Silicon surfaces, self-assembled monolayers

Table 3 summarizes examples of organic surface treatments used for changing the surface electronic properties. As a first example we consider self-assembly of organo-silanes on oxidized silicon. These self-assembled monolayers are known to be stable and well ordered [63, 64]. Modification of the monolayer properties can be achieved in a predetermined fashion by changing the chemical structure of the... 

Figure Bl.22.3. RAIRS data in the C-H stretching region from two different self-assembled monolayers, namely, from a monolayer of dioctadecyldisulfide (ODS) on gold (bottom), and from a monolayer of octadecyltrichlorosilane (OTS) on silicon (top). Although the RAIRS surface selection rules for non-metallic substrates are more complex than those which apply to metals, they can still be used to detemiine adsorption geometries. The spectra shown here were, in fact, analysed to yield the tilt (a) and twist (p) angles of the molecular chains in each case with respect to the surface plane (the resulting values are also given in the figure) [40].

The first surface-initiated enzymatic polymerization reported was the synthesis of amylose brushes on planar and spherical surfaces [145]. For this, silica or silicone surfaces were modified with self-assembled monolayers of (3 amino-propyl)trimethoxysilane or chlorodimethylsilane, respectively. To these functionalities, oligosaccharides were added via (a) reductive amidation of the oligosaccharides to surface-bound amines, (b) conversion of the oiigosaccharide to the according aldonic acid lactone and reaction with surface bound amines, and (c) incorporation... 

In addition to Grignard chemistry and hydrogermylation, for which analogs also exist for silicon, there is another type of functionalization chemistry that has been carried out on germanium surfaces that of alkanethiol attachment to H-terminated Ge. Alkanethiols are well known for their use in self-assembled monolayers (SAMs) on gold surfaces. 

Effenberger, F., Gotz, G., Bidlingmaier, B. and Wezstein, M. Photoactivated preparation and patterning of self-assembled monolayers with 1-alkenes and aldehydes on silicon hydride surfaces. Angewandte Chemie-International Edition 37, 2462 (1998). 

The left picture shows aggregates of silicon oxide particles with a diameter of 0.9 pm (see example 1.1). At the bottom an atomic force microscope image of cylindrical CTAB micelles adsorbed to gold(lll) is shown (see example 12.3, width 200 nm). The right image was also obtained by atomic force microscopy. It shows the surface of a self-assembled monolayer of long-chain alkylthiols on gold(lll) (see fig. 10.2, width 3.2 nm). 

In the 1940s, it was demonstrated in the pioneering work of Zisman and coworkers [8] that the LB technique is not the only way to create an organized organic monolayer on a solid substrate. It was demonstrated that when a compatible substrate is exposed to a solution of an amphiphilic compound, the dissolved molecules form a self-assembled monolayer on the substrate surface. Such films maintain their structural integrity after they are removed from solution. The most common examples of such films are organosulfur films on gold substrates [9] and alkyltrichlorosilane films on silicon dioxide substrates [10]. Compared with the LB films, the self-assembled films are somewhat less ordered. On the other hand, these films are easier to prepare, since they do not require special instrumentation and can easily be deposited on both planar and non-planar substrates. Also, in many cases the amphiphilic molecules which make the self-assembled film are chemisorbed on the substrate. Such films are more stable when heated or exposed to solvents than are typical LB films, which are held to the substrate by non-covalent interactions. 

The higher order alkylchlorosilanes (C8 and C18) have historically been treated in the same way as organosilanes. The reaction inevitably occurs in the liquid phase and is usually followed by a curing step. The extremely low surface of the silicon wafers and the deposited Si02 layers, used for self-assembled-monolayers does not allow a spectroscopic quantification of the surface species. A completely different type of analysis techniques is used here mainly to determine the quality (roughness and uniformity), the adherence (parallel or at random) and the hydrofobicity of the coated layer. 

Qnclin S Ravoo BJ Reinhoudt DN, Engineering silicon oxide surfaces using self-assembled monolayers, Angew. Chem. Int. Ed., 2005, 44, 6282-6304. 

Bottom-gate, top-contact (Fig. 4.2a) and a bottom-gate, bottom-contact (Fig. 4.2b) TFT configurations are used to evaluate the FET performance of our semiconductors. The devices are built on an n-doped silicon wafer (gate electrode) with a 100-nm thermal silicon oxide (SiC>2) dielectric layer which is modified with a self-assembled monolayer of octyltrichlorosilane (OTS-8) to promote molecular ordering in the semiconductor layer. For the top-contact device the semiconductor layer ( 20-50 nm) is deposited on the OTS-8-modified SiC>2 surface by spin coating. A... 

Another approach in generating molecular insulating layers without the need of chemical conversion after deposition is the use of preliminarily modified molecules which can form dense self-assembled monolayers. To create dense self-assembled monolayers with sufficient robustness and insulating properties, a modified alkyltrichlorosilane with an aromatic end-group (18-phenoxyoctadecyl)tri-chlorosilane (PhO-OTS chemical structure Fig. 6.15a) was synthesized and tested [50]. The SAMs were created in a one-step process from vapor phase or solution. On self-assembly on a natively oxidized silicon surface the n-n interaction between the phenoxy end-groups of adjacent molecules creates an intermolecular top-link, leading to a more closely packed surface compared to monolayer than when linear end groups are used. 

Fig. 10.19. Steps for nanotransfer printing a thin layer of Au on to a silicon wafer using a self-assembled monolayer (SAM) surface chemistry. Plasma oxidizing the surface of the wafer generates OH groups. Solution or vapor phase exposure of the wafer to 3-mercaptopropyltrimethoxysilane yields a SAM with exposed thiol groups. Contacting...

The grafting to method was used to anchor polymer chains onto the surface of silica particles and silicon wafers (Scheme 1). The synthetic procedure starts with covalent grafting of GPS to the surface. A self-assembled monolayer of GPS on silicon wafer surfaces was prepared according to the procedure suggested by Luzinov [32], For this, Si wafers were immersed in a 1% GPS toluene solution for 15 h under dry Ar atmosphere (< 1 ppm H2O). After treatment, GPS modified wafers were washed 3 times in dry toluene under dry Ar atmosphere to avoid polymerisation of non-grafted GPS and precipitation of particles. Afterwards, the silanized wafers were washed 2 times with ethanol in ultrasonic bath for 5 min followed by drying with nitrogen flux. The thickness of the GPS layer was determined by null ellipsometiy. 

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