Research in the Barron Group

 

Chemical Control over Materials Formation

 

Soft-Chemistry
 
Phase Control by Molecular Design
CVD of GaS
Applications of cubic-GaS
CVD of other 13/16 Thin Films
 
New Routes to Materials
Ambient Metal Exchange: Geology in a Test Tube
Functionally Graded Interfaces
 
Ambient Growth of Inorganic Thin Films
Inhibition of Cement
Insulators
 
Crystal Engineering

 

Soft-Chemistry
 

It is increasingly recognized that progress in the preparation of novel material requires the development of new synthetic techniques. The ceramic method (cycles of grinding one or more materials and heating) is relatively crude when compared to the sophisticated methodologies available in organic synthesis. The synthesis of organic compounds relies primarily on group-by-group assembly of the whole molecule. In contrast, the formation of inorganic materials by ceramic methods is controlled by diffusion of ionic and atomic species through both reactants and products. Aware of the limitations of the ceramic method as a technique for the production of advanced materials, increasing efforts have been directed towards the development of mild chemistry-based approaches that occur at low temperatures. These methods, loosely grouped under the name "chemie douce" (soft-chemistry), pay close attention to the structure, stability, and mechanisms of product formation

 
Over the last decade we have been interested in synthesis of inorganic materials by non-aggressive routes, i.e., the use of chemical (kinetic) control rather than thermodynamic "brute force".
 
 
Top of Page
 
Phase Control by Molecular Design 
 
CVD of GaS
 
The concept of molecules-to-materials has long been suggested as a mild chemistry-based route to materials. The concept is based on the proposal that a molecule containing pre-formed bonds similar to that in the desired phase will allow for the low energy controlled synthesis of the desired product. However, the very presence of a Ga-As bond in a molecule does not control the formation of GaAs since GaAs is a thermodynamically stable phase! We proposed that in order for the concept of molecular control to be demonstrated, a new "metastable" phase should be made from a molecule and that phase should only be able to be made using that molecule.
 
The initial goal for this concept was gallium sulfide. Hexagonal gallium sulfide (GaS) has a layered ...S-Ga-Ga-S... repeat unit made up of six-membered Ga3S3 cycles. If a molecule with no structural relationship to these features is used then any crystalline phase formed should be structurally distinct. Using our "library" of tert-butyl gallium sulfide compounds we investigated the chemical vapor deposition of gallium sulfide thin films (Chem. Mater., 1993, 5, 1344).
 
Chemical vapor deposition of gallium sulfide from [(tBu)2Ga(StBu)]2 yields poorly crystalline hexagonal GaS, while amorphous GaS is formed from [(tBu)GaS]7. In contrast, the use of [(tBu)GaS]4 yields cubic GaS (Chem. Mater., 1992, 4, 11). Cubic-GaS is also prepared from other cubane derivatives, but not by any other route, thus meeting the requirement of molecular control.
 
In order to demonstrate the structural relationship between the cubane precursor, [(tBu)GaS]4, and the cubic GaS, followed the processes in a step-by-step manner. The structure of [(tBu)GaS]4 was determined, and found to be identical, at 25 °C and 250 °C (the vapor transport temperature) by X-ray diffraction and gas phase electron diffraction (Organometallics, 1995, 14, 690), respectively. The vapor phase decomposition was shown by vapor phase laser photochemistry to occur without cleavage of the Ga4S4 core (Organometallics, 1995, 14, 690) and finally, cubane-like structures have been imaged by STM on the CVD growth surface.

Top of Page

 

Applications of cubic-GaS
 
Our observation that our new cubic phase of GaS was isostructural and lattice matched to GaAs led to our investigation of the ability of cubic-GaS to provide excellent surface passivation of GaAs devices (Appl. Phys. Lett., 1993, 62, 711 and Appl. Phys. Lett., 1993, 63, 625). Our success in this area led to the fabrication of a new class of GaAs based field effect transistors; the field effect transistor with an insulating sulfide heterojunction or "FETISH" (Science, 1994, 263, 1751).

 

Top of Page

CVD of other 13/16 Thin Films
 
Structural control over the phase of a deposited film is also demonstrated for InS thin films (Adv. Mater. Optics. Electron., 1992, 1, 229). The use of the selenide and telluride cubanes as CVD precursors does not provide a route to cubic phases of GaE due to cleavage of the Ga4E4 cores. The decomposition pathway, however, results in the formation of specific (meta-stable for GaTe) phases (Chem. Mater., 1997, 9, 3037). In addition to controlling the structure of the deposited film, we have demonstrated that control of the film morphology is possible and that gallium and indium selenide nanoparticles may be grown by CVD (Chem. Vapor Deposition, 1996, 2, 182).

 

 
 
Top of Page

New Routes to Materials

 
An early example of using mild chemical reactions for the synthesis of a solid state materials was the reaction of indium(III)chloride with tris(trimethyl-silyl)phosphine as a route to InP (J. Chem. Soc., Chem. Commun., 1989, 359). Our method was later shown to be a route to InP nanoparticles.
 
As part of a study of unusual synthetic methods, we synthesized polycrystalline chalcopyrite semiconductors and their solid solution by microwave irradiation (Science, 993, 260, 1653 and Chem. Mater., 1995, 7, 699).
 
 
Ambient Metal Exchange: Geology in a Test Tube
 
Reaction of the carboxylate-alumoxane with a metal acetylacetonate complex [e.g., M(acac)3], results in the formation of a doped-alumoxane. Upon thermolysis these doped-alumoxanes result in homogenous mixed metal oxides as a consequence of the atomic level mixing produced by a unique substitution reaction. The incoming metal ion is exchanged for an aluminum ion within the alumoxane nano-clusters and the elimination of Al3+ as Al(acac)3 (Chem. Mater., 1996, 8, 2331).
 
This work has been extended to the formation of crystalline phases in good to excellent phase purity (J. Am. Ceram. Soc., 2000, 83, 1777). Aluminates prepared in this manner include: CaAl12O19 (hibonite), Y3Al5O12 (YAG), LaAl11O18, Ce2Al3O8, NdAlO3, Er6Al10O24, and Al2TiO5. Furthermore, a new distorted perovskite phase (a = 5.47 Å, c = 13.3 Å) of LaAlO3 is formed.
 

Top of Page

 

Functionally Graded Interface
 
The fusion of chemically dissimilar materials is a major problem in composite fabrication and the formation of protective coatings. One method, that has recently gained general acceptance, to create a stable interface between chemically dissimilar materials is the creation of a graded interface. We have developed a number of techniques for creating functionally graded interfaces, including those between a metal and a ceramic (Thin Solid Films, 1992, 207, 138) and a ceramic and carbon (J. Am. Ceram. Soc., 1990, 73, 3696). One approach to this problem we have suggested is to determine, through theoretical calculations, the optimum materials match as an aide to minimizing experimentation (Mat.Res. Soc. Symp. Proc., 1990, 193, 149).

Top of Page

Ambient Growth of Inorganic Thin Films 

Inhibition of Cement
 
Cement is a material ubiquitous to modern society. Cementitious materials have been in use for over 2000 years and to this day are considered essential to both the construction and oil industries. Despite their mundane low-tech status, cements are among the least well understood materials due to their complex nature. In the oil industry, bore-holes of ever increasing depths are strengthened by cement. This requires control over setting to allow the cement to be pumped down in a "liquid" form. A wide range of chemicals are employed to delay the setting time. Recent work has concentrated on a series of organic phosphonates, in particular nitrilotris(methylene)triphosphonic acid {N[CH2PO(OH)2]3, H6ntmp}, which have been found to be highly efficacious. It has been proposed that the retarding effects are due to complexation of calcium both on the surface of the calcium silicates (e.g., tricalcium silicate) and in solution (gypsum).
 
We have employed a combination of solid state NMR spectroscopy, SEM, XRD and XPS to investigate the mechanism of inhibition for a range of inhibition agents.
 
Previous research has suggested that inhibition occurs through either (a) surface binding to minerals to stop hydration, or (b) solution binding of Ca ions to inhibit re-growth (setting). Our studies have demonstrated an alternative mechanism occurs for some inhibition agents. This new proposal involves the dissolution of Ca by a chelate agent, that subsequently re-precipitates an insoluble layered material onto the mineral surface. Inhibition, thus is a function of water diffusion through this layer.
 
We have crystallographically characterized one such phase: [Ca(H4ntmp)(H2O)].3.5(H2O), which exists as a series of planar sheets which are coplanar and contain inter-sheet contacts via hydrogen bonded water of crystallization.
 
Based upon the above results we propose that the reaction of Ca(OH)2 or CaO within a Ca containing mineral with H6ntmp results in dissolution of calcium with subsequent precipitation of [Ca(H4ntmp)(H2O)]. Heterogeneous precipitation of [Ca(H4ntmp)(H2O)] onto the surface of the mineral particles, as opposed to homogeneous crystal growth, would be enhanced by the ability of terminal phosphonate moieties on the [Ca(H4ntmp)(H2O)] oligomer to bind to the mineral surface. The layered sheet-like coating of [Ca(H4ntmp)(H2O)] would inhibit both further dissolution of calcium and also diffusion of water through to the mineral surface.
 
One aspect of this research is the in-situ creation of a composite structure. We are interested in the applicability of this dissolution - regrowth technique as a general route to nanocomposite materials.

Top of Page

Insulators
 
Silicon dioxide (SiO2) forms the basis of planar Si chip technology. Insulator coatings for electronic and photonic devices layers are most frequently formed by thermal oxidation of Silicon (Si) in the temperature range 900 to 1200 °C. SiO2 is also deposited by chemical vapor deposition (CVD) techniques at lower temperatures (200 to 900 °C) on various substrates. The growth of insulator films at low temperatures is very attractive for most device applications due to reduced capital cost, high output and technological constraints associated with the growth of dielectric thin films using conventional high-temperature growth/deposition techniques. Deposition of SiO2 insulator layers from solutions is previously known using organo-metallic solutions. In this procedure, the insulator layer is applied onto the substrate either by dipping the substrate into the solution or by spinning the substrate after a small amount of the solution is applied onto the surface. In both cases the substrate is then placed in an oven to drive off the solvent.
 
Researchers have described processes for deposition of SiO2 layers on silicon surfaces using a room temperature (30 to 50 °C.) solution growth. The growth of liquid-phase deposited (LPD) SiO2, for deposition of SiO2 on the surface of soda lime silicate glass, is based on the chemical reaction of H2SiF6 with water to form hydrofluoric acid and solid SiO2. One of the major disadvantages of SiO2 LPD method is a very low deposition rate of about 8 nm/hour, which makes it impractical for growing insulator layers for most semiconductor device applications.
 
We are presently investigating the catalyzed growth of insulating materials by LPD using a combination of transition metal catalysis and thin film techniques. Our eventual goal is to develop wholly inorganic analogs to bio-mineralization processes.

 

Top of Page

Crystal Engineering
 
X-ray crystallography is now a vital tool for inorganic chemistry. The presence of the CCD system in the Texas Center for Crystallography at Rice University allows for rapid collection of data. However, in collaboration with Prof. Simon Bott at the University of Houston, we are interested in using crystallography as a tool for the understanding and prediction of extended structures in inorganic systems.
 
A recent example of our research in this area involves the crystal packing of alcohol amines formed by the reaction of primary amines with 1,2-epoxy-3-phenoxypropane (J. Mater. Chem., 2000, 11, 24). Through a combination of our own data and that in the Cambridge Crystallographic Database, compounds of the general type X-CH(OH)CH2N(H)R were investigated and a rationalization for the packing of racemic mixtures non-centrosymmetric space groups was proposed.
 
 
 
The rapid data collection abilities of the CCD, and the observation of severe site disorder in solid solutions of coordination compounds, led to an accuracy assessment of the refinement of partial metal disorder in solid solutions of Al(acac)3 and Cr(acac)3 (J. Chem. Soc., Dalton Trans., in press).

 Top of Page

 

Return to Research in the Barron Group

 

Return to Barron Group Home Page