Patent Application
20170217785
This disclosure relates generally to perovskites and more particularly to inorganic halide perovskite nanowires.
Halide perovskites have been demonstrated to be a promising class of materials for optoelectronic applications, including high-efficiency photovoltaic cells, light-emitting diodes, lasers, and photodetectors. Advantages of these compounds include their excellent charge-transport properties and broad chemical tunability. While recent studies have been mostly focused on hybrid organic-inorganic compounds, the study of their inorganic analogues, like AMX3 (A=Rb, Cs; M=Ge, Sn, Pb; X=Cl, Br, I), is limited.
Previous studies on the all-inorganic halide perovskites have revealed that these materials have great potential in optoelectronic applications. CsGeX3 are known for their nonlinear optical properties and are potentially useful for nonlinear optics in the mid-infrared and infrared regions. CsSnI3-xFx has been demonstrated to be an effective hole-transport material and is able to replace the problematic organic liquid electrolytes in dye-sensitized solar cells. Theoretical calculations on ASnX3 (A=Cs, CH3NH3, NH2CH═NH2; X=Cl, I) suggested that their electronic properties are strongly depend on the structure of the inorganic SnX6 octahedral cage, which implies good prospects for the all-inorganic halide perovskites.
However, most of these studies were based on polycrystalline perovskite films deposited on substrates using vapor-phase co-evaporation or solution deposition of a mixture of AX and BX2. The uncontrolled precipitation or evaporation of the perovskite produces large morphological variations, making it a non-ideal platform for understanding these materials' fundamental properties.
One innovative aspect of the subject matter described in this disclosure can be implemented in a nanowire comprising an inorganic halide perovskite. In some embodiments, the inorganic halide perovskite comprises ABX3, where A is Cs or Rb, where B is Sn or Pb, and where X is Cl, Br, or I. In some embodiments, the nanowire has a cross-sectional dimension of less than 1000 nanometers.
Another innovative aspect of the subject matter described in this disclosure can be implemented in a method including providing a first solution comprising cesium oleate or rubidium oleate in a first organic solvent. A second solution comprising a lead halide and a surfactant in a second organic solvent is provided. The halide is selected from a group consisting of chlorine, bromine, and iodine. The first solution and the second solution are mixed. A reaction between the cesium oleate or the rubidium oleate and the lead halide forms a plurality of nanowires comprising an inorganic lead halide perovskite. In some embodiments, nanowires of the plurality of nanowires comprise ABX3, where A is Cs or Rb, where B is Pb, and where X is Cl, Br, or I.
Another innovative aspect of the subject matter described in this disclosure can be implemented in a method including depositing lead iodide on a substrate. The lead iodide is contacted with a solution of a cesium halide or a rubidium halide in a first alcohol. The halide is selected from a group consisting of chlorine, bromine, and iodine. A reaction between the lead iodide and the cesium halide or the rubidium halide forms a plurality of nanowires comprising an inorganic lead halide perovskite. In some embodiments, the nanowires of the plurality of nanowires comprise ABX3, where A is Cs or Rb, where B is Pb, and where X is Cl, Br, or I.
Another innovative aspect of the subject matter described in this disclosure can be implemented in a method including depositing a cesium halide or a rubidium halide on a substrate. The halide of the cesium halide or the rubidium halide is selected from a group consisting of chlorine, bromine, and iodine. The cesium halide or the rubidium halide is contacted with a solution of a tin halide or a lead halide in a first alcohol. The halide of the tin halide or the lead halide is selected from a group consisting of chlorine, bromine, and iodine. A reaction between the cesium halide or the rubidium halide and the tin halide or the lead halide forms a plurality of nanowires comprising an inorganic halide perovskite. In some embodiments, nanowires of the plurality of nanowires comprise ABX3, where A is Cs or Rb, where B is Sn or Pb, and where X is Cl, Br, or I.
Details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages will become apparent from the description, the drawings, and the claims. Note that the relative dimensions of the following figures may not be drawn to scale.
Reference will now be made in detail to some specific examples of the invention including the best modes contemplated by the inventors for carrying out the invention. Examples of these specific embodiments are illustrated in the accompanying drawings. While the invention is described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to the described embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. Particular example embodiments of the present invention may be implemented without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention.
Various techniques and mechanisms of the present invention will sometimes be described in singular form for clarity. However, it should be noted that some embodiments include multiple iterations of a technique or multiple instantiations of a mechanism unless noted otherwise.
The terms “about” or “approximate” and the like are synonymous and are used to indicate that the value modified by the term has an understood range associated with it, where the range can be ±20%, ±15%, ±10%, ±5%, or ±1%. The term “substantially” is used to indicate that a value is close to a targeted value, where close can mean, for example, the value is within 80% of the targeted value, within 90% of the targeted value, within 95% of the targeted value, or within 99% of the targeted value.
Controlled synthesis of materials with high quality and well-defined morphology not only benefits fundamental research but also offers great promise for practical applications Examples include the development of semiconducting quantum dots (QDs), one-dimensional (1D) nanowires (NWs), and two-dimensional (2D) nanosheets, which can have optical and electrical properties superior to those of their bulk counterparts. Semiconductor NWs, in particular, currently attract widespread interest due to the great potential to advance fundamental and applied research toward new classes of inherently 1D photonic and electronic nanostructures. In terms of inorganic halide perovskites, with the exception of single-crystalline QDs, there have been no reports of 1D or 2D nanostructures.
In some embodiments, a nanowire of a nanosheet comprises an inorganic halide perovskite. A perovskite is a material with the same crystal structure as calcium titanium oxide (CaTiO3). The general chemical formula for a perovskite is ABX3, where A and B are two cations of different sizes, and X is an anion that bonds to both the A and B cations. The A cations are larger than the B cations. In some embodiments, the nanowire has a chemical formula ABX3, where A is cesium (Cs) or rubidium (Rb), wherein B is tin (Sn) or lead (Pb), and where X is a halide such as chlorine (Cl), bromine (Br), or iodine (I).
In some embodiments, a nanowire has a cross-sectional dimension of less than 1000 nanometers (nm). For example, if the nanowire has a circular cross-section, the diameter of the nanowire is less than 1000 nm. For example, if the nanowire has a rectangular or square cross-section, a length of an edge of the rectangle or square is less than 1000 nm. In some embodiments, the nanowire has a length of about 100 nm to 30 microns.
In some embodiments, a nanosheet has a thickness of about 1 nanometer to 100 nanometers. In some embodiments, a nanosheet has a dimension on its planar surface of about 250 nanometers to 5 microns. For example, the nanosheet is circular, the diameter (i.e., the dimension on its planar surface) would be about 250 nanometers to 5 microns.
In some embodiments of the inorganic halide perovskite nanowire fabrication methods described below, long chain amines and short chain amines are used as ligands that cap the nanostructures during growth of the nanostructures. In some embodiments, organic acids perform a similar function. By combining different long and short chain amines, the growth of the structure can be directed such that nanowires or nanosheets are formed.
At block 120, a second solution comprising a lead halide and a surfactant in a second organic solvent is provided. The halide is selected from a group consisting of chlorine, bromine, and iodine. The surfactant may comprise an amine. For example, in some embodiments, the surfactant comprises a surfactant selected from a group consisting of octylamine, oleylamine, oleic acid, and combinations thereof. Other amines with long carbon chains (e.g., octadecylamine) may also be used. In some embodiments, using octylamine (a type of short carbon chain amine) as a surfactant may increase the nanowire yield compared to other surfactants. The second organic solvent may comprise a non-coordinating solvent or a coordinating solvent. When a coordinating solvent is used, the size distribution (e.g., the cross-sectional dimension) of the nanowires may be wider than when using a non-coordinating solvent. In some embodiments, the second organic solvent comprises ODE. In some embodiments, the second organic solvent comprises oleylamine. In some embodiments, block 120 is performed in an inert gas environment.
At block 130, the first solution and the second solution are mixed. For example, the first solution may be poured into the second solution. A reaction between the cesium oleate or the rubidium oleate and the lead halide forms a plurality of nanowires comprising an inorganic lead halide perovskite. In some embodiments, block 130 is performed at about 130° C. to 250° C. In some embodiments, the mixture is held at about 130° C. to 250° C. for about 5 minutes to 20 hours. In some embodiments, block 130 is performed in an inert gas environment. In some embodiments, the plurality of nanowires formed at block 130 comprises ABX3, wherein A is Cs or Rb, wherein B is Pb, and wherein X is Cl, Br, or I.
For example, when fabricating CsPbBr3 or CsPbCl3 nanowires, different combinations of different amines may be used as surfactants. In some embodiments, octylamine and oleylamine are used. In some embodiments, decylamine or dodecylamine combined with oleylamine is used. In some embodiments, an acid is not used in the fabrication of CsPbBr3 or CsPbCl3 nanowires. In some embodiments, when fabricating CsPbI3 nanowires, oleylamine and oleic acid are used as surfactants.
Synthesis of CsPbX3 NWs using an embodiment of the method 100 shown in
The preparation of CsPbX3 NWs was performed under air-free conditions using Schlenk techniques by reacting cesium oleate with lead halide in the presence of oleic acid/octylamine and oleylamine in octadecene (ODE) at 130° C. to 250° C. A Cs-oleate solution was prepared by loading 0.4 g Cs2CO3 and 1.2 mL oleic acid (OA) in flask along with 15 mL ODE, degassing and drying under vacuum at 12° C. for 1 hour, and then heating under N2 to 150° C. until all Cs2CO3 reacted with OA.
5 mL ODE and 0.18 mmol PbX2 were loaded into a flask and degassed under vacuum for 1 hour at 120° C. Certain amounts of oleylamine (OLA) and oleic acid/octylamine were injected at 120° C. under N2 (typically, 0.3 mL OLA, 0.4 mL OA for CsPbBr3, 0.5 mL OLA, 0.8 mL OA for CsPbI3). The temperature was raised to 150° C. and 1 hour was allowed to elapse for complete dissolution of the PbX2 salt.
For the synthesis of CsPbBr3 and CsPbCl3 nanowires, the solution was kept at 150° C., and 0.6 mL of as-prepared Cs-oleate solution was quickly injected. After a certain duration, the reaction mixture was cooled by a water bath. For CsPbI3 nanowires, higher temperatures (>180° C.) were needed for relative uniform nanowire growth. In a typical case, after complete dissolution of PbI2 salt, the temperature was raised to 250° C., and 0.6 mL of as-prepared Cs-oleate solution was injected. After 5 minutes to 10 minutes, the reaction mixture was cooled by a water bath. To isolate the purify the CsPbX3 nanowires, the crude solution was cooled down with a water bath and aggregated NWs were separated by centrifugation at 6000 rpm for 5 minutes and washed twice with hexane.
To analyze the NWs' formation mechanism, the reaction was quenched to room temperature at different points in time, and the respective intermediates were separated by centrifugation and examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM).
These morphologies do not represent discrete intermediates formed at specific reaction times, but evolve sequentially from each other. Consequently, different intermediates can coexist in the product at a given time during the reaction. The growth of CsPbI3 NWs requires elevated temperatures (T>180° C.) and demonstrates much faster kinetics. As such, the reaction is less controllable and the size distribution of the NWs was wider, ranging from tens to hundreds of nanometers. The CsPbCl3 NWs have also been synthesized at 150° C., but the proportion of the NWs in the product at different reaction stages was always relatively low.
Catalyst-free, solution-phase syntheses can be used to prepare nanostructures with low aspect ratio, such as rods and dots. The formation of high aspect ratio NWs in solution may be achieved by oriented attachment of nanocrystals or by anisotropic growth driven by high monomer concentrations with the assistance of surfactant capping. It is believed that the formation of the CsPbX3 NWs here is likely not due to a dipole-driven one-dimensional oriented attachment of NCs, since no dimers or “oligomers” of NCs are observed in the products, and during the aging of a colloidal solution of the nanocrystals, there was no nanorod formation due to the dipole-driven attachment. In order to obtain a better understanding of the NWs' growth mechanism, further experiments have been conducted to investigate the influence of temperature, time, surfactants, and precursor concentration on the morphology of the product. A control experiment done by changing the reaction solvent from ODE to oleylamine shows much slower kinetics but with higher yield of NWs, which suggests the NW formation most likely proceeds through a surfactant-directed 1D growth mode.
Some parameters controlling nanowire formation are the reaction temperature, concentration of the reactants, and concentration and composition of the stabilizing agents. In the case of CsPbBr3 nanowire synthesis, if the reaction temperature is too high, the stable phase during reaction will be a highly symmetric cubic phase, which lacks the inherent anisotropy of crystal structure, and thus can only yield nanocubes and large crystals. Notably, the reported reaction temperature is the temperature of the oil/salt bath, the actual reaction temperature of the reaction system will be lower.
In order to gain better understanding of the growth mechanism, control experiments were performed. First, when only using oleylamine as surfactant, it turns out that the PbBr2 does not fully dissolve. This demonstrates that oleic acid is needed for coordinating with Pb2+ and decomposing the precursor to form monomers. Another control experiment was carried out by changing the reaction solvent from ODE to oleylamine while maintaining all other conditions. The reaction shows much slower kinetics and the yield of the nanowires is much higher (even though the size distribution becomes larger). ODE is a non-coordinating solvent, while oleylamine can serve as a capping ligand for Pb2+. The slower kinetics of the control experiment implies that the binding of oleylamine to Pb2+ can reduce the reactivity of the Pb2+ precursor, and maintain a higher monomer concentration after the nucleation stage, which is needed for anisotropic growth. Also, the significant increase of the yield of nanowires in the control experiments implies that oleylamine may preferentially bind to certain facets of CsPbX3, and favor the anisotropic structure growth along certain direction.
CsPbX3 bulk crystals exhibit a cubic perovskite structure in the highest temperature phase. Upon lowering the temperature, CsPbI3 undergoes one phase transition, cubic-orthorhombic (328° C.), with a color change from dark to yellow. CsPbBr3 has two phase transitions, cubic-tetragonal (130° C.)-orthorhombic (88° C.), with hardly any color change (orange). CsPbCl3 shows three successive phase transitions, cubic-tetragonal (47° C.)-orthorhombic (42° C.)-monoclinic (37° C.), with hardly any color change (pale yellow).
Both the yellow color of the crystal and the XRD pattern confirm that the CsPbI3 NWs are in the orthorhombic phase. The HR-TEM images show that the CsPbI3 NWs are single-crystalline, with uniform <100> growth direction. The exact phase of CsPbCl3 cannot be determined with the X-ray diffractometer that was used because the resolution of the instrument cannot differentiate the closely spaced peaks.
The optical properties of the CsPbX3 (X=Br, I) NWs were studied by measuring the UV-vis absorption and PL spectra of each material dispersed on a substrate, which is shown in
The narrow PL spectrum of CsPbBr3 (
The CsPbI3 PL spectrum (
In some embodiments, a method of fabricating nanowires (e.g., such as a method described herein) may yield cesium lead halide nanowires of a specific halogen that have uniform diameters and/or desirable optical properties. In some embodiments, the method may have a higher yield of nanowires for a specific halogen compared to other halogens. In such a case, cesium lead halide nanowires of a specific halogen (e.g., bromine) may be fabricated. An anion-exchange process can be performed on the CsPbBr3 nanowires to replace some of all of the bromine with another halogen, such as chlorine or iodine, for example.
At block 502 of the method 500 shown in
At block 504, the plurality of CsPbBr3 nanowires is contacted with a solution including a long-chain ammonium chloride or a long-chain ammonium iodide (i.e., a long-chain ammonium halide). Examples of long-chain ammonium chlorides and iodides include oleylammonium chloride and oleylammonium iodide. Additional examples of long-chain ammonium halides (i.e., chlorides and iodides) include C8 through C24 (C8-C24) linear alkyl ammonium halides and di-, tri-, and tert-alkyl ammonium halides. At least some of the Br in the nanostructure is replaced with Cl or I during an anion-exchange reaction.
In some embodiments, the solvent of the solution comprises a non-polar solvent. In some embodiments, the solvent comprises a C8 though C24 (C8-C24) olefin or octadecene (ODE). In some embodiments, a concentration of the long-chain ammonium chloride or long-chain ammonium iodide in the solution is about 0.1 milligrams per milliliter (mg/mL) to 10 mg/mL or about 0.1 mg/mL to 5 mg/mL. In some embodiments, the plurality of CsPbBr3 nanowires is contacted with the solution for about 1 minute to 7200 minutes, about 60 minutes to 5760 minutes, or about 240 minutes to 4320 minutes. In some embodiments, block 504 is performed in an inert gas environment. In some embodiments, the solution is at a temperature of about 40° C. to 80° C.
The length of time for which the plurality of CsPbBr3 nanowires is contacted with the solution determines, in part, the degree to which Br is replaced with Cl or I in the anion exchange process. Generally, the longer the period of time that the nanowire is contacted with the solution, the greater the degree to which the anion exchange process occurs with Br being replaced with Cl or I. The concentration of the long-chain ammonium chloride or long-chain ammonium iodide in the solution determines, in part, the degree to which Br is replaced with Cl or I in the anion exchange process. Generally, the higher the concentration of the long-chain ammonium chloride or long-chain ammonium iodide in the solution, the faster the anion exchange (i.e., Br being replaced with Cl or I) will occur. The concentration of the long-chain ammonium chloride or long-chain ammonium iodide in the solution is specified so that the anion exchange process does not occur too quickly and damage the morphology of the nanowires. For example, if Br is completely replaced with Cl or I in 10 minutes or less, the morphology of the nanowires may be damaged. In some embodiments, lower concentrations of the long-chain ammonium chloride or long-chain ammonium iodide in the solution and short exposure times may be used to generate a composition of the nanowires in which a small amount of Br is exchanged with Cl or I.
In some embodiments, nanowires of the plurality of nanowires have the same morphology and crystal structure before and after block 504. For example, in some embodiments, a CsPbBr3 nanowire has an orthorhombic crystal structure before block 504 and an orthorhombic crystal structure after at least some of the bromine has been exchanged with chlorine or iodine after block 504. In some embodiments, nanowires of the plurality of nanowires comprise or consist of single crystals.
In some embodiments to the method 500 shown in
The methods 500 and 510 described above with reference to
Anion exchange processes were performed on CsPbBr3 NWs using embodiments of the method 500 shown in
CsPbBr3 NWs were fabricated by loading 5 mL octadecene and 0.2 mmol PbX2 into a flask and degassing under vacuum for 20 minutes at 120° C. 0.8 mL dried octylamine and 0.8 mL dried oleylamine were injected at 120° C. under N2 successively. After the injection of octylamine, the solution gradually turned a bit milky, and with the injection of oleylamine, the solution turned clear. The temperature was then raised to 135° C. The solution was stirred for 20 minutes and became opaque white. The solution was kept at 135° C., and 0.7 mL of as-prepared Cs-oleate solution was injected. After 40 minutes to 60 minutes, the reaction mixture was cooled by a water bath. The NWs were isolated by centrifugation at 6000 rpm for 5 minutes and washed once with hexane. The precipitated NWs were then re-dispersed in hexane/toluene. The NWs had uniform diameters of 8 nm to 12 nm. The NWs had lengths of a few microns.
The anion exchange reaction was performed under air-free conditions using Schlenk techniques. PbX2 or an oleylammonium halide was used as the anion source and mixed with octadecene (5 mL) in a flask and kept under vacuum at 100° C. for 20 minutes. Surfactants such as oleic acid, oleylamine, and mixtures thereof were injected at 100° C. under N2 flow. After complete dissolution of the anion source, the temperature was lowered to 40° C. to 80° C. and CsPbBr3 NWs (0.01 mmol to 0.025 mmol) dispersed in hexane/toluene were injected to initiate the anion-exchange reaction. After reaction, the NWs were isolated by centrifugation at 6000 rpm for 5 minutes and washed once with hexane. The obtained precipitated NWs were re-dispersed in hexane/toluene for further analysis.
As shown in
The size and shape of the NWs was also preserved in both the Br—I and Br—Cl exchanges as shown in
The CsPbBr3 NWs anion-exchanged with chlorine and iodine had a tunable photoluminescence spanning over nearly the entire visible spectral region (409 nm to 680 nm). The as-grown CsPbBr3 NWs had an emission peak (UV excitation, λ=365 nm) at 519 nm, which gradually red shifted with increased I-content to reach a final value of 680 nm. When the I-precursor was replaced with the Cl-precursor, the photoluminescence peaks of the CsPbBr3 NWs blue shifted to shorter wavelengths to reach a shortest wavelength of 409 nm.
At block 802 of the method 800 shown in
In some embodiments, the plurality of halide perovskite nanostructures is a plurality of inorganic halide perovskite nanostructures. In some embodiments, the plurality of halide perovskite nanostructures comprises a plurality of larger-sized halide perovskite nanostructures, the plurality of halide perovskite nanowires, and a first plurality of halide perovskite nanoparticles. The plurality of halide perovskite nanowires includes a first plurality of halide perovskite nanowires, with nanowires of the first plurality of halide perovskite nanowires having diameters of 2 nm to 3 nm. In some embodiments, the plurality of halide perovskite nanostructures comprises the plurality of nanowires (e.g., nanowires having diameters of about 8 nm to 10 nm), nanoplates, and nanoparticles. For example, the halide perovskite nanostructures may be fabricated using embodiments of the method 100 shown in
Nanowires of the first plurality of halide perovskite nanowires are thin or small-diameter halide perovskite nanowires. The ligands used in the fabrication process determine in part the diameters of the halide perovskite nanowires generated. For example, using dodecylamine, oleylamine, and oleic acid surfactants when fabricating halide perovskite nanowires can generate nanowires having a diameter of about 2 nm to 3 nm. In some embodiments, other acids, such as hexanoic acid ligands or octanoic acid, are used instead of oleic acid. In some embodiments, decylamine is used instead of dodecylamine.
At block 804, the first plurality of nanostructures is separated from the first plurality of nanowires and the first plurality of nanoparticles. Nanostructures of the first plurality of nanostructures having a larger size (e.g., diameter) than the first plurality of nanowires and the first plurality of nanoparticles means that they have more mass and have less colloidal stability. In some embodiments, the nanowires of the plurality of nanowires having a larger size are also removed in this operation.
In some embodiments, the separation is performed with a centrifuge. A centrifuge applies an effective gravitational force to the liquid including the plurality of nanostructures. Under a gravitational force (or an effective gravitational force), the size and density of a nanostructure and the rate at which the nanostructure separates from a plurality of nanostructures are correlated; nanostructures having a larger size and density separate more quick from other nanostructures having a smaller size and/or density.
At block 806, a first specified amount of an anti-solvent is added to the liquid in which the first plurality of nanowires and the first plurality of nanoparticles are suspended. Again, the halide perovskite nanowires and nanoparticles may be dispersed or suspended in a non-polar liquid/solvent, and the ligands on the halide perovskite nanowires and nanoparticles make them soluble in a non-polar liquid. The anti-solvent decreases the solubility of the nanowires and nanoparticles in the liquid. In some embodiments, the anti-solvent is a polar solvent. In some embodiments, the anti-solvent does not dissolve the halide perovskite nanowires or nanoparticles.
In some embodiments, the anti-solvent has a polarity index of about 4 to 5. The polarity index is a measure of the polarity of the solute-solvent interactions. In some embodiments, the polarity index of the anti-solvent should not be higher than about 5 because the anti-solvent might then damage the halide perovskite nanowires. Also, if the polarity index of the anti-solvent is too high, adding a small first specified amount of the anti-solvent to the liquid will decrease the solubility of the halide perovskite nanowires to a degree such that the halide perovskite nanowires cannot be separated from the halide perovskite nanoparticles. In some embodiments, the polarity index of the anti-solvent should not be lower than about 4 because the solubility of the halide perovskite nanoparticles would not be decreased enough to separate them from the halide perovskite nanowires after adding a first specified amount of the anti-solvent to the liquid. In some embodiments, the anti-solvent is selected from a group consisting of ethyl acetate (polarity index=4.4), methyl ethyl ketone (polarity index=4.7), and methyl isobutyl ketone (polarity index=4.2).
At block 808, the first plurality of halide perovskite nanowires is separated from first nanoparticles of the first plurality of nanoparticles. The first nanoparticles have a first size (e.g., diameter) and sizes larger than the first size. In some embodiments, the first nanoparticles have more mass than nanowires of the first plurality of halide perovskite nanowires. In some embodiments, the separation is performed with a centrifuge. The first nanoparticles having more mass than nanowires of the first plurality of halide perovskite nanowires means that they will separate from the nanowires in the centrifuge.
For example, the halide perovskite nanowires and nanoparticles in the liquid may be centrifuged for about 2 minutes to 8 minutes, or about 5 minutes, at about 3000 rpm to 9000 rpm, or about 6000 rpm. The halide perovskite nanoparticles having the first size and sizes larger than the first size gather at the bottom of a centrifuge chamber after centrifugation. The halide perovskite nanowires remain suspended in the supernatant. In some embodiments, nanoparticles having a size smaller than the first size remain suspended in the supernatant. If some halide perovskite nanoparticles remain suspended in the supernatant, the centrifugation process may be repeated one to five more times.
For example, as shown in
At block 812, the first plurality of nanowires is separated from second nanoparticles of the first plurality of nanoparticles. The second nanoparticles have a second size, with the second size being smaller than the first size. In some embodiments, the separation is performed with a centrifuge. In some embodiments, the separation process at block 812 is similar to or the same as the separation process at block 808.
An embodiment of the method 800 shown in
Colloidal synthesis methods were used to fabricate ultrathin CsPbBr3 NWs. 5 mL ODE, 0.2 mmol PbBr2, and 4.3 g 1-dodecylamine were loaded into a flask and degassed under vacuum for 20 minutes at 100° C. 0.8 mL dried oleylamine and 0.2 mL dried oleic acid were injected at 160° C. under Ar successively. The solution was kept at 160° C. for 20 minutes for full dissolution of the precursor. Afterwards, 0.7 mL of Cs-oleate solution was quickly injected. After 20 minutes, the reaction mixture was cooled by a water bath.
The yield of the ultrathin NWs was generally low (e.g., only about several percent). The stepwise purification method was used to improve the purity of the sample to over 90%. By using different volumes of ethyl acetate (EA) as the anti-solvent, ultrathin NWs could gradually be separated from other impurities and larger NWs.
After the fabrication of the CsPbBr3 NWs, the product from the reaction was centrifuged at 6000 rpm for 5 minutes. The supernatant from the centrifugation was kept for further purification. 20 mL ethyl acetate was added to the supernatant as an anti-solvent (the volume ratio of the original supernatant to anti-solvent was about 1:4), and the clear supernatant solution immediately became cloudy. Afterwards, the solution was centrifuged at 6000 rpm for 5 min, and the supernatant was kept for further purification. Three similar operations were performed by adding extra ethyl acetate to the purified supernatant with an overall volume ratio of the original supernatant to anti-solvent being about 1:7, 1:10, and 1:35 in each successive operation.
To achieve wide chemical tunability, an anion-exchange process can be applied to the ultrathin CsPbBr3 NWs in which BP anions are replaced with either Cl− or I− ions. Halide exchange reactions were carried out at room temperature using PbX2 (X=Cl, I) as precursors. For example, a stock solution of anhydrous toluene (5 mL), PbX2 (0.188 mmol, X=Cl or I), OA (0.5 mL), and OAm (0.5 mL) was made. 0.05 mL to 0.6 mL of 10% diluted precursor solution was added to the purified ultrathin CsPbBr3 nanowire solution at room temperature.
The blue emission from the ultrathin CsPbBr3 NWs can be tuned through purple to red emission with different conversion degree. After the anion-exchange, the morphology of the ultrathin wires was largely preserved. The slight size expansion (Br—I exchange) and contraction (Br—Cl exchange) can also be observed from TEM images.
When an excess amount of anion-exchange precursor is added to the CsPbBr3 system, a competitive reaction mechanism other than anion-exchange reaction can be observed. This reaction mechanism damages to the ultrathin NWs and forms nanocubes and irregular nanocrystals. Coordinating an excessive amount of ligands on the ultrathin wires may gradually etch and dissolve the NWs, subsequently leading to formation thermodynamically more favorable morphologies, such as cubes or irregular crystals, for example. This competitive reaction can be largely suppressed by reducing the amount of the precursor. However, occasionally asymmetric red emission tails observed in the PL emission spectra imply that a small portion of the ultrathin NWs are still damaged during the anion-exchange process.
At block 904, a second solution comprising a tin halide and a surfactant in a second organic solvent is provided. The halide is selected from a group consisting of chlorine, bromine, and iodine. The surfactant may comprise an amine or a phosphine. For example, in some embodiments, the surfactant comprises a surfactant selected from a group consisting of octylamine, oleylamine, oleic acid, and combinations thereof. Other amines with long carbon chains (e.g., octadecylamine) may also be used. In some embodiments, the surfactant comprises trioctylphosphine. The second organic solvent may comprise a non-coordinating solvent or a coordinating solvent. In some embodiments, the second organic solvent comprises ODE. In some embodiments, the second organic solvent comprises oleylamine. In some embodiments, at block 904 a second solution comprising a tin halide and a surfactant and no second organic solvent is provided. For example, SnI2 in trioctylphosphine may be provided. In some embodiments, block 904 is performed in an inert gas environment.
At block 906, the first solution and the second solution are mixed. For example, the first solution may be poured or injected into the second solution. A reaction between the cesium oleate and the tin halide forms a nanowire in some embodiments and a nanoplate in some embodiments comprising an inorganic tin halide perovskite. In some embodiments, the mixture is held at about 130° C. to 250° C. for about 5 minutes to 20 hours. In some embodiments, block 906 is performed in an inert gas environment. In some embodiments, a plurality of nanowires formed at block 906 comprises ABX3, wherein A is Cs, wherein B is Sn, and wherein X is Cl, Br, or I. The method 900 may also be used to fabricate tin halide perovskite nanobelts/nanoribbons and nanosheets. In some embodiments of the method 900, no trioctylphosphine is present. In some embodiments of the method 900, all operations are performed in an inert gas atmosphere with little to no water being present.
In some embodiments, a fabrication process for inorganic tin halide perovskite nanowires and nanoplates includes providing a first solution comprising a cesium salt dissolved in organic amines and other organic solvents, providing a second solution comprising a tin halide and a surfactant in an organic solvent, and mixing the first solution and the second solution, a reaction between the solvated cesium precursor and the tin halide precursor forming a plurality of nanowires or nanoplates comprising an inorganic tin halide perovskite.
In some embodiments, a method of fabricating inorganic tin halide perovskite nanowires includes dissolving cesium carbonate in octanoic acid and octylamine. In some embodiments, tin is dissolved in trioctylphosphine. The tin dissolved in trioctylphosphine is injected in the cesium solution.
In some embodiments, depositing lead iodide on a substrate comprises depositing a solution of lead iodide in a solvent on the substrate and then evaporating the solvent. Depositing the solution on the substrate may be performed with a spin coating process, for example. In some embodiments, the solvent comprises a solvent selected from a group consisting of dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). In some embodiments, evaporating the solvent comprises annealing the substrate at about 70° C. to 100° C. for about 10 minutes to 20 minutes.
In some embodiments, the substrate comprises a material that the solution of lead iodide in a solvent wets. In some embodiments, the substrate comprises a glass. In some embodiments, the glass is oxygen plasma treated. An oxygen plasma treatment may clean the glass and improve its wetting properties (i.e., decrease the hydrophobicity of the glass and reduce the contact angle between the glass and the solution). In some embodiments, a polymer may be disposed on a surface of the substrate. The lead iodide solution may wet the polymer. For example, in some embodiments, the polymer comprises a hydrophilic polymer. In some embodiments, the polymer comprises a polymer selected from a group consisting of poly(3,4-ethylenedioxythiophene) (PEDOT), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and polyaniline.
At block 1104, the lead iodide is contacted with a solution of a cesium halide or a rubidium halide in a first alcohol. In some embodiments, the lead iodide is contacted with the solution of the cesium halide or the rubidium halide in the first alcohol at about 20° C. to 80° C. for about 6 hours to 18 hours. For example, the substrate with the lead iodide disposed thereon may be immersed in a solution of the cesium halide or the rubidium halide in the first alcohol. The halide of the cesium halide is a halide selected from a group consisting of chlorine, bromine, and iodine. A reaction between the lead iodide and the cesium halide or the rubidium halide forms a plurality of nanowires comprising an inorganic lead halide perovskite. The first alcohol may comprise a short chain alcohol. For example, in some embodiments, the first alcohol comprises methyl alcohol or ethyl alcohol. In some embodiments, the plurality of nanowires comprises ABX3, wherein A is Cs or Rb, wherein B is Pb, and wherein X is Cl, Br, or I. In some embodiments, block 220 is performed in an inert gas atmosphere.
In some embodiments, after block 1104, the plurality of nanowires is washed with a second alcohol. The second alcohol is used to clean the plurality of nanowire without damaging the. The second alcohol may be an alcohol that is less polar than methyl alcohol. For example, in some embodiments, the second alcohol comprises isopropyl alcohol.
Is some embodiments of the method 1100, tin iodide is deposited on the substrate at block 1102. Using tin iodide in the method 1100 would form inorganic tin halide perovskite nanowires.
An embodiment of the method 1100 shown in
460 mg PbI2 was dissolved in 1 mL anhydrous dimethylformide (DMF) and stirred at 70° C. overnight before further use. The PbI2 solution was spin coated onto the PEDOT:PSS-coated glass substrates at 1000 rpm for 120 seconds and then annealed at 100° C. for 15 minutes. The PbI2 film was submerged in a glass vial with 8 mg/mL CsBr solution in methanol, with the PbI2 side facing up. The capped reaction vial was heated at 50° C. for 12 hours, and the substrate was removed and washed in anhydrous isopropanol for 30 seconds. The sample was then dried by heating to 50° C. for 5 minutes. For the synthesis of CsPbCl3 (or CsPbI3) nanowires, the CsBr-methanol solution was replaced with a 6 mg/mL CsCl-methanol (or 4 mg/mL CsI-methanol) solution. The entire growth process was carried out in a nitrogen-filled glove box.
From the scanning electron microscopy (SEM) images shown in
The elemental composition and atomic structure of the nanowires were quantified to ensure they conformed to the expected, room-temperature orthorhombic phase of CsPbBr3. This was important as the reaction mixture contained a small amount of iodide from the initial PbI2 film, making the chance of a Br—I alloy non-negligible. The X-ray diffraction (XRD) pattern of the CsPbBr3 growth substrate shows strong diffraction peaks which can be assigned to the pure orthorhombic crystal structure (space group Pbnm), and does not contain impurity peaks from either the PbI2 or CsBr starting materials (
Single crystal nanowires of CsPbCl3 were successfully synthesized by replacing CsBr with CsCl during the reaction, with the XRD pattern confirming the expected tetragonal phase of the as-grown CsPbCl3.
Attempts were also made to synthesize CsPbI3 nanowires to achieve red emission and full coverage of the visible spectrum.
A dense mesh of rough nanowires was obtained upon replacing CsBr with CsI (
Three parameters affect the CsPbBr3 nanowire growth process: CsBr-methanol concentration, reaction time, and reaction temperature. A specific CsBr-methanol concentration window is needed to synthesize the CsPbBr3 perovskite composition. Pure CsPbBr3 can be obtained under medium concentration (6 mg/mL to 10 mg/mL), without formation of the undesired CsPb2Br4 or Cs4PbBr6 stoichiometries. The dominate product was found to be CsPb2Br5 at low concentration (<4 mg/mL), and a mixture of Cs4PbBr6 and CsPbBr3 at high concentration (>12 mg/mL). One reaction scheme consistent with these results is:
The right side the equation depicts the reaction of CsPbBr3 with surplus CsBr to form Cs4PbBr6 at high concentrations of CsBr. The left side shows the release of CsBr from CsPbBr3 at low concentrations to form CsPb2Br5. This hypothesis is consistent with the PbBr2—CsBr phase diagram: all three compositions are thermodynamically stable at room temperature. Additionally, it has already been shown that certain chemical potentials (or concentrations) of CsBr correspond certain compositions.
During the initial stages of the reaction, the film color transitioned rapidly to red, then gradually to yellow. It is hypothesized that an intermediate CsPbIxBr3-x forms initially, then evolves to pure CsPbBr3, as confirmed by XRD patterns at different growth times. The peaks corresponding to PbI2 disappeared within the first 2 minutes, indicating that PbI2 dissolves/reacts rapidly. For the intermediate products from 2 minutes to 10 minutes, the (001) peak was shifted ˜0.2° below that of pure CsPbBr3; this is indicative of lattice expansion caused by the formation of CsPbIxBr3-x. Notably, researchers have reported intermediate CH3NH3PbI2Br products during the growth of single crystal CsPbBr3 nanostructures. Therefore, inorganic perovskite nanowires likely possess similar growth dynamics to hybrid perovskites.
Attempts to grow CsPbBr3 using PbBr2 and to grow CsPbCl3 using PbCl2 were made in order to simplify the synthetic process. However, neither provided as high of quality nanoscale products as PbI2. When a film of PbBr2 was used, the morphology of the as-grown CsPbBr3 was found to be amorphous or polycrystalline rather than single crystalline as with PbI2. For PbCl2, limited solubility in DMF made spin coating a sufficiently thick film difficult.
It was determined that mild heating was useful for the formation of CsPbX3 nanowires, as it speeds the dissolution-recrystallization process. One consequence of heating the reaction is the introduction of dislocations during nanowire growth. Mild heating likely enhances the solubility of the PbI2 precursor in methanol, thus driving the resolution-recrystallization process and ultimately forming CsPbBr3 nanostructures away from the film. For the nanowire geometry growth mechanism, screw dislocation-driven growth was proposed for the formation of CH3NH3PbX3 nanowires as the spontaneous growth of single-crystal hollow tubes was observed. As similar single-crystal tube structures in the CsPbBr3 growth process were observed, it is reasonable to hypothesize that inorganic perovskite nanowire growth is driven by an analogous mechanism.
In some embodiments, depositing cesium halide or the rubidium halide on a substrate comprises depositing a solution of the cesium halide or the rubidium halide in a solvent on the substrate and then evaporating the solvent. Depositing the solution on the substrate may be performed with a spin coating process, for example. In some embodiments, the solvent comprises a solvent selected from a group consisting of methyl alcohol and dimethyl sulfoxide (DMSO). In some embodiments, evaporating the solvent comprises annealing the substrate at about 150° C. or less.
In some embodiments, the substrate comprises a material that the solution of the cesium halide or the rubidium halide in the solvent wets. In some embodiments, the substrate comprises a glass. In some embodiments, the substrate comprises silicon. In some embodiments, the glass is oxygen plasma treated. An oxygen plasma treatment may clean the glass and improve its wetting properties. In some embodiments, a polymer may be disposed on a surface of the substrate. The cesium halide or the rubidium halide solution may wet the polymer. For example, in some embodiments, the polymer comprises a hydrophilic polymer. In some embodiments, the polymer comprises a polymer selected from a group consisting of poly(3,4-ethylenedioxythiophene) (PEDOT), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and polyaniline.
At block 1504, the cesium halide or the rubidium halide is contacted with a solution of a tin halide or a lead halide in a first alcohol. In some embodiments, the cesium halide or the rubidium halide is contacted with the solution of the tin halide or a lead halide in the first alcohol at less than about 100° C. for about 30 minutes to 150 minutes. For example, the substrate with the cesium halide or the rubidium halide disposed thereon may be immersed in a solution of the tin halide or the lead halide in the first alcohol. The halide of the tin halide or the lead halide is selected from a group consisting of chlorine, bromine, and iodine. A reaction between the cesium halide or the rubidium halide and the tin halide or the lead halide forms a plurality of nanowires comprising an inorganic halide perovskite. The first alcohol may comprise an anhydrous long chain alcohol. For example, in some embodiments, the first alcohol comprises propanol (e.g., 1-propanol or 2-propanol) or butanol (1-butanol, 2-butanol, 2-methyl-1-propanol, or 2-methyl-2-propanol). In some embodiments, the plurality of nanowires comprises ABX3, wherein A is Cs or Rb, wherein B is Sn or Pb, and wherein X is Cl, Br, or I. In some embodiments, block 1504 is performed in an inert gas atmosphere.
In some embodiments, after block 1504, the plurality of nanowires is washed with a second alcohol. The second alcohol is used to clean the plurality of nanowires without damaging them. The second alcohol may comprise an anhydrous long chain alcohol. For example, in some embodiments, the second alcohol comprises propanol (e.g., 1-propanol or 2-propanol) or butanol (1-butanol, 2-butanol, 2-methyl-1-propanol, or 2-methyl-2-propanol). For example, in some embodiments, the second alcohol comprises isopropyl alcohol.
Using an embodiment of the method 1500 shown in
The CsSnI3 nanowires were synthesized on clean substrates that were loaded with a layer of CsI and allowed to react in a solution of SnI2 in anhydrous 2-propanol. Specifically, to grow CsSnI3 nanowires, Si or SiO2 substrates were cleaned. After cleaning, the following operations were performed in an argon-filled glove box with an oxygen level of <0.1 ppm and a H2O level of <2.0 ppm. A saturated solution of CsI in anhydrous methanol was prepared by allowing the solution to stir for at least 1 hour. Afterwards, the clean substrates were heated on a clean hotplate to 100° C. and allowed to equilibrate for 10 minutes. 70 μL of the saturated CsI solution was pipetted dropwise onto the hot substrates, which wetted the surface completely, without spilling. For substrates of different size, the volume of the CsI/MeOH was scaled to appropriately to wet the surface of the substrate without spilling. The substrates were allowed to completely dry for up to 30 minutes on the hotplate. Anhydrous dimethyl sulfoxide (DMSO) also can be used as a substitute for anhydrous methanol for this step. Afterwards, the CsI-coated substrates were placed in a clean 20 mL vial with the CsI-coated side facing up. The vial containing the CsI-coated chip was heated to 60° C. before the reaction began.
Separately, a saturated stock solution of SnI2 in anhydrous 2-propanol was prepared (6.6 mmol/L) by stirring overnight, and the solution was diluted to 4 mmol/L to 0.3 mmol/L by diluting with anhydrous 2-propanol. To begin the reaction, 1 mL of SnI2/2-propanol solution was pipetted onto the warm CsI-coated substrates, and the vial was capped and allowed to react for 90 minutes to 120 minutes. The substrates darkened with the nucleation and growth of CsSnI3 in the SnI2/2-propanol solution.
To stop the reaction, the substrate was lifted out of the solution and tipped towards the corner. Any excess solution was wiped away with a cloth to absorb the growth solution while minimizing the deposition of salts on the chip after evaporation of the growth solution. Optionally, the sample can be quickly washed in anhydrous 2-propanol and dried in the same manner. After synthesis, the sample was stored and transported in a sealed centrifuge tube to minimize air/humidity exposure.
The CsSnI3 nanowires were characterized.
Using the methods described above, inorganic halide perovskite nanowires can be fabricated that are large enough (i.e., a large enough diameter or a large enough cross-sectional dimension) to support photonic lasing. In order to achieve lasing with some halide perovskite compositions, nanowires with diameters greater than about 180 nm are needed. For example, some embodiments of the method 1100 shown in
Photonic lasing arises from the unique ability of a nanowire to act as both gain medium and laser cavity. Forming the nanowire from a stable, highly absorptive and emissive material allows for stimulated emission to occur upon reaching a sufficient carrier density. The nanowire geometry defines the laser cavity which is bounded on either end by the nanowire end facets. The difference in refractive index between the nanowire and its environment (atmosphere, substrate, etc.) generates significant end facet reflectivity as well as providing efficient wave guiding along the length of the wire. For CsPbBr3 nanowires, lasing was achieved via optical excitation from a femtosecond pulsed laser. At excitation densities below the lasing threshold, spontaneous emission dominates, and the nanowire was uniformly emissive. Upon surpassing the threshold, however, stimulated emission takes over and a periodic pattern was observed, which is caused by interference of the coherent emission from the two end facets of the nanowire.
The dependence of the PL spectral response on increasing excitation fluence is shown in
Analysis of the individual peak widths from the spectra in
Time-resolved PL was used to measure the carrier population dynamics of the laser cavity above and below the lasing threshold. Below the lasing threshold, the nanowire PL signal decays biexponentially with the rapid component (154±2 ps, 81%) contributing much more than the slower component (970±20 ps, 19%). The rapid component is attributed to surface state recombination and the longer component to bulk recombination. When the excitation fluence is increased past the lasing threshold, an additional decay component is observed (10.4±0.3 ps, 87%), which decays more rapidly than the other components (110±2 ps, 11%; 720±20 ps, 2%) as well as the instrument response (˜30 ps). This rapid decay corresponds to the stimulated emission process depleting carriers that would otherwise undergo spontaneous emission. The lasing threshold and Q-factor place these nanowires in the upper tier of nanowire lasers and clearly demonstrate the effectiveness of CsPbBr3 as a gain medium.
The mechanism for optical gain by stimulated emission in perovskite nanostructures is still under investigation. In CsPbX3 quantum dots (QDs), support for both single exciton and biexciton lasing has been demonstrated. In the case of biexciton-driven stimulated emission, a 50 meV red-shifted emission band was assigned to biexciton recombination, in agreement with expected biexciton binding energies in CsPbX3 QDs. While a similar asymmetric red-shifted emission band was observed, the red-shift of 69 meV is larger than expected for biexciton binding energies in non-colloidal CsPbBr3 nanowires. Instead, it is postulated that an electron-hole plasma (EHP) mechanism is responsible for stimulated emission, as proposed in well-known compositions such as ZnO and GaN as well as in recent CH3NH3PbX3 nanowires. An EHP is formed in bulk crystals when the carrier density surpasses the Mott density. Here, the carrier density at the lasing threshold was estimated to be ˜1×1018 cm−3 by FDTE simulation, an order of magnitude greater than the bulk CsPbBr3 Mott density of ˜2×1017 cm−3. In addition, the formation of an EHP is expected to lead to a blue-shift in the cavity modes due to a decrease in the refractive index that arises from exciton absorption saturation. A blue-shift of 2 nm with increasing excitation fluence was observed, and it is therefore reasonable that an EHP mechanism is responsible for stimulated emission in CsPbX3 nanowires. Further study is needed, however, to elucidate the stimulated emission carrier dynamics.
Cesium lead halide materials have been shown to be stable when exposed to moisture or heat, making them attractive for real-world application. The CsPbBr3 nanowires are stable for days under ambient atmosphere and illumination without loss of morphology. Additionally, the material composition remains intact over the course of months and does not separate into PbI2 and other byproducts as has been demonstrated for some hybrid materials. CsPbBr3 nanowires are also stable under constant, pulsed excitation above the lasing threshold. Nanowires were excited to 1.2 Pth under either a closed nitrogen atmosphere or ambient atmosphere (50±1% relative humidity) and the integrated emission intensity was monitored. The results indicate significant operating lifetimes even under atmospheric conditions. In both cases, initial burn-in was observed where the integrated emission amplitude exceeds 100%. Under nitrogen, nanowire lasing continues uninterrupted for longer than one hour or over a billion excitation cycles, and the nanowire was undamaged after this time. Under atmospheric conditions, the burn-in time was accelerated, but the decrease in lasing intensity did not occur until longer than 25 minutes of continuous exposure to high-energy pulsed excitation. In contrast, the long-term lasing stability of methyl ammonium lead halide nanowire lasers has never been demonstrated. The stable, continuous lasing operation observed here is promising for future applications as it suggests both high photo- and thermal stability under both ideal and sub-optimal environmental conditions.
Lasing was also demonstrated for CsPbCl3 nanowires. Upon focused excitation, lasing occurred near 430 nm with the emergence of narrow peaks similar to CsPbBr3 nanowires, as shown in
Embodiments of the methods described herein (e.g., the method 100 shown in
In an example embodiment, the colloidal synthesis of CsPbBr3 perovskite nanoplates was performed using air-free techniques. First, a cessium oleate solution was prepared. Briefly, 0.4 g Cs2CO3 and 1.2 mL OA were loaded into a 3-neck flask along with 15 mL ODE, degassed under vacuum at 120° C. for 1 h, following a second degassing phase at 150° C. under Ar until all Cs2CO3 reacted with OA.
ODE (5 mL) and PbBr2 (0.069 g) were loaded into 25 mL 3-neck flask and dried under vacuum for 1 hour at 120° C. Dried oleylamine (0.5 mL) and dried OA (0.5 mL) were injected at 120° C. under Ar. After complete solubilisation of a PbBr2 salt, the temperature was changed to 90° C. to 130° C. and hot (˜100° C.) Cs-oleate solution (0.4 mL, 0.125 M in ODE, prepared as described above) was quickly injected. The reaction mixture then was immediately cooled by the ice-water bath. The temperatures of 90° C. to 130° C. tend to strongly favor asymmetric growth producing quasi 2D geometries.
The nanoplates were extracted from the crude solution by centrifuging at 8500 RPM for 5 minutes. Lower temperature reactions (where the crude solution concentration is smaller) and where thinner nanoplates are formed demand longer centrifugation times and cooling the solution to 17° C. (above freezing point of ODE). After centrifugation, the supernatant was discarded and the particles were redispersed in hexane forming stable colloidal solutions. Further cleaning of the perovskite nanoplates can be performed.
The ionic nature of the metathesis reaction dictates the rapid nucleation and growth kinetics of the resulting nanocrystals. The reaction temperature plays a critical role in determining the shape and thickness of the resulting nanoplates. Reactions conducted at 150° C. produce mostly symmetrical nanocubes with green-color photoluminescent (PL) emission. Reactions conducted at lower temperatures present blue-shifted PL spectra. For example, at 130° C. lower symmetry nanoplates with cyan emission are formed. At 90° C. and 100° C., very thin nanoplates were detected along with lamellar structures ranging 200 nm to 300 nm in length. As TEM images showed, nanoplates grow along and inside these lamellar structures, suggesting that organic mesostructures serve as growth directing soft templates that break the crystal's inherent cubic symmetry and dictate the 2D growth. Such a mechanism is not without precedent, where a similar soft templating mechanism was reported for wurtzite CdSe nanoplates. Reaction temperatures as low as 70° C. resulted in almost transparent suspensions, where TEM showed amorphous micron size sheets with almost no crystals present. These objects may be unreacted precursors. Interestingly, reactions at temperatures of 170° C. to 200° C. produced larger nanocubes and at longer reactions times high aspect ratio nanowires. Recent reports suggest these geometries evolve sequentially from each other.
Further details regarding the embodiments described herein can be found in the following references, all of which are herein incorporated by reference:
In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of invention.
This application claims priority to U.S. Provisional Patent Application Ser. No. 62/290,009, filed Feb. 2, 2016, to U.S. Provisional Patent Application Ser. No. 62/342,094, filed May 26, 2016, to U.S. Provisional Patent Application Ser. No. 62/399,845, filed Sep. 26, 2016, and to U.S. Provisional Patent Application Ser. No. 62/428,939, filed Dec. 1, 2016, all of which are herein incorporated by reference.
This invention was made with government support under Contract No. DE-AC02-05CH11231 awarded by the U.S. Department of Energy. The government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 62290009 | Feb 2016 | US | |
| 62342094 | May 2016 | US | |
| 62399845 | Sep 2016 | US | |
| 62428939 | Dec 2016 | US |
