Patent Application
20150001450
Embodiments of the present invention relate generally to methods of crystallizing colloidally suspended nanocrystals via heating and in an alternative embodiment, ball milling methodologies.
Semiconductor nanocrystals, or quantum dots, can be produced in a variety of ways. When produced in a colloidal suspension, they are often formed as a connected atomic species. However, in some instances, they do not form a perfect lattice, and are therefore not considered to be completely crystalline. This is an artifact of the previous methods utilized in colloidal synthesis. For instance, in previous attempts at colloidal growth and suspension of semiconductor nanocrystals, the different atomic species present in the solution can move throughout the solvent. In some cases, these atomic species will attach to the nanocrystal surface during formation of the nanocrystal. In doing so, the bonding site may not be ideal for forming a well-structured lattice that is characteristic of semiconductor nanocrystals.
This lattice structure, however, is an important aspect of the characteristics of semiconductor nanocrystals. The electronic band structure of a semiconductor nanocrystal is largely affected by the lattice structure. Further, flaws in the lattice structure more easily allow impurity atoms within the semiconductor nanocrystal due to the lack of proper ordering within the structure.
Embodiments of the invention disclosed herein may include a method of crystallizing a semiconductor nanocrystal population comprising: suspending the semiconductor nanocrystal population in a high boiling point solvent to form a solution; and heating the solution to a temperature of approximately 100° C. to approximately 400° C.
Embodiments of the invention may also include a crystallized semiconductor nanocrystal population made by a method, the method comprising: suspending a semiconductor nanocrystal population in a high boiling point solvent to form a solution; and heating the solution to a temperature of approximately 100° C. to approximately 400° C.
Embodiments of the invention may also include a method of crystallizing a semiconductor nanocrystal population comprising: drying the semiconductor nanocrystal population into a powder; placing the powder into a ball mill; and ball milling the powder for a duration of time.
Embodiments of the invention may also include a crystallized semiconductor nanocrystal population made by a method, the method comprising: drying a semiconductor nanocrystal population into a powder; placing the powder into a ball mill; and ball milling the powder for a duration of time.
Embodiments of the present invention include methods of crystallizing semiconductor nanocrystals more effectively, creating a more consistent lattice structure. Previous attempts have utilized heat in an effort to give sufficient energy to the lattice to enable the atoms to migrate and shift into a more consistent crystalline lattice. To some extent, this is similar to annealing processes, wherein thermal energy is used to shift the atoms into a more crystalline shape. In previous attempts to ‘anneal’ into a crystalline structure, heat was typically applied to the powder form of the semiconductor nanocrystals. However, when heat is applied to the powder form, semiconductor nanocrystals typically grow beyond the desired size as atoms near to the nanocrystals, including those from neighboring nanocrystals, tend to bond together. Since semiconductor nanocrystals' properties are largely based on the size of the nanocrystal, this can cause severe problems in trying to tune the crystallized nanocrystals.
In one embodiment, as illustrated in
In many cases, the semiconductor nanocrystal population may be provided already suspended in a volatile solvent with a relatively low boiling point, as many applications require such a suspension. However, these solutions have too low of a boiling point to effectively heat the solution to the point of crystallization. For instance, hydrazine is one example of such a solvent. Hydrazine has a boiling point of approximately 114° C. This is too low for most desired annealing processes. Accordingly, the semiconductor nanocrystal population can be suspended in a relatively higher boiling point solvent. This solvent may include tri-octyl phosphine (TOP), oleic acid, n-methylformamide, any other formamide derivative, and a variety of organic solvents. These solvents are illustrative, but not meant to be limiting. In some instances, the solvent should have a boiling point of approximately 100° C. or more. In further embodiments, the boiling point may be at least approximately 250° C.
Once the semiconductor nanocrystal has been suspended in a high boiling point solvent, the resulting solution may be heated to a temperature capable of crystallizing the population (S2). In some embodiments, this temperature may include a range of approximately 100° C. to approximately 400° C. In a further embodiment, the range can include approximately 250° C. to approximately 400° C. This temperature range is the reason that the above described lower boiling point solvents may not be compatible with the methods disclosed herein. However, in a further embodiment, a lower boiling point solvent such as hydrazine may be used for this heating step; however, the solution would need to be in a pressurized environment in order to avoid boiling of the solvent.
This method has multiple advantages over prior methods. For example, when the nanocrystals are heated in close proximity to one another at a high enough temperature, the atoms have enough energy to move around and reposition into a well formed crystalline lattice structure, without the typical downfall of grain growth that occurs by heating the powder. In a further embodiment, the solution containing the semiconductor nanocrystal population may benefit from being more dilute. A dilute solution of nanocrystals can add to the advantages of this method by further separating individual nanocrystals from one another, further reducing the possibility of grain growth between nanocrystals. Further, during the increase in energy from heating in solution and the more ordered structure of the resulting lattice, impurity atoms tend to migrate towards the surface of the nanocrystals and may even disassociate from the nanocrystal and be driven into solution, resulting in a more pure semiconductor nanocrystal of the desired material system.
Following heating of the solution, in one embodiment the semiconductor nanocrystal population may be removed from the solution (S3). This can eliminate any impurities left behind in the solution, as well as provide a more useful medium for the nanocrystals. Following removal from the high boiling point solution, the semiconductor nanocrystal population may be resuspended in a volatile solvent (S4). As one example, the volatile solvent may include hydrazine. Upon suspension in the volatile solvent, a crystallized population of semiconductor nanocrystals is provided. This population of crystallized semiconductor nanocrystals is also provided in a solvent which may be useful for many applications. A further embodiment includes the crystallized semiconductor nanocrystal population made according to the above disclosed method.
In an alternative embodiment, as illustrated in
In order to determine how long to ball mill the powder, which uses kinetic energy to crystallize the powder, one can measure the level of crystallization at predetermined periods throughout the milling process. For instance, differential scanning calorimetry (DSC) can be performed to measure the level of crystalinity. Using techniques such as DSC, the crystallization of the starting powder can be measured in order to estimate the length of time needed to ball mill. Further, the milled powder can be tested periodically during milling to determine if the desired crystallization has yet occurred, stopping the process if it has or continuing the ball milling if not. Once the powder has been ball milled to the desired specifications, the crystallized nanocrystals may be resuspended in a volatile solvent (P4), for instance hydrazine. In a further embodiment, a crystallized semiconductor nanocrystal population made by the above disclosed methods is included.
The methods disclosed herein can be useful for any number of types of nanocrystals, including all types of semiconducting nanocrystals and quantum dots.
The foregoing description of various aspects of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such variations and modifications that may be apparent to one skilled in the art are intended to be included within the scope of the present invention as defined by the accompanying claims.
This application claims the benefit of co-pending U.S. Provisional Application Ser. No. 61/840,631, filed 28 Jun. 2013, which is hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 61840631 | Jun 2013 | US |
