multifunctional templates are discussed.

Abnormal saturation magnetization dependency on W content for Co-W thin films

A series of hexagonal close-packed (HCP) Co-W thin films were deposited by sputtering on surface oxidized silicon substrates at 300 ◦C. A linear dependency of saturation magnetization (Ms) on W content was found up to about 9 at. pct W, and then it underwent an increased Ms vs. W content curve as compared to the initial linear dependency. The thermal magnetization technique was used to confirm that the increased Ms behaviour is correlated to the phase separation of the Co-W thin films. The phase separation behaviour was also found to be dependent on W content and the reason was discussed in detail. Finally an interesting composition range was suggested to be about 13 at. pct–17 at. pct W for the Co-W thin films, in which they exhibit much higher magnetic anisotropy energy than Co-Cr thin films and improved phase separation.

Photothermally Induced Microchemical Functionalization of Organic Monolayers

A prototype photothermal procedure: Alkylsiloxane monolayers on surface-oxidized silicon substrates are processed in gaseous bromine using a focused Ar+-laser operated at λ=514 nm. Monolayer bromination remains confined to a micrometer-sized reaction zone, where laser irradiation results in a local temperature rise (see figure).

Laser-induced local dehydroxylation on surface-oxidized silicon substrates: mechanistic aspects and prospects in nanofabrication

Processing of silicon oxide surfaces with a focused laser beam at a wavelength of 514 nm has been investigated. Laser processing of native silicon samples (dox=1–2 nm) allows the fabrication of reactive templates with laterally varying hydroxyl group density. Very similar results were also obtained on thermally oxidized silicon samples (dox≈100 nm), whereas respective experiments on quartz plates have failed. These results support a photothermal mechanism where laser irradiation causes a local temperature rise and initiates dehydroxylation. In agreement with a photothermally induced dehydroxylation reaction, a thermokinetic analysis of the experimental data points to a highly activated process. In conjunction with site-selective functionalization routines this opens up an avenue towards functional surface structures with lateral dimensions significantly below the optical diffraction limit.

Control of the Structure of Self-Spreading Lipid Membrane by Changing Electrolyte Concentration

We have controlled the structure of self-spreading lipid bilayer membranes prepared on surface-oxidized silicon substrates by changing electrolyte concentration. Analysis of the fluorescence intensity, considering the optical interference effect, clarified the stacking structure of the lipid membrane. By varying the electrolyte concentration, we can vary the number of single multilamellar lobes adsorbed on the underlying self-spreading bilayer. This dependence of the stacking ability on the electrolyte concentration was investigated on the basis of changes in the bilayer-lobe interaction energies, including van der Waals, electrostatic double layer, and hydration interaction energies. Theoretical estimation suggests that the observed electrolyte concentration dependence can be explained by the combination of the van der Waals attractive interaction energy and the repulsive double-layer interaction energy.

Relation between Particle Growth in Solution and Composition of Mixed Titania/Vanadium Oxide Films: Implications for Chemical Bath Deposition

Composite titania/vanadium oxide thin films were prepared by chemical bath deposition. The reaction solution consisted of aqueous solutions of ammonium vanadate and a titanium peroxo complex. Deposition could be achieved on surface-oxidized silicon substrates at reaction temperatures of 333 K. The films consisted of titania as well as vanadium(IV) and (V) oxide nanoparticles. For the elucidation of the deposition mechanism the reaction solutions were investigated by dynamic light scattering to monitor the particle agglomeration in the reaction solution and UV/Vis spectroscopy to gain information about the change in chemical composition. The deposition mechanism was discussed in terms of attachment of colloidal particles. Thereby an optimum size exists for the colloids in which deposition can take place, whereas below no deposition occurs and above only the sedimentation of larger agglomerates on the surface is possible. This kinetic control leads to rather uniform films with compositions in a narrow range.

Direct laser patterning of octadecylsiloxane monolayers on surface-oxidized silicon substrates: Indications for a photothermal excitation mechanism

A focused laser beam at a wavelength of 514 nm has been used to pattern octadecylsiloxane monolayers grown on distinct substrates which expose a silicon oxide surface. Patterning experiments on conventional silicon substrates with a native oxide layer result in a decomposition of the monolayer along well-confined lines. The line widths generally are well below the focal laser spot diameter of 2.5 ± 0.3 μm indicating a nonlinear dependence of the overall process on the laser intensity. Very similar results were obtained using thermally oxidized silicon substrates. In contrast, experiments to pattern coated quartz plates failed even if the laser power and hence the local irradiance at the surface was raised to much higher values. Altogether, these experiments strongly support a photothermal excitation mechanism, that is, a decomposition of the monolayer as a result of the laser induced local temperature rise.

Laser-assisted decomposition of alkylsiloxane monolayers at ambient conditions: rapid patterning below the diffraction limit

Rapid patterning of octadecylsiloxane monolayers at ambient conditions is demonstrated using a focused laser beam at a wavelength of 514 nm. Surface-oxidized silicon substrates have been coated and subsequently processed at distinct laser powers over a wide range of writing speeds up to 25 mm/s. The method allows for a well-confined local decomposition of the monolayer with an unexpectedly high lateral resolution which is significantly below the diffraction limited laser spot diameter of the optical setup. In particular, at a 1/e2 focal spot diameter of about 2.5 μm line widths close to 200 nm are reached. Complementary experiments at a spot diameter of about 1.2 μm yielded irregular lines with a minimum width close to 100 nm. The underlying highly superlinear dependence of the patterning process on the laser intensity is attributed to the interplay between the laser-induced local temperature rise and the thermally activated decomposition of the organic coating. A simple thermokinetic analysis of the data allows one to estimate effective kinetic parameters of the decomposition process and reproduce the experimentally observed functional dependence of the line width on the incident laser power and the writing speed.

Laser-assisted fabrication of submicron-structured hydrophilic/ hydrophobic templates for the directed self-assembly of alkylsiloxane monolayers into confined domains

A highly focused laser beam at a wavelength of 514 nm is used to pattern hydrophilic, surface-oxidized silicon substrates. Irradiation converts the affected surface areas from hydrophilic to hydrophobic and hence provides a means to prepare hydrophilic/hydrophobic patterns. The patterns are stable for weeks or longer, even if the samples are stored at ambient conditions. Contrary to ordinary avenues to hydrophilic/hydrophobic patterns no coating is required. This makes the procedure extremely simple and robust. Routine patterning can be carried out over large areas at fast writing speeds under ambient conditions. At a focal spot diameter of about 2.5 μm, hydrophobic lines with a width down to 0.4 μm are prepared indicating a highly superlinear dependence of the overall process on the laser intensity. The patterned substrates might be useful in a broad variety of applications. In particular, as shown here, they represent suitable templates for the directed self-assembly of organic monolayers.

Deposition of Composite Titania/Vanadia Thin Films by Chemical Bath Deposition.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Deposition of Composite Titania/Vanadia Thin Films by Chemical Bath Deposition

Composite titania/vanadia thin films were prepared on sulfonate-terminated self-assembled monolayers and surface-oxidized silicon substrates by chemical bath deposition. Films consist of homogeneously distributed titania and vanadia nanoparticles. No concentration gradient of these components was observed.

Infrared absorption study of CO chemisorption on copper island films

The infrared absorption spectra of chemisorbed CO at 90 K on copper island films deposited by electron-beam evaporation onto surface-oxidized silicon substrates have been measured at normal incidence of radiation. The integrated absorption intensity of the C–O stretch mode at saturation of CO adsorption depends on both the mass thickness of copper and the substrate temperature during copper deposition. The observed CO absorption features are quite different from those observed on bulk single crystals and thick polycrystalline films. The FE-SEM images reveal that at a given mass thickness the islands are larger when deposited at 296 K than when deposited at 473 K. This difference is reflected in the mass thickness dependence of the absorption intensity; the maximum absorption intensity is obtained at a thickness of 9 nm when deposited at 296 K, whereas the corresponding thickness is 16 nm when deposited at 473 K. A feature common to both films is partial aggregation of the islands. The integrated absorption intensity of the C–O stretch mode observed on the 16 nm film is 5 times larger than that observed on the bulk Cu(1 0 0) surface. This absorption enhancement is attributed to the transverse collective electron resonance of the copper islands.