Abstract:
The present invention comprises new materials, material structures, and processes of fabrication of such that may be used in technologies involving the conversion of light to electricity and/or heat to electricity, and in optoelectronics technologies. The present invention provide for the fabrication of a clathrate compound comprising a type II clathrate lattice with atoms of silicon and germanium as a main framework forming lattice spacings within the framework, wherein the clathrate lattice follows the general formula Si 136−y Ge y , where y indicates the number of Ge atoms present in the main framework and 136−y indicates the number of Si atoms present in the main framework, and wherein y&gt;0.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 7/535,414 filed on Apr. 16, 2008 and issued on May 19, 2009 which is a continuation of prior filed International Application, Serial Number PCT/US2006/041086 filed Oct. 20, 2006, which claims priority to U.S. provisional patent application No. 60/728,508 filed Oct. 20, 2005 which is hereby incorporated by reference into this disclosure. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under Grant No. DE-FG02-04ER46146 awarded by the United States Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The direct conversion of solar energy to electricity is a technology which holds great promise for contributing to the solution of future global energy needs. However, currently, the technology has neither reached the efficiency nor reduction in cost necessary to contribute significantly as a source of energy on a large scale. The most effective route to realization of large scale solar energy conversion is to find vastly improved materials from which the necessary devices may be fabricated. Typically, these materials may either ( a ) convert light to electricity with greatly improved efficiency or ( b ) greatly reduce the cost of device fabrication, or both. 
     Although solar energy holds great promise as a source of power for the future, major advances in materials development must be made. While some advances in solar cell efficiencies have been made in the last few decades, new materials that offer superior efficiencies could offer the boost needed to make photovoltaics a global widespread source of energy. Also, the high costs of producing solar cells are a significant barrier to the widespread use of photovoltaic energy conversion. 
     The direct conversion of heat to electricity through thermoelectric power generation also holds great promise for lightening the burdens of increasing energy needs. However, the materials utilized by this technology are the limiting factor in achieving higher efficiencies. A material&#39;s usefulness as a thermoelectric is measured by its dimensionless figure of merit ZT. Current state of the art thermoelectric materials possess ZT between 1 and 1.5. Though many advances have been made in recent years, the current values of ZT are still too low for use in large-scale power generation and/or refrigeration. 
     Thus, a need exists in the art for new materials for use in the direct conversion of solar and heat energy to electricity. 
     SUMMARY OF INVENTION 
     As many technologies are limited by the materials they utilize, and as improvements upon existing materials are often not enough to provide significant advancements in the technologies, new materials, structures, and/or approaches must be found. 
     The present invention addresses the need for higher efficiency materials for energy conversion applications, specifically for photovoltaic and thermoelectric energy conversion applications. The present invention also addresses the need for optoelectronic materials that may be easily integrated into current electronics technologies. The present invention offers new directions in addressing these problems, in as much as structural aspects of these new materials may be adjusted offering control over the materials properties, which are specifically useful for photovoltaic, optoelectronic, and/or thermoelectric applications. 
     The present invention comprises specific materials composed of the type II clathrate crystal structure, consisting of a lattice framework composed of nano-scale polyhedra that form cavities that may encapsulate guest atoms, as well as methods of controlling the structural aspects of these materials, and methods of producing these materials in bulk and thin film form, for use in photovoltaic (PV), thermoelectric (TE), and/or optoelectronic applications. The structures and the optical and electrical properties of these materials may be manipulated in a controlled fashion in order to optimize the materials properties for the desired application. 
     Specifically, frameworks composed of Si, Ge, or Si-Ge alloys are within the scope of the present invention. The atoms of the main framework may be substituted by other atoms, as described below, in order to optimize the electrical, thermal, mechanical, and structural properties in these materials. Also, the guest atoms may be made to selectively occupy the different cavities in varying concentrations, again in order to optimize the electrical, thermal, mechanical, and structural properties in these materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which: 
         FIG. 1(   a ) is an illustration of the crystal structure of the type II clathrate, indicating the guest atoms inside the two polyhedra.  1 ( b ) is a representation of the polyhedra, E 28  (top) and E 20  (bottom), that form the framework in type II clathrates. 
         FIG. 2  is a graph illustrating the powder x-ray diffraction patterns of Na x Si 136  for three values of x showing the high purity of these type II Si clathrates and the ability to vary x. 
         FIG. 3  is a graph illustrating the refinement of powder X-ray diffraction data for Na 1 Si 136 . 
         FIG. 4  is a graph illustrating the refinement of powder X-ray diffraction data for Na 8 Si 136 . 
         FIG. 5  is a graphical illustration of the electrical resistivity of Na 1 Si 136  (low alkali content) and the fully occupied Cs 8 Na 16 Si 136  (high alkali content, inset) showing the wide variation of the electrical properties of type II clathrates determined by the guest atom content. Na 1 Si 136  behaves as an electrical insulator whereas Cs 8 Na 16 Si 136  possesses metallic behavior. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In addition to intense basic scientific investigation, currently clathrate materials are attracting attention for their potential use in numerous technological applications, including thermoelectrics, superconductivity, optoelectronics, photovoltaics, hard and low compressibility materials, magnetic materials, and energy storage. It is the unique structural aspects of these materials, and how these aspects relate to their physical properties, that raise such wide interest. Most of the work on inorganic clathrates to date has been focused on what is known as the type I structure. 
     The present invention comprises materials that form with what is known as the type II clathrate crystal structure, and includes controlled manipulation of the structure and/or composition, methods of producing the materials in bulk and thin film form, and use of these materials in photovoltaic applications. With reference to  FIG. 1   a , these materials are composed of a covalently bonded framework, which forms “polyhedra” that can encapsulate guest atoms inside. The invention pertains to type II clathrate materials that have the general form A x E 136  (0&lt;x&lt;24), in which E represents the atoms forming the framework of the structure, and A represents the guest atoms contained inside the framework polyhedra. As illustrated with reference to  FIG. 1   b , there are two distinct polyhedra that form the building blocks of the structure, denoted E 20  (twenty vertices at which the E atoms reside; sixteen E 20  cages per conventional unit cell of E 136 )  15  and E 28  (twenty-eight vertices at which the E atoms reside; eight E 28  per E 136 )  20 , and their size and the local environment of the guest atom inside is distinctly different. In accordance with the present invention guest atoms  25 , may or may not reside within lattice spacings formed by the polyhedra of the clathrate lattice. 
     The crystal structure of the type II clathrate consists of structural units A x E 136  in which E represents the atoms forming the framework of the structure, and A represents the guest atoms, or voids, contained inside the framework polyhedra. In the present invention, E can include, but is not limited to Si, Ge, or Sn, and A includes, but is not limited to Na, K, Rb, Cs, Ca, Sr, Ba, or Eu. The present invention allows the control of the various structural aspects of the type II clathrate structure, in order to control and/or optimize the material&#39;s physical properties for photovoltaic applications. The manipulation of the band gap, carrier (or doping) concentration and photo-electric efficiency are described in the following paragraphs. These methods can be used individually; however they are intended to be used in any combination to optimize the desired physical properties of the compound. Clathrate materials are currently generating interest for several applications, as noted above, however the combination of the methods explained below comprises a new way to optimize the necessary properties for the appropriate application. The methods of producing the materials are also described below. 
     The present invention allows for the formation of alloys in the main framework of the structure. Specifically, Si-Ge alloys of the general type A x Si 136-y Ge y  are formed in accordance with the present invention. For the conversion of light to electricity, these alloys show superior performance over either pristine Si or Ge type II clathrate frameworks, in as much as the Si-Ge alloy type II clathrates possess a direct band gap in the electronic band structure, in the visible light range, whereas the pristine clathrates possess indirect band gaps. Direct gap semiconductors allow more efficient absorption of light. The fact that these are Si based materials allows for the integration into current Si based photovoltaic and optoelectronic technologies. The formation of alloys also has beneficial effects on the thermoelectric properties of these materials, in as much as the thermal conductivity is reduced by mass fluctuation scattering of phonons, due to the difference in mass between Si and Ge. 
     The present invention allows for the guest atom concentration in the type II clathrates to be varied, by adjustment of the synthesis processing. Thus in the general formula A x E 136  the value of x may be varied from 0&lt;x&lt;24. As illustrated with reference to  FIG. 2 ,  FIG. 3 ,  FIG. 4  and  FIG. 5 , the electrical properties of type II clathrates depend sensitively on the value of x, where for low x the materials are insulating or semiconducting, and for high x the materials show metallic properties. This allows for tuning the carrier concentration and the band gap in order to optimize these materials for photovoltaic application. This approach, combined with formation of solid solutions allows for a more complete tunability towards optimization for photovoltaic applications. 
     In addition to varying the total guest atom content x, in the present invention the guest atoms are made to selectively occupy the two distinct polyhedra E 20  and E 28 . For example, for fixed x, say x=8, the guests may be distributed among the polyhedra, possible configurations include: all sixteen guests in the E 20  cages, with none in the E 28  cages; all eight guests in the E 28  cages with none in the E 20  cages; or four guests in the E 20  cages and four guests in the E 28  cages. As the two polyhedra that the guests may occupy comprise distinct crystallographic sites, with different local environments, the selective occupation of the sites allows for adjustment of the structure of the materials, and also allows for the control of the physical properties of these materials. This allows for the control of the effective charge transfer from the guest atoms to the cages, offering a route for adjusting the electrical properties. Also, the thermal properties are affected by the concentration of the guest atoms. 
     The present invention allows for the substitution of the framework E atoms by other atoms, resulting in a chemical formula of A x E 136-y M y , where the substitutional atom M includes, but is not limited to Ni, Pd, Pt, Cu, Ag, Au, Zn, Al, Ga, In, P, As, and Sb, and E is Si, Ge, or Sn. The substitution of framework atoms also allows for the controlled adjustment of the electrical and thermal properties. In particular, framework substitution may be used to form semiconducting materials and to have accurate control over the electrical transport properties in these materials. 
     The present invention allows for the production of type II clathrate materials in bulk or film form for various electronic and/or thermoelectric applications. The methods utilized in the invention allow for bulk quantities of high purity type II clathrates to be produced as described below. 
     In a first embodiment, intermediate Zintl compounds are used. Some examples include, but are not limited to NaSi, NaGe, NaSn, KSi, KGe, KSn, RbSi, RbGe, RbSn, CsSi, CsGe, CsSn, BaSi 2 , BaGe 2 , BaSn 2 , SrSi 2 , SrGe 2 , SrSn 2 , Na 1-x K x Si 2 , Na 1-x K x Ge 2 , Na 1-x K x Sn 2 , Na 1-x Rb x Si 2 , Na 1-x Rb x Ge 2 , Na 1-x Rb x Sn 2 , Na 1-x Cs x Si 2 , Na 1-x Cs x Ge 2 , Na 1-x Cs x Sn 2 , K 1-x Rb x Si 2 , K 1-x Rb x Ge 2 , K 1-x Rb x Sn 2 , K 1-x Cs x Si 2 , K 1-x Cs x Ge 2 , K 1-x Cs x Sn 2 , Rb 1-x Cs x Si 2 , Rb 1-x Cs x Ge 2 , Rb 1-x Cs x Sn 2 , NaSi 1-y Ge y , KSi 1-y G y , RbSi 1-y Ge y , CsSi 1-y Ge y , Na 1-x K x Si 1-y Ge y , Na 1-x Rb x Si 1-y Ge y , Na 1-x Cs x Si 1-y Ge y , K 1-x Rb x Si 1-y Ge y , K 1-x Cs x Si 1-y Ge y , and Rb 1-x Cs x Si 1-y Ge y  (0&lt;x, y&lt;1), or any variation, combination, mixture, alloy, or solid solution of any of these compounds. It is important to note that other intermediate compounds not listed may also be used. The intermediate Zintl compounds are formed by reaction of the appropriate high purity elements in the proper mole ratios at 650° C. in tungsten crucibles, sealed in a steel canister under high purity nitrogen gas at a pressure of one atmosphere. The steel canister is itself sealed under nitrogen at a pressure slightly less than atmosphere in a quartz tube. The resulting intermediate Zintl compounds are extremely sensitive to air and moisture and all handling must be performed in an inert atmosphere. 
     In a particular embodiment to form the type II clathrate, the appropriate intermediate Zintl compound is ground to a very fine powder, placed in a quartz crucible, which is then inserted into a quartz tube that is sealed at one end. The tube is then attached to a vacuum valve coupling allowing for connection to a high vacuum system, keeping the sample under nitrogen atmosphere. The gas is then evacuated from the tube, and the intermediate Zintl compound is heated to approximately 275° C. for several hours to remove any excess alkali metal that may be present. The intermediate Zintl compound is then slowly heated from this temperature to an appropriate temperature for the synthesis of the type II clathrate, the temperature depending on the composition to be achieved. For example, in the case of NaSi a typical temperature is 420° C. to form the clathrate Na x Si 136 . The guest atom content is controlled by the amount of time the material is heated in the furnace. For example, a Na x Si 136  material heated for eight hours at 420° C. results in x=8, while a Na x Si 136  material heated for twenty-four hours at 420° C. results in x=1. 
     In an additional embodiment to form the type II clathrate, the intermediate Zintl NaSi compound is rapidly heated through the decomposition temperature (˜360° C.) at the rate of several hundred ° C./min. This is achieved by inserting the quartz tube containing the sample into a tube furnace preheated to 800° C., but removing the sample before decomposition of the clathrate begins (at ˜450° C. under vacuum). This results in a Na x Si 136  clathrate with x close to 24. The Na content is then varied by further heating under vacuum at 360° C. to 425° C. By scaling up to larger crucible and tube sizes, as much as 500 mg of high quality Na x Si 136  is produced in a single synthesis run. 
     In a second embodiment, stoichiometric amounts of high purity elements are combined in a tungsten or tantalum crucible sealed in a steel canister under nitrogen gas at a pressure of one atmosphere. The steel canister is itself sealed under nitrogen in a quartz tube. The mixture is reacted at 800° C. for 48 hours, then cooled to 650° C. and held at this temperature for seven days. The compound is then cooled to room temperature. This produces high purity type II clathrates with general compositions of A 24 E 136-y M y , A 8 B 16 E 136-y M y , A 8 B 16 Si 136-y Ge y  and A 8 B 16 Si 136-y-z Ge y M z , where A and B include, but are not limited to Na, K, Cs, Rb, Ca, Ba, Sr, Eu, M includes, but is not limited to Ni, Pd, Pt, Cu, Ag, Au, Zn, Al, Ga, In, P, As, and Sb, and E is Si, Ge, or Sn. 
     In a third embodiment, the guest atom content in type II clathrates of general composition A 24 E 136-y M y , A 8 B 16 E 136-y M y , A 8 B 16 Si 136-y Ge y  and A 8 B 16 Si 136-y-z Ge y M z  is varied, resulting in the general compositions AE 136-y M y , A x B m E 136-y M y , A x Si 136-y Ge y  A x B m Si 136-y Ge y  and A x B m Si 136-y-z Ge y M z . Type II clathrates of the composition A 24 E 136-y M y , A 8 B 16 E 136-y M y , A 8 B 16 Si 136-y Ge y  and A 8 B 16 Si 136-y Ge y M z  are first synthesized using the method outlined in the previous paragraph. These are referred to as “fully occupied” or “stoichiometic,” in as much as all of the polyhedra are occupied by guest atoms. These compounds are then ground to fine powders and annealed at temperatures between 300° C. and 500° C. under high vacuum of 10 −5  to 10 −6  torr. The process of grinding and annealing is repeated as necessary, and the temperature is increased each time as necessary. This vacuum annealing or “degassing” removes a portion of the guest atoms from the polyhedra, leaving these polyhedra unoccupied. Thus, the products of this procedure are type II clathrates of general composition A x E 136-y M y , A x B m E 136-y M y , A x Si 136-y Ge y  A x B m Si 136-y Ge y  and A x B m Si 136-y-z Ge y M z . 
     The invention allows for the production of type II clathrates in thin and thick film form for photovoltaic and optoelectronic applications. Most of the growth methods used in film and coating technology known in the art can be used to fabricate type II clathrate films. Specifically, the method of single or dual laser ablation, offers the ability to maintain precise control over the composition or stoichiometry of these films. Various methods for the formation of type II clathrate films in accordance with the present invention by laser ablation are within the scope of the present invention, including, but not limited to: 
     Ablate a compact bulk target of NaSi→NaSi film→Type II clathrate film 
     Ablate a compact bulk target of NaSi in a Na vapor environment→Type II clathrate film 
     Ablate a compact bulk target of Na x Si 136-y Ge y →Type II clathrate film 
     Ablate compact bulk targets of other alkali compounds in similar manner, e.g. NaGe, CsGe, KSi, etc. 
     The film can then be deposited on any substrate at a deposition temperature of between 0° C. to 900° C. 
     In a particular embodiment, the film growth process in accordance with the present invention includes the steps of, placing the material target to be ablated on a rotational stage in a vacuum system (˜10 −6  Torr) and focusing a pulsed laser beam (preferably a uv laser) onto the target through an optical window on the vacuum system. The laser-target interaction produces a plasma plume that expands away from the target. A substrate (silicon, glass, etc.) is then placed 4-10 cm in front of the target to collect the evaporated material in the form of a film. The substrate can be heated in the temperature range of 20-900° C. to promote crystallization and film adhesion to the substrate. 
     If incorporation of additional Na into the growing film is needed, Na may be thermally evaporated in a boat to generate a high Na vapor pressure in the vicinity of the substrate. 
     In another embodiment utilizing dual-laser ablation, a second laser of suitable wavelength (most likely an infrared laser) is spatially overlapped with the first laser on the target with a suitable delay between the laser pulses of the two lasers (˜50 ns) to minimize particulate ejection and to maximize the material plume energy. 
     It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween. Now that the invention has been described.