A microwave nanofabrication process can produce a wide range of semiconductor and other nanoparticles in a matter of minutes that would days to achieve the same quality and quantity using conventional processes.
The Regents of the University of California (Oakland, CA) have patented a method for the synthesis of high quality colloidal nanoparticles in fifteen minutes using microwaves in a high heating rate process. Irradiation of single mode, high power, microwave is a particularly well suited technique to produce high quality semiconductor nanoparticles. The use of microwave radiation effectively automates the synthesis, and more importantly, permits the use of a continuous flow microwave reactor for commercial preparation of high quality colloidal nanoparticles, according to U.S. Patent 7,615,169. Colloidal nanoparticles can be rapidly synthesized under high power microwave radiation and provide industrial scalability with no sacrifice to structural integrity or optical quality.
The microwave method provides for large-scale, safe, convenient, reproducible, energy-conserving synthesis of highly-dispersive inorganic nanoparticles with narrow size distribution, according to inventors Geoffrey Fielding Strouse (Tallahassee, FL), Jeffrey A. Gerbec (Goleta, CA), and Donny Magana (Tallahassee, FL). The system produces mainly inorganic materials, and their diameters are on the order of nanometers (nm). The main crystal may be single crystal, polycrystal, alloys with or without phase separation due to stoichiometric variations, or core-shell structures or doped nanostructures.
Over the past decade, numerous advances have been made in the synthetic procedures for formation and isolation of high quality inorganic nanoparticles. These materials are finding applications in a wide range of disciplines, including optoelectronic devices, biological tagging, optical switching, solid-state lighting, and solar cell applications.
One of the major hurdles for industrialization of these materials has been the development of a reproducible, high quantity synthetic methodology that is adaptable to high throughput automation for preparation of quantities of >100's of grams of single size (<5% RMS) crystalline quantum dots of various composition to be isolated.T
he general synthetic approach for preparation of colloidal semiconductor nanoparticles employs a bulky reaction flask under continuous argon flow with a heating mantle operating in excess of 240.degree. C. The reaction is initiated by rapid injection of the precursors, which are the source materials for the nanoparticles, at high temperatures and growth is controlled by the addition of a strongly coordinating ligands to control kinetics. And to a more limited extent, domestic microwave ovens have been used to synthesize nanoparticles. The high temperature method imposes a limiting factor for industrial scalability and rapid nanomaterial discovery for several reasons: (1) random batch-to-batch irregularities such as temperature ramping rates and thermal instability; (2) time and cost required for preparation for each individual reaction; and (3) low product yield for device applications.
While recent advances in the field have developed better reactants, including inorganic single source precursors, metal salts, and oxides; better passivants, such as amines and non-coordinating solvents; and better reaction technologies, such as thermal flow reactors; the reactions are still limited by reproducibility. Coupled to this problem is the lack of control over reaction times, which require continuous monitoring. In the case of III-V compound semiconductors, the synthetic pathways have rates of growth on the order of days, while in the case of II-VI's, size control is very difficult and depends on the ability to rapidly cool the reaction. In these cases, the reaction depends on heating rate, heat uniformity over the reaction vessel, stirring and rapid and uniform cool-down. These problems and others have been solved according to the inventors.
The duration of nanoparticle reactions have been optimized to 15 minutes at a maximum temperature of 280.degree. C. It has been shown that under the influence of microwave radiation, the crystallinity becomes dependant on the power of the radiation in concert with the temperature of the reaction. The microwave reactor allows the precursors to be prepared at or near room temperature (RT) and loaded into a reaction vessel prior to its introduction into an RT chamber of the microwave reactor. The reaction vessel is then heated to temperatures between 200.degree. C. and 300.degree. C. with active cooling. The microwave reactor operates at 2.45 GHz and can be adapted to a continuous flow or autosampler system. Incorporation of integrated absorption and fluorescent detectors allow the reaction stream to be continuously monitored for applications where high throughput, high volume preparation of colloidal semiconductor nanoparticles is desired.
The figure is a flowchart that illustrates the processing steps for synthesizing nanoparticles used in the preferred embodiment of the present invention. These steps are typically performed using single reaction vessel, continuous flow reactor or stopped flow reactor. When the nanoparticles synthesized by the method are semiconductor nanoparticles, there is no limitation in their composition, but typical examples are single substances of Group 14 elements, such as C, Si, Ge, or Sn, single substances of Group 15 elements, such as P (black phosphorus), single substances of Group 16 elements, such as Se or Te, and group compounds.
Block 38 represents the step of preparing one or more constituent elements at or near room temperature, wherein the room temperature is below 100.degree. C. Preferably, a dielectric constant of a main one of the constituent elements is 20 or higher.
Block 40 represents the step of heating the prepared constituent elements to an elevated temperature using high-rate heating, in order to create a reaction mixture. Preferably, the heating step is performed using microwave irradiation, and the high heating rate comprises a rate of 30.degree. C./min or higher.
Block 42 represents the step of stabilizing the reaction mixture at the elevated temperature. Preferably, the elevated temperature is greater than 240.degree. C.
Block 44 represents the step of cooling the stabilized reaction mixture to a reduced temperature using high-rate cooling. Preferably, the high cooling rate comprises a rate of 125.degree. C./min or higher.