We have been interested in fabricating structures which can have dimensions on the scale of molecules (˜10 Å). By producing lithographic structures on the molecular-scale length, we obtain a new class of materials with unusual chemical properties. The materials contain pores which have been shown to restrict the mobility and adsorption of molecules. This restriction is the basis of size- and shape-selective chemistry which is used in many modern separation and catalytic processes.Features in our materials are significantly smaller than the smallest nano-structures produced with electron beam lithography. To date 20–50 Å features have been produced with electron beam lithography which uses ionization from beam-solid interactions to drive radiation chemistry in a resist layer. These dimensions are near fundamental limits which come from beam-solid interactions and the response of the resist system. Further limitations occur when the nanostructural pattern must be transferred from the resist layer into a useful metal or semiconductor structure. Methods have not been devised to fully exploit the accuracy with which resist patterns can be defined, and the smallest useful transferred structures have dimensions of ˜80–100 Å.To produce the smallest of all lithographic structures, we use a new method for pattern generation. Instead of using radiation chemistry to define a pattern, we use the organization of layers in deposited superlattices.

Lithographic techniques have been used to prepare transmission electron microscopy samples of MoS2, allowing examination of the edge surface with single-layer resolution. We observe that these surfaces are easily disordered by chemical treatments common in the catalysis industry. In some cases treatment in H2/H2S leads to an exfoliation of the layered structure, a process which can be observed as it occurs in the microscope.

The structural and electronic properties of a new class of superlattices consisting of layers of a-Si:H 8–1200 thick interleaved with a-Ge:H, a-Si1−xCx:H or a-SiNx:H are reviewed.

Optical absorption in crystalline MoS2 has been measured from 0.7 to 3.5 eV using photothermal deflection and transmission spectroscopies. We have determined the indirect band gap in both natural and synthetic material. Photon energies below the band gap show defect absorption which is strongly concentrated at the edge surfaces. These surface defects have the same optical properties in microcrystals and lithographically textured single crystals.

Using a new room temperature method for the synthesis of alloys, the oxidation of metal polyanions with transition metal cations in solution, it is possible to vary smoothly and continuously the electrical resistivity over nine orders of magnitude. This method of alloy synthesis consists of the high temperature reaction of elements to form Zintl phase type materials, extraction of metal polyanions from the Zintl phase followed by oxidation of the metal polyanions by a metal cation in solution to give amorphous alloys. In the systems M2SnTe4 (M= Mn, Fe, Co, Cu) and Co2−xMnxSnTe4 both the resistivity and optical bandgap of the alloy appear to correlate with the degree of electron transfer from anion to cation in the alloy, i.e. the electronegativity difference between the metal anion and metal cation.

Lithographic techniques were used to expose edge surfaces in layered molybdenum disulfide single crystals. This microstructuring produced ideal samples for the study of the surface morphology and electronic structure of this catalytically important material. The optical absorption that was measured at mid-gap increased by two orders of magnitude after texturing. This increase resulted from reduced molybdenum at surface defects that are located on edge planes, as shown by photoemission spectroscopy. This information cannot easily be obtained on conventional crystals with predominantly basal plane surfaces.

Microfabrication techniques have been used to prepare a new class of molecular scale microporous materials. These materials are formed by chemically etching slots into alternate layers of a lithographically exposed amorphous superlattice cross section. The slot width is accurately controllable from 10 Å to more than 500 Å.

The optical and catalytic properties of molybdenum sulfide edge planes have been studied in single crystals and disordered powders. We observe an infrared optical absorption due to the edge planes in the single crystal platelets. This absorption is proportional to the hydrodesulfurization catalytic activity in unsupported powders which were annealed at various temperatures to change their degree of crystallinity. This provides a direct correlation between the electronic structure of these materials as measured by optical absorption and their catalytic activity. This result shows that the optical and catalytic properties of the edge sites are similar in the amorphous and crystalline materials and we derive turnover numbers and absorption cross sections for these sites.

Chemical and physical studies of molybdenum disulfide show that the extreme anisotropy which dominates the bulk properties of this layered compound is also very important in the understanding of surface properties. We observe that the basal surfaces in single crystals are chemically and optically inert. The edge surfaces are highly reactive to both oxygen and metallic impurities. Photoemission spectroscopy and optical absorption show that the edge surfaces have electronic states within the forbidden gap. The density of these surface defects is correlated with the hydrodesulfurization catalytic activity.

Lithographic fabrication techniques have been extended to the molecular dimension by using the layer pattern in deposited amorphous superlattices as an etching template for the creation of slot structures. Fabrication considerations are discussed for the formation of 10–50Å slots structures suitable for studying shape selective chemistry with fluoresence spectroscopy.

We have investigated molecular confinement in materials with nanometer sized pores, fabricated from amorphous superlattices of a-Si:H/a-SiOx:H, using 1,10-bis(1-pyrenyl)decane as a fluorescent probe. In large (46 Å) pores, the emission and excitation spectra and their time dependence are similar to microcrystalline material, showing the presence of molecules aggregated in the ground state. This dimer emission decreases sharply as the pore size is decreased, showing that the minimum size for aggregate formation is about 20 Å.

Using lithographic techniques, we have prepared two-dimensional pores with widths controllable in the range 10- 100 \AA{}. These materials are formed from amorphous superlattices consisting of alternating layers of silicon and silicon dioxide by use of a selective etch. The micropores adsorb hydrocarbons from solution in a size-selective manner. The effects of confinement in these molecular-scale structures are observed by fluorescence spectroscopy: We demonstrate that pyrene molecules trapped in 20-\AA{} pores lack the mobility necessary to form dimers.

We have examined the interfaces in amorphous silicon hydride/amorphous silicon nitride multilayers with layer thicknesses varying from 7 to 200 Å. Infrared measurements show that these interfaces are heavily hydrogenated, with 1015 cm−2 excess hydrogen atoms per interface pair. Electroabsorption spectroscopy indicates that the interfaces are charged and asymmetrical, indicating that the interface properties depend upon the order of deposition. We attribute these effects to interface defects which are induced by the severe mismatch in bond density at the interface. Raman measurements show that the interface region is highly disordered, with bond angle fluctuations of 13°. Effects due to the interface region extend 10–20 Å into the a-Si:H layers.

The size and distribution of built-in electric fields in $a\ensuremath{-}\mathrm{Si}:\mathrm{H}/a\ensuremath{-}\mathrm{Si}{\mathrm{N}}_{x}:\mathrm{H}$ layered amorphous semiconductor materials have been determined by use of electroabsorption spectroscopy. Strong asymmetries are present between the interfaces, leading to internal fields as large as 4 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ V/cm in material with thin (12 \AA{}) layers. These fields are due to an interface charge present when amorphous silicon is deposited onto silicon nitride, which we attribute to strain-relieving defects caused by structural mismatch.

Electroabsorption spectroscopy shows that large (4×105 V/cm) electric fields exist in a‐Si: H/a‐SiNx: H superlattice materials. These fields are due to interface charges which are asymmetrically distributed, showing that interface properties strongly depend upon the order of deposition. The charge density observed is compared with that obtained from optical, resistivity and photoluminescence measurements.