Hydrogen-terminated Silicon Substrates Research Articles

1,2-Epoxyalkane: Another Precursor for Fabricating Alkoxy Self-Assembled Monolayers on Hydrogen-Terminated Si(111).

This work describes the UV alkoxylation of a series of 1,2-epoxyalkanes on the hydrogen-terminated silicon (H-Si) substrate. The formation of alkoxy self-assembled monolayers (SAMs) and the nature of bonding at the surface of H-Si were examined using water contact angle goniometer,spectroscopic ellipsometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy. UV exposure to 1,2-epoxyalkane mesitylene solution for 60 min formed alkoxy-SAMs onto H-Si with hydrophobic properties. The local molecular environment of the alkyl chains transitioned from a disordered, liquid-like state to an ordered, crystalline-like structure with increasing thechain length. XPS and FTIR indicated that the reaction of H-Si with 1,2-epoxyalkane produced Si-O-C linkages.TheSi-H bond homolysis and electron/hole were the plausible mechanistic routes forthe grafting of 1,2-epoxyalkanes.

Vacuum Ultraviolet Treatment of Acid- and Ester-Terminated Self-Assembled Monolayers: Chemical Conversions and Friction Reduction.

We have prepared COOH- and COOCH3-terminated self-assembled monolayers (SAMs) from undec-10-enoic acid (UDA) and methyl undec-10-enoate (MUDO) molecules on hydrogen-terminated silicon (H-Si) substrates through ultraviolet (UV) irradiation. The as-prepared UDA- and MUDO-SAMs were exposed to 172 nm vacuum-UV (VUV) light in a high vacuum environment (HV, <10-3 Pa) for different periods. The presence of COO components at the surfaces of these SAMs without prior oxidation would simplify the understanding of the origin of the chemical conversions and the changes of surface properties, as the prior oxidation would change the surface properties and generate different oxygenated groups. After the HV-VUV treatment, the abundance of COOH and COOCH3 components of these SAMs decreased without significant dissociation of their C-C backbones. Degradation of these components occurred through dissociating their C-O bonds, resulting in different C═O components. Also, the occurrence of Norrish type pathways resulted in a slight decrease of carbon content and produced CH3 components. We have applied the HV-VUV lithography to control the abundance of COOH and COOCH3 components in well-defined areas and to investigate the friction differences between the irradiated and masked areas. The irradiated areas exhibited lower friction than the masked areas without observing significant height contrasts between these areas. The reduction in friction was attributed to the conversion of the COOH and COOCH3 components to less adhesive components such as C═O and CH3. These experiments suggest the HV-VUV treatments as an approach for low damage dry surface modifications and reductive lithographic techniques at surfaces terminated by acid and ester groups.

Immobilization of Reduced Graphene Oxide on Hydrogen-Terminated Silicon Substrate as a Transparent Conductive Protector.

Silicon is a promising electrode material for photoelectrochemical and photocatalytic reactions. However, the chemically active surface of silicon will be easily oxidized when exposed to the oxidation environment. We immobilized graphene oxide (GO) onto hydrogen-terminated silicon (H-Si) and reduced it through ultraviolet (UV) and vacuum-ultraviolet (VUV) irradiation. This acted as an ultrathin conductive layer to protect H-Si from oxidation. The elemental evolution of GO was studied by X-ray photoelectron spectroscopy, and it was found that GO was partially reduced soon after the deposition onto H-Si and further reduced after UV or VUV light irradiation. The VUV photoreduction demonstrated ca. 100 times higher efficiency compared to the UV reduction based on the irradiation dose. The saturated oxygen-to-carbon ratio (RO/C) of the reduced graphene oxide (rGO) was 0.21 ± 0.01, which is lower than the photoreduction of GO on SiO2 substrate. This indicated the H-Si played an important role in assisting the photoreduction of GO. No obvious exfoliation of rGO was observed after sonicating the rGO-covered H-Si sample in water, which indicated rGO was immobilized on H-Si. The electrical conductivity of H-Si surface was maintained in the rGO-covered region while the exposed H-Si region became insulating, which was observed by conductive atomic force microscopy. The rGO was verified capable to protect the active H-Si against the oxidation under an ambient environment.

Low Temperature, Selective Atomic Layer Deposition of Cobalt Metal Films Using Bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and Alkylamine Precursors

The atomic layer deposition (ALD) of cobalt metal films is described using the precursor bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and tert-butylamine or diethylamine. Platinum, copper, ruthenium, Si(100) with native oxide, thermal SiO2, hydrogen-terminated silicon, and carbon-doped oxide substrates were used with growth temperatures between 160 and 220 °C. Plots of growth rate versus pulse lengths showed saturative, self-limited behavior at ≥3.0 s for bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and ≥0.1 s for tert-butylamine. An ALD window was observed between 170 and 200 °C, with a growth rate of 0.98 A/cycle on platinum substrates. A plot of thickness versus the number of cycles at 200 °C on platinum substrates was linear between 25 and 1000 cycles, with a growth rate of 0.98 A/cycle. A 98 nm thick film grown at 200 °C showed crystalline cobalt metal by X-ray diffraction. Atomic force microscopy of 10 and 98 nm thick cobalt metal films grown on platinum substrates at 200 °C showed rms roughness val...

Polymer brushes on hydrogen-terminated silicon substrates via stable Si[sbnd]C bond

We demonstrate a straightforward and efficient method for the creation of polymer brushes on hydrogen-terminated silicon substrates through the UV-induced photopolymerization. The surface grafting polymerization is applicable to a series of monomers, allowing the direct formation of homogeneous polymer coatings ranging from hydrophilic poly(2-isopropenyl-2-oxazoline) (PIPOx), amphiphilic poly(N-isopropyl acrylamide) (PNIPAM), to hydrophobic polystyrene (PS) and poly(4-(1H,1H,2H,2H-perfluorohexyl)oxymethylstyrene) (PPHMS) on Si(100) and Si(111) surfaces via stable SiC bonds. Polymerization kinetic investigation indicates a linear increase of polymer layer thickness with the polymerization time. Moreover, the as-prepared polymer brushes exhibit superior stability against basic conditions in contrast to those that were formed on silicon substrates via conventional SiOC bond.

Vacuum ultraviolet trimming of oxygenated functional groups from oxidized self-assembled hexadecyl monolayers in an evacuated environment

Vacuum ultraviolet light irradiation in dry air generates active oxygen species, which have powerful oxidation abilities. These active oxygen species (O) can oxidize the alkyl moieties of polymers, and generate new oxygenated groups such as OH, CHO and COOH groups. Reducing the oxygen content in the exposure environment decreases the rate of oxidation processes. In this study, we examined the influences of the 172nm VUV irradiation in a high vacuum (HV,<10−3Pa) environment on the chemical constituents, surface properties and morphological structure of well-defined VUV/(O)-modified hexadecyl (HD-) self-assembled monolayer (SAM) prepared on hydrogen-terminated silicon (H-Si) substrate. After VUV light irradiation in a HV environment (HV-VUV), the chemical constituents and surface properties were changed in two distinct stages. At short irradiation time (the first stage), the CO and COO groups decreased rapidly, while the CO groups slightly changed. The dissociation of nonderivatizable groups (such as ether (COC) and ester (CCOOC) groups) compensated the dissociated OH, CHO, CCOC and COOH groups. With further irradiation (the second stage), the quantities of the oxygenated groups slightly decreased. The carbon skeleton (CC) of SAM was scarcely dissociated during the HV-VUV treatment. These chemical changes affected the surface properties, such as wettability and morphology.

Photochemical Preparation of Alkoxy Self-assembled Monolayers on Si from 1,2-Epoxyalkane Molecules

We have immobilized 1,2-epoxydodecane (EDD) on hydrogen-terminated silicon (H-Si) substrates by UV photochemical grafting. The surface properties of the fabricated samples showed the formation of h...

Superlubricity-activated thinning of graphite flakes compressed by passivated crystalline silicon substrates for graphene exfoliation

A special thinning phenomenon is observed through molecular dynamics, where compression of AB-stacked graphite flakes between two hydrogen-terminated silicon substrates leads to the exfoliation of graphene layers. We have used multiple molecular dynamics simulations to study how this thinning phenomenon is affected by parameters such as size, number of graphene layers, and the crystalline orientation of the substrate surface. It is shown that this thinning phenomenon occurs through the activation of an inter-layer superlubricity regime, caused by torque-induced spontaneous rotations of the layers which are initiated by in-plane shear modes of graphite during compression.

Functionalization of Azide-Terminated Silicon Surfaces with Glycans Using Click Chemistry: XPS and FTIR Study

Efficient functionalization of silicon substrates is important for the development of silicon-based sensors. Organic monolayers directly bonded to hydrogen-terminated silicon substrates via Si–C bonds display enhanced stability toward hydrolytic cleavage. Here, we show that monolayers presenting a high density of terminal azide groups are amenable to bioconjugation with alkynyl-derivatized glycans via a copper-catalyzed azide–alkyne 1,3-dipolar cycloaddition. The prerequisite azide-functionalized silicon surface is fabricated via hydrosilylation of undecylenic acid with hydrogen-terminated silicon substrate followed by reaction of the thus formed monolayer of acid groups with short, bifunctional oligoethylene oxide chains carrying an amine function at one terminus and an azido group at the other. The possibility to functionalize these azido-surfaces with alkynyl-derivatized glycans such as propargyl mannose through a click protocol is demonstrated and evidenced using X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. In addition, the interaction of these mannose-adorned silicon substrates with glycan binding proteins such as Lens culinaris lectin is investigated. The data establishes clearly the specificity of the interaction of this newly fabricated silicon surface for mannose-selective proteins as well as its reusability, thereby demonstrating its potential as a sensor.

Controlled grafting of cationic poly[(ar-vinylbenzyl)trimethylammonium chloride] on hydrogen-terminated silicon substrate by surface-initiated RAFT polymerization

Controlled grafting of well-defined cationic poly[(ar-vinylbenzyl)trimethylammonium chloride] [poly(VBTAC)] brushes on a hydrogen-terminated silicon substrate (SiH) was performed via surface-initiated RAFT polymerization. The azo-initiator was immobilized on the SiH surface via a three step process involving (i) coupling of the t-butyloxycarbonyl (t-BOC) protected allylamine to the SiH surface under UV irradiation, (ii) conversion of the t-BOC protected allylamine groups into the free amine groups by trifluoroacetic acid, and (iii) the amide reaction of allylamine with the 4,4′-azobis(4-cyanopentanoyl chloride) initiator. The living polymerization produced silicon substrate coated with well-defined cationic poly(VBTAC) with a target molecular weight and a grafting density as high as 0.93chains/nm2.

Lithographically patterned silicon nanostructures on silicon substrates

The paper reports on controlled formation of silicon nanostructures patterns by the combination of optical lithography and metal-assisted chemical dissolution of crystalline silicon. First, a 20nm-thick gold film was deposited onto hydrogen-terminated silicon substrate by thermal evaporation. Gold patterns (50μm×50μm spaced by 20μm) were transferred onto the silicon wafer by means of photolithography. The etching process of crystalline silicon in HF/AgNO3 aqueous solution was studied as a function of the silicon resistivity, etching time and temperature. Controlled formation of silicon nanowire arrays in the unprotected areas was demonstrated for highly resistive silicon substrate, while silicon etching was observed on both gold protected and unprotected areas for moderately doped silicon. The resulting layers were characterized using scanning electron microscopy (SEM).

Gold nanoparticle deposition on Si by destabilising gold colloid with HF

Gold nanoparticles from commercially available colloids were deposited onto a hydrogen-terminated silicon substrate without the use of a polyelectrolyte linker by the addition of HF acid. The deposition density was shown to be controlled over three orders of magnitude by varying the colloid concentration, and finer control is achieved by varying the deposition time. In order to minimise agglomeration, however, we show that deposition times should be minimised since nanoparticle agglomeration increases rapidly over the first 2min after the addition of HF. To increase nanoparticle density without increasing agglomeration, we show that successive depositions of short times linearly increase the deposition density without increasing the agglomeration of nanoparticles.

Biofunctionalization on alkylated silicon substrate surfaces via "click" chemistry.

Biofunctionalization of silicon substrates is important to the development of silicon-based biosensors and devices. Compared to conventional organosiloxane films on silicon oxide intermediate layers, organic monolayers directly bound to the nonoxidized silicon substrates via Si-C bonds enhance the sensitivity of detection and the stability against hydrolytic cleavage. Such monolayers presenting a high density of terminal alkynyl groups for bioconjugation via copper-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC, a "click" reaction) were reported. However, yields of the CuAAC reactions on these monolayer platforms were low. Also, the nonspecific adsorption of proteins on the resultant surfaces remained a major obstacle for many potential biological applications. Herein, we report a new type of "clickable" monolayers grown by selective, photoactivated surface hydrosilylation of α,ω-alkenynes, where the alkynyl terminal is protected with a trimethylgermanyl (TMG) group, on hydrogen-terminated silicon substrates. The TMG groups on the film are readily removed in aqueous solutions in the presence of Cu(I). Significantly, the degermanylation and the subsequent CuAAC reaction with various azides could be combined into a single step in good yields. Thus, oligo(ethylene glycol) (OEG) with an azido tag was attached to the TMG-alkyne surfaces, leading to OEG-terminated surfaces that reduced the nonspecific adsorption of protein (fibrinogen) by >98%. The CuAAC reaction could be performed in microarray format to generate arrays of mannose and biotin with varied densities on the protein-resistant OEG background. We also demonstrated that the monolayer platform could be functionalized with mannose for highly specific capturing of living targets (Escherichia coli expressing fimbriae) onto the silicon substrates.

Surface characterization of quinhydrone-methanol and iodine-methanol passivated silicon substrates using X-ray photoelectron spectroscopy

Hydrogen-terminated silicon substrates were passivated with quinhydrone-methanol (QHY/ME) and iodine-methanol (I 2 /ME), and the chemical changes occurring at the surface were investigated using X-ray photoelectron spectroscopy (XPS). The XPS surface studies demonstrate that QHY/ME passivation provides reduced oxidation, less carbon contamination, and a chemically inert surface. Electrical characterization also demonstrates higher minority carrier lifetimes of QHY/ME passivated substrates as compared to I 2 /ME passivated substrates. The quality of surface treatment was also characterized using the contact angle measurement, which confirms the presence of a hydrophobic organic layer on the surface after QHY/ME passivation. Si 2p XPS spectra of the QHY/ME, I 2 /ME samples, and H-terminated silicon. With a peak at 102.9 eV for I 2 /ME, it is evident that it provides poor surface passivation than QHY/ME where no surface oxidation is observed.

Directing the Formation of Nanostructured Rings via Local Oxidation

We provide compelling evidence that ring formation in solutions of thiol-passivated Au nanoparticles is driven by breath figure dynamics. A method for the controlled placement of rings of nanoparticles on a solid substrate, which exploits variations in substrate wettability to fix the positions of the submicrometer water droplets formed in the breath figure process, has been developed. This is achieved by heterogeneously patterning hydrogen-terminated silicon substrates with oxide regions that act as adsorption sites for the droplets. The droplets in turn template the formation of thiol-passivated Au nanoparticle rings during spin-casting from volatile solvents.

Synthesis of theophylline derivatives and study of their activity as antagonists at adenosine receptors

The synthesis of oligo(ethylene glycol)-alkene substituted theophyllines in positions 7 and/or 8 is described. The binding activity at adenosine receptors of selected derivatives was studied. Compound 2 showed high affinity for human A 2B receptor ( K i = 4.16 nM) with a selectivity K iA2A/ K iA2B of 24.1, and a solubility in water of 1 mM. The alkenyl substituent in some of the theophylline derivatives allows for covalent attachment of them onto hydrogen-terminated silicon substrate surfaces via hydrosilylation. Alternatively, an azido group was incorporated to an oligo(ethylene glycol)theophylline derivative as an anchor for tethering the molecules on ethynyl presenting surfaces via click reaction.

Atomic Layer Deposition of Antimony Oxide on Hydrogen-terminated Silicon Substrates

Atomic layer deposition thin antimony oxide films on silicon substrates was investigated. Growth of small crystallites could be obtained by using triisopropylantimony (SbiPr3) and water. The deposition of thin continuous films succeeded after employing ozone and SbiPr3. The growth rate was significantly higher for the higher deposition temperature: 0.16 nm/cycle at 300 {degree sign}C compared to 0.06 nm/cycle at 200 {degree sign}C. X-ray photoelectron spectroscopy detected no impurities in the film other than adventitious carbon at surface. A composition ratio of antimony to oxide of 27/73 was found, suggesting the occurrence of the pentavalent oxide Sb2O5. The oxide is prone to sputter reduction during depth profiling. A chemical shift in the binding energy of the Sb photoemission peaks from the oxide to the Sb0 state clearly arises already after gentle argon sputtering. At temperatures up to 40 {degree sign}C the as-deposited films showed remarkably resistance to buffered oxide etch and nitric acid.

Atomic Layer Deposition of Antimony Oxide on Hydrogen-terminated Silicon Substrates

not Available.

Effects of Low Temperature O2 Treatment on the Electrical Characteristics of Amorphous LaAlO3 Films by Atomic Layer Deposition

Amorphous LaAlO3 films were deposited on hydrogen-terminated silicon substrates by atomic layer deposition (ALD) at 300 oC. The precursors were lanthanum tris(N,N'-diisopropylformamidinate), trimethylaluminum (TMA) and water. Capacitance-voltage measurements made on ALD MoN/LaAlO3/Si stacks showed humps especially at low frequencies. They were effectively removed by O2 treatment at 300 o C without affecting the dielectric constant (κ~15). The O2 treatment can be carried out either after deposition of a LaAlO3 film, or after each ALD cycle. The O2 treatment also lowered the leakage current from 80 mA cm-2 to 1 mA cm-2 for EOT = 1.3 nm. This indicates that oxygen vacancies are the main defects in as-deposited LaAlO3. Oxygen treated LaAlO3 is one of the best candidates for future high-κ dielectric material due to its low leakage, low defect density and abrupt interface with silicon.

Nanostructured ultra-low-κ porous fluoropolymer composite films via plasma co-polymerization of hydrophobic and hydrophilic monomers and subsequent hydrolysis treatment

A new approach to fabricate porous nanostructured fluoropolymer composite films with a low dielectric constant (κ) was put forward at the first time. Initially, a film (pp-HDFD–PEGMA film) composed of dense, uniform, and well-defined nanospheres was controllably deposited on hydrogen-terminated silicon substrates by simultaneous plasma polymerization and deposition of a fluorine-containing hydrophobic monomer, 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluoro-1-decene (HDFD), and a hydrophilic monomer poly(ethylene glycol) methacrylate (PEGMA), using a pulsed plasma polymerization technique. Then, by hydrolysing the plasma co-deposited film in aqueous hydrochloric acid solution to effectively remove the soluble nanospheres or fragments which mainly derived from PEGMA, a nanoporous fluorocarbon film was achieved. Subsequently, a top poly(tetrafluoroethylene) layer was deposited via the magnetron sputtering process to cap and complete an encapsulated structure. The resulting bilayer composite film consisting of a layer of nanostructured fluorocarbon porous film and a layer of encapsulation fluorocarbon polymer has a κ value of 1.8. The morphology investigation of the plasma co-deposited film prior and after acid-treatment by field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) confirmed the form of the nanospheres and nanoporous structure, while the chemical composition and structure analysis by X-ray photoelectron spectroscopy (XPS) revealed that after the acid-treatment, the porous nanostructured film are composed predominantly of mainly fluorocarbon polymer.