Patent Application
20110017286
The synthesis of nanocrystal heterostructures, having two or more components within each particle, is desirable both for creating multi-functional materials and for controlling electronic coupling between nanoscale units (Yin, Y.; Alivisatos, A. P. Nature, 437:664-670 (2005); Jun et al., J. Chemical Communications, 1203-1214 (2007); Cozzoli et al., Chemical Society Reviews, 35:1195-1208 (2006); Casavola et al., European Journal of Inorganic Chemistry, 837-854 (2008)). As the complexity of colloidal nanocrystal heterostructures increases beyond simple spherical core-shell morphologies, their electronic structure and physical properties will strongly depend on the spatial organization of the two materials within each nanocrystal. Colloidal nanocrystals possessing anisotropic shapes provide a platform for selective chemical modification based on the relative reactivities of the different crystalline facets exposed at the surface. This enables the synthesis of multi-component nanostructures through the nucleation and growth of a secondary material on specific facets of the nanocrystals (Cozzoli et al., Chemical Society Reviews, 35:1195-1208 (2006); Casavola et al., European Journal of Inorganic Chemistry, 837-854 (2008); Shi et al., Nano Letters, 6:875-881 (2006); Milliron et al., Nature, 430:190-195 2004; Mokari et al., Science, 304:1787-1790 (2004); Kudera et al., Nano Letters, 5:445-449 (2005); Shieh et al., Journal of Physical Chemistry B, 109:8538-8542 (2005); Talapin et al., Nano Letters, 7:2951-2959 (2007)). While the methodology of sequential growth has been applied to a wide range of material combinations, its drawback is that the desired heterogeneous nucleation on the existing nanocrystal surface often competes with homogenous nucleation of separate nanocrystals of the secondary material.
An alternative method for synthesizing nanocrystal heterostructures, which circumvents separate nucleation, is the transformation of a portion of the nanocrystal into a new composition or structural phase (Sun, Y.; Xia, Y., Science, 298:2176-2179 (2002); Yin et al., Science, 304:711-714 (2004); Cable, R. E.; Schaak, R. E., Journal of the American Chemical Society, 128:9588-9589 (2006); Mews et al., Journal of Physical Chemistry, 98:934-941 (1994); Dloczik, L.; Koenenkamp, R. Journal of Solid State Electrochemistry, 8:142-146 (2004); Son et al., Science, 306:1009-1012 (2004)). In ionic nanocrystals, cation exchange reactions have been used to alter the composition of the material by replacing the cations within the nanocrystal lattice with a different metal ion (Robinson et al., A. P. Science, 317:355-358 (2007); Mews et al., Journal of Physical Chemistry, 98:934-941 (1994); Dloczik, L.; Koenenkamp, R. Journal of Solid State Electrochemistry, 8:142-146 (2004); Son et al., Science, 306:1009-1012 (2004); Wark et al., Journal of the American Chemical Society, 130:9550-9555 (2008); Camargo et al., Langmuir, 23:2985-2992 (2007); Pietryga et al., Journal of the American Chemical Society, 130:4879-4885 (2008)). For example, the addition of a small molar excess of Ag+ cations to cadmium chalcogenide nanocrystals (CdS, CdSe, CdTe) leads to their complete conversion to the corresponding silver chalcogenide (Son et al., Science, 306:1009-1012 (2004)). Remarkably, the shape of anisotropic nanocrystals such as rods and tetrapods is preserved after cation exchange when their dimensions are greater than the reaction zone for exchange (˜4 nm), indicating that the cohesion of the crystal is maintained during the diffusion and exchange of cations. The relative rigidity of the anion sublattice enables the partial transformation of the nanocrystal to create a heterostructure where the two compounds share a common anion. Adjusting the ratio of substitutional cations to those within the nanocrystals can be used to control the relative volume fraction of the two crystals within the binary heterostructures (Robinson et al., A. P. Science, 317:355-358 (2007)). The spatial arrangement of materials within the nanocrystal will depend on a number of kinetic and thermodynamic factors such as the relative activation barriers for cation exchange to initiate at different facets of the nanocrystal and the energetic stability of interfaces as reaction fronts proceed through the nanocrystal. In the case of Ag+ exchange in CdS nanorods, the reorganization of Ag2S and CdS regions via cation diffusion causes significant changes in the morphology of the heterostructures as the fraction of Ag2S increases within each nanorod (Robinson et al., A. P. Science, 317:355-358 (2007); Demchenko et al., ACS Nano, 2:627-636 (2008)). Low amounts of Ag+ produce small Ag2S regions dotting the surface of the nanocrystals, whereas greater amounts of Ag+ lead to alternating segments of CdS and Ag2S along the nanorod. The large lattice strain between CdS and Ag2S is believed to play an notable role in forming the striped pattern observed for this system. Thus, it is interesting to examine a case where the lattices of the cation exchange pair have little mismatch between them.
The use of colloidal nanocrystals in solar cell devices is a highly active area of research. Colloidal semiconductor nanocrystals are attractive materials as the active layer in solar cells as they allow solution-phase processing to be used, which may significantly lower fabrication costs. Previous solar cell devices incorporating nanocrystals have either used blends of nanocrystals with organic polymers or bilayers of two different types of semiconductor nanocrystals. In these cases, the contact between the two active components (electron-accepting and electron donating) was not well-defined and could vary with device batch. Forming well-defined and strong contact between the electron-accepting and electron-donating components of a nanocrystal-based solar cell device is desirable for increasing performance. The present inventors have produced binary nanocrystal heterostructures, which contain both electron-accepting and electron-donating regions within a single nanocrystal. In this case, the connectivity between the two materials is well-defined and can be controlled. Such a configuration has many benefits, which can lead to power conversion efficiencies closer to theoretical limits. These benefits include: more efficient charge separation, reduction in surface trap states common at the interface of two heterogeneous materials, and improved charge mobility.
Embodiments of the invention address these and other problems, individually and collectively.
Embodiments of the invention include the synthesis of CdS—Cu2S nanorod heterostructures synthesized by partial Cu+ cation exchange. The Cu2S regions primarily occur at one or both ends of the nanorods and appear to nucleate and grow along a single crystallographic direction. The values of CdS—Cu2S interface formation energies provided by theoretical modeling suggest that the asymmetric CdS—Cu2S heterostructures that are observed are produced by selective Cu2S nucleation on the (000
One embodiment of the invention is directed to a composite nanorod comprising: a linear body including three or less alternating regions including a first region and a second region, wherein the first region comprises a first (ionic) material comprising a first ionic material and the second region comprises a second (ionic) material comprising a second ionic material. The first and second ionic materials may be ionic semiconducting materials.
Another embodiment of the invention is directed to a method comprising: forming a mixture comprising nanorods comprising a first (ionic) material comprising first ions, coordinating molecules, and second ions in a solvent; and forming composite nanorods in the solvent, wherein each composite nanorod comprises a linear body comprising a first region comprising the first material and a second region comprising a second (ionic) material, wherein the second material comprises the second ions, and wherein the linear body including three or less alternating regions.
Another embodiment of the invention is directed to a composite nanorod comprising: a linear body including a first region and a second region, wherein the first region comprises a first (ionic) material comprising cadmium sulfide and the second region comprises a second (ionic) material comprising copper sulfide.
Another embodiment of the invention is directed to a method comprising: forming a mixture comprising nanorods comprising a first (ionic) material comprising first ions, coordinating molecules, and second ions in a solvent; and forming composite nanorods in the solvent, wherein each composite nanorod comprises a linear body comprising a first region comprising the first material and a second region comprising a second (ionic) material, wherein the first material comprises cadmium sulfide and the second material comprises copper sulfide.
Other embodiments of the invention are directed to devices including such composite nanorods.
Some embodiments of the invention are directed to binary nanorod heterostructures, which are rod shaped nanocrystals comprising at least two semiconducting materials. The process can start with nanorods composed of a single material and uses a simple chemical transformation to convert the nanorods to the binary heterostructures. Referring to
These and other embodiments of the invention are described in further detail below.
a) and 1(b) show a schematic of the cation exchange process for making CdS—Cu2S nanorods.
a)-4(d) show bright field transmission electron microscopy (TEM) images before and after complete Cu+ cation exchange, along with size distributions of the nanorods.
a)-5(d) show composite energy-filtered images of the CdS and Cu2S portions of the binary nanorods, where the ends of the nanorods have been converted to Cu2S. A high-resolution TEM image of a CdS—Cu2S nanorod heterostructure is also shown
a)-6(e) show Cu-EFTEM images for three CdS—Cu2S binary nanorod samples along with histograms of the asymmetry of the length of the Cu2S segments and total Cu2S length fraction within individual nanorods for each of the samples shown.
As used herein, the term “nanorod” is used herein to mean any linear nanostructure. An exemplary nanorod according to an embodiment of the invention may exist only as a nanorod may exists as an arm or other part of a larger two or three dimensional particle such as a tetrapod particle or other type of particle.
In some embodiments of the invention, there can be three of less alternating regions (or layers) per nanorod, and these alternating regions may be formed in a liquid medium. Adjacent alternating regions may comprise different materials. The alternating regions can comprise copper sulfide and cadmium sulfide.
The composite nanoparticles according to embodiments can be used for any suitable purpose. For example, they can be used to label biological materials, as electronic components in photovoltaic devices or light-emitting diodes, in electronic devices. etc.
The partial transformation of ionic nanocrystals through cation exchange has been used to synthesize nanocrystal heterostructures. The present inventors demonstrate that the selectivity for cation exchange to take place at specific facets of the nanocrystal plays a notable role in controlling the resulting morphology of the binary heterostructure. In the case of copper I (Cu+) cation exchange in cadmium sulfide (CdS) nanorods, the reaction starts preferentially at the ends of the nanorods such that copper sulfide (Cu2S) grows inwards from either end. The epitaxial attachments of Cu2S to the end facets of CdS nanorods minimize the interface formation energy, making these interfaces stable throughout the exchange reaction. Additionally, as the two end facets of wurtzite CdS nanorods are crystallographically nonequivalent, asymmetric heterostructures can be produced. The creation of asymmetric, elongated nanostructures, such as the CdS—Cu2S nanorods presented herein, is desirable for novel nanocrystal-based devices, including solar cells, which utilize the separation and extraction of photo-generated charge carriers.
Cation exchange provides a facile method for systematically varying the proportion of two chemical compositions within a single nanocrystal. It has been shown that cation exchange can be used to fully (and reversibly) convert CdS (or alternatively CdSe or CdTe) nanocrystals to Cu2S nanocrystals by a complete replacement reaction of the Cd2+ cations with Cu+ cations. The resultant material is the copper-anion analog of the starting material. The size and shape of the nanocrystal can be preserved when the nanocrystal has minimum dimensions greater than about 4 nm. The high mobility of cations in the CdS (as well as CdSe and CdTe) lattice suggests that partial cation exchange may lead to interesting patterns of segregated domains of copper chalcogenide within a cadmium chalcogenide nanorod. Thus, it is possible to convert a previously formed nanorod of a single chemical composition into a nanorod heterostructure, such as CdS—Cu2S by partial cation exchange. The exchange reaction is selective for the end facets of the anisotropic nanorods such that the Cu2S regions grow inwards from either end. This facet selective chemical reaction can be extended to convert CdSe and CdTe nanorods into Cu2Se or Cu2Te containing nanorod heterostructures.
The above-described nanorods may be formed using any suitable process. In some embodiments, before the mixture is formed, precursor nanorods may first be formed in solution. For example, the prercursor nanorods can be formed using the methods described in U.S. Pat. Nos. 6,225,198 and 6,306,736. The nanorods may be purely linear structures, or may be arms in two or three-dimensional nanostructures, such as in a nanotetrapod. Such precursor nanorods may consist only of one material (e.g., only CdS) such as one compound semiconductor material. The material in the precursor nanorods may correspond to a first material. The first materials may contain ions (e.g., Cd2+), which are exchanged during the composite nanorod formation process.
After the precursor nanorods are formed, they may remain in the solution in which they were formed. Alternatively, the precursor nanorods may be in a dry state, and may then be mixed with a solvent to form a solution. For example, to produce the aligned nanorod film shown in
Once the first solution is formed, coordinating molecules and second ions may be added to the solution. The second ions (e.g., Cu+) may be in the form of an ionic compound (a copper salt) prior to being added to the solution. The ionic compound may be mixed with a second solvent having coordinating molecules (e.g., methanol) to form a second solution, which may be added to the first solution comprising the precursor nanorods comprising the first material (e.g., CdS). When the ionic compound is added to the first solution, the ions forming the ionic compound may dissociate in solution.
The reaction of the ionic compound (e.g., a copper salt) and nanorods (e.g., CdS nanorods) occurs at room temperature to form the composite nanorods (e.g., Cu2S/CdS nanorods). In the formation of Cu2S—CdS composite nanorods, second ions such as Cu+ ions can replace some Cd2+ ions in the precursor CdS nanorods. Alternatively, a suitable temperature range for performing the reaction is between −40° C. to 75° C.
Illustratively, the first solution may comprise cadmium sulfide (CdS) nanorods in toluene, and the second solution may comprise a copper salt such as tetrakis(acetonitrile)copper(I)hexafluorophosphate ([MeCN]4Cu(I)PF6) in methanol. It is desirable to keep this salt solution in an inert atmosphere, such as argon, to prevent oxidation of the copper salt solution. Because the reaction between the precursor nanorods and the ionic compound can be fast in some instances, lowering the temperature may slow down the reaction such that the two solutions can fully mix before the reaction occurs. Good mixing is desirable to ensure that the reaction occurs evenly (to the same extent) among the nanorods in solution (although there will always be some distribution). Otherwise, the cation exchange reaction that forms copper sulfide (Cu2S) within the CdS nanorods by exchange of Cd2+ with Cu+ can occur before the two solutions are well-mixed, leading to different conversion fractions of the CdS nanorods in solution to Cu2S.
The mixture used to form the composite nanorods can have a second ion/first ion weight or molar ratio between 0 and 2 in some embodiments. For example, in the case that the second ion is Cu+ and the first ion is Cd2+ and the nanorods are CdS, increasing the Cu+/Cd2+ ratio will increase the fraction of Cu2S within the nanorods, as shown schematically in
Coordinating molecules added to the solution can be used to either to facilitate or hinder the ion exchange process. Molecules that preferentially solvate the second ion will inhibit the reaction, whereas molecules that preferentially solvate the first ion will promote the exchange reaction. For example, in the case that the second ion is Cu+ and the first ion is Cd2+, methanol or other alcohols facilitate the reaction by preferentially solvating Cd2+. However, the presence of coordinating molecules such as alkyl amines and thiols was found to inhibit the reaction by coordinating to the Cu+ ions in solution. As noted above, the coordinating molecules may be in a second solution comprising the second ions (e.g., Cu+). The second solution could optionally include polar solvents such as acetonitrile, acetone, dimethylsulfoxide (DMSO), and N,N-dimethylformamide (DMF).
The first solution including the precursor nanorods may include any suitable solvent. The solvent may comprise an organic solvent. For example, the solvent may include saturated or unsaturated cyclic (or linear) hydrocarbons alone, or in combination with other molecules. In some cases, the solvent comprises at least one of hexanes, benzene, toluene, cyclohexane, octane or decane. Other examples of suitable solvents include halogenated solvents such as chloroform or tetrachloroethylene.
Rapid stirring is desirable in some embodiments. The solution is desirably well-mixed before the reaction occurs. In the case where either the second ion, first ion, or the nanorods are sensitive to oxygen and water, the reaction can be performed in an inert atmosphere, such as argon or nitrogen. For example, the exclusion of oxygen and water is desirable to prevent oxidation of the Cu+ ions in solution. However, after the reaction has occurred, the nanocrystals can be exposed to air. In cases where the materials are not sensitive oxygen or water, the reaction can be performed in air.
An exemplary composite nanorod according to an embodiment of the invention may have alternating regions, which alternate axially down the linear body of a nanorod. The alternating regions may have different materials and may be in any suitable form. For example, the alternating regions may be in the form of alternating layers of different ionic compounds such as Cu2S and CdS. The ionic compounds may include other types of materials including CdSe, ZnS, ZnSe, PbS, PbSe, HgS, FeS2, ZnO, CuO, Cu2O, CdTe, GaAs, InP, etc.
The rate of addition of the second ion to the nanocrystals containing the first ion can be used to control the percentage of asymmetric nanorod heterostructures produced. For example, by slowly injecting Cu+ ions at a constant rate of 0.15 mL/minute into a solution of CdS nanorods, the fraction of asymmetric CdS—Cu2S nanorods is greatly increased compared to fast addition of the Cu+ solution. It is also possible to vary the rate of injection over time. The rate of injection of second ions can be controlled using a syringe pump or similar apparatus.
The cation exchange reaction is reversible. As an example, CdS nanorods can be partially or wholly converted to Cu2S by Cu+ cation exchange by using methanol, which preferentially coordinates to Cd2+ over Cu+. These nanorods can then be partially or wholly converted back to CdS through reverse exchange using Cd2+ cations. In the reverse exchange, a coordinating solvent or molecule is needed which preferentially solvates Cu+ cations over Cd2+. Tributylphosphine was found to be a suitable molecule for promoting the reverse exchange to convert Cu2S to CdS.
Although CdS and Cu2S are described in detail herein as examples of the first material and the second material, the first and second materials may be other materials in other embodiments of the invention. Also, there may be more than two distinct materials in more than two distinct regions in a single linear body in a nanorod in other embodiments of the invention. For example, the first, second, third, etc. materials may comprise semiconductors such as compound semiconductors. Suitable compound semiconductors include Group II-VI semiconducting compounds such as MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, and HgTe. Other suitable compound semiconductors include Group III-V semiconductors such as GaAs, GaP, GaAs—P, GaSb, InAs, InP, InSb, AlAs, AlP, and AlSb.
The first ions and the second ions may include any suitable type of ions with any suitable charge states. The first and second ions are typically metal ions. For example, in the examples below, the first ion may be Cd2+, while the second ion may be Cu+. The first ion and the second ion may have different charges or the same charge.
The second ion may be derived from a precursor compound. In some embodiments, the precursors used to may include Group II, III, IV, V, and/or VI elements. For example, in embodiments of the invention, a region with material to be formed may include a Group II-VI compound semiconductor, which can be the reaction product of at least one precursor containing a Group II metal containing precursor and at least one precursor containing a Group VI element, or a precursor containing both a Group II and a Group VI element. Thus, the second ion may be an ion of a Group II or Group VI element. In other embodiments of the invention, the region of material to be formed may include a Group III-V compound semiconductor, which can be the reaction product of at least one precursor containing a Group III element and at least one precursor containing a Group V element, or a precursor containing both a Group III and a Group V element. The second ion in this example may be an ion of a Group III or V element.
The preferred embodiments that are described below are illustrated in the context of converting a CdS nanorod into a Cu2S—CdS composite nanorod. The skilled artisan will readily appreciate, however, that the materials and methods disclosed herein will have application for a number of other nanorod or nanostructure materials.
There is theoretically no limitation on the lengths of the composite nanorods. There can be limitations on the maximum length of CdS nanorods that one can grow (˜200 nm). In the examples presented below, the nanorods were 20-60 nm in length and each nanorod contained 0, 1, or 2 Cu2S segments. The length of each segment can be any fraction of the length of the entire nanorod. Longer nanorods could be produced in other embodiments of the invention.
In the nanorods shown in
Nanorods with three or less distinctive regions can be advantageous. For example, if a nanorod has two outer regions sandwiching an inner region of a different material, the outer regions can serve as contacts to electrodes in an electrical device. Furthermore, asymmetric nanorods containing two distinctive regions at either end according to embodiments of the invention are strong candidates for use in nanocrystal-based solar cell devices. Photoexcitation of these structures can lead to charge separation at the interface of the two regions. Each region can then be used to transport a charge of opposite sign (i.e. electron or hole) to its respective electrode.
The electronic energy levels of the semiconducting CdS and Cu2S are in a Type II band alignment such that charge separation can occur when electrons are excited under irradiation of visible and ultraviolet light. This material pair has already been demonstrated in working solar cell device using bulk films of CdS and Cu2S (M. A. Green, Solar Cells. Kensington, New South Wales: University of New South Wales (1998), A. L. Fahrenbruch, R. H. Bube, Fundamentals of Solar Cells. Academic Press: New York (1983)). Solar cells based on films containing layers of separate CdS and Cu2S nanocrystals have also been produced (Y. Wu, C. Wadia, W. Ma, B. Sadtler, A. P. Alivisatos, Nano Letters 8:2551 (2008)). The electronic energy levels of conduction and valence bands in the materials can be controlled by varying the diameter of the initial cadmium sulfide nanorods due to quantum size effects. The energy level alignment can also depend on the relative proportions of CdS and Cu2S materials within the nanorod, which can be tuned in the cation exchange process.
Nanorod based solar cells have been shown to have higher efficiency than those made with spherical nanocrystals (W. U. Huynh, J. J. Dittmer, A. P. Alivisatos, Science 295: 2425 (2002). By vertically aligning the nanorods, this efficiency could be enhanced further as it would allow for better charge transport along the nanorods to the electrodes of the device (see
As noted above, in embodiments of the invention, partial cation exchange can be used to synthesize nanocrystal heterostructures. Partial cation exchange is a novel route to synthesize nanocrystal heterostructures, including two materials that are epitaxially connected within an individual nanocrystal. This single-step, chemical transformation enables the systematic tuning of the composition and properties of colloidal semiconductor nanocrystals. Upon the addition of copper or silver salts to a colloidal solution of cadmium sulfide (CdS) nanorods, the spontaneous replacement of cadmium cations within the CdS lattice for copper (silver) cations occurs, while preserving the original dimensions of the nanorods. The exchange process produces crystalline regions of copper sulfide (silver sulfide) within the CdS nanorods, creating binary nanorod heterostructures. The morphology of the heterostructures depends on the cation species used and the fraction of the nanorod converted. High-resolution transmission electron microscopy of the binary nanorods shows that the crystal lattices are epitaxially connected at their interface. This well-defined contact leads to strong electronic coupling between the materials, evidenced by fluorescence spectroscopy. The absorption and emission of light of the binary nanorods can be tuned throughout the visible and near-infrared regions making these novel nano-scale architectures desirable for a variety of optoelectronic applications including near-infrared emission and solar energy conversion.
Nanorods including Ag2S and CdS have been made. One difference between nanorods including Cu2S and CdS is that the former focuses on the spontaneous organization of the two materials to form a periodic pattern within the nanorod. In comparison, in some embodiments, the latter focuses on the ability to make a binary nanorod where one half is one material and the other half is a different material. In the former, non-selective nucleation of the Ag2S material leads to the formation of Ag2S regions throughout the nanorod. In the latter case, the selective nucleation of Cu2S at one end of the nanorod leads to an asymmetric structure.
The chemical process for making the two different structures can be similar, with the difference between the two being whether silver ions, Ag+, or copper ions, Cu+, are added to the solution of cadmium sulfide, CdS, nanorods. However, the resulting nanorod heterostructures are notably different. In the former case, the Ag2S regions are very mobile within a CdS nanorod. The two materials organize to form alternating regions of CdS and Ag2S along the nanorod with a defined periodicity. The materials “self-assemble” in the rod. In the former example, a simple process of cation exchange can be used to make a more complex nanocrystal that is normally achieved through multiple deposition steps. However in the latter case, the Cu2S forms on just the ends of the CdS nanorods through selective exchange of these facets and works its way towards the middle. There is no reorganization of the Cu2S regions. In the latter example, it is possible to make a binary nanorod heterostructure, which contains two epitxially connected materials.
Most microscale electronic devices use a heterostructure of two semiconductor materials with an epitaxial interface between them (an epitaxial interface normally leads to good electronic coupling or communication between the two materials). CdS—Cu2S nanorods can satisfy this requirement. Having this asymmetry, where one end of the nanorod is one material and the other is a different material, makes it very potentially useful for nano-scale devices.
Another aspect is the applications of the two types of nanorods, CdS—Ag2S and CdS—Cu2S nanorods. Both have potential uses in solar cells, but for very different reasons (i.e. the two materials would use different processes to collect solar energy and turn it into electrical energy). In the latter case, the absorption of visible photons from the sun will create an electron-hole pairs (negative charge-positive charge pair) within CdS—Cu2S nanorods. The relative alignment of the electronic levels of these two materials allows for charge separation to occur at their interface and for the CdS to transport the electron (negative charge) and the Cu2S material to transport the positive charge. The extraction of these charges at opposite electrodes can be used to generate a current. In the former case, the periodic arrangement of CdS and Ag2S materials creates a linear array of Ag2S quantum dots separated by confining regions of CdS. Such structures are of interest for colloidal quantum dot solar cells, where the sparse density of electronic states within a dot may lead to multiple exciton generation.
I. Synthesis of CdS nanorods. Colloidal CdS nanorods were synthesized using standard techniques developed for cadmium chalcogenide nanorods (Peng, Z. A.; Peng, X. Journal of the American Chemical Society, 124:3343-3353 (2002)). The reactions were performed under air-free conditions and the CdS nanocrystals were stored in an argon-filled glove box.
II. Cation exchange of CdS nanorods. Cu+ cation exchange was used to convert CdS nanorods into CdS—Cu2S binary nanorods and Cu2S nanorods. The reactions were performed inside an argon-filled glove box at room temperature. The extent of conversion depends on the Cu+/Cd2+ ratio, where an excess of Cu+ ions (Cu+/Cd2+>2 as two Cu+ ions replace one Cd2+ ion for charge balance) leads to full conversion to Cu2S. The molar concentration of Cd2+ ions for each CdS nanorod solution was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) of acid-digested samples. Typical molar extinction coefficients for Cd2+ within the CdS nanorod solutions were 3×106 mole/cm2 at 300 nm measured by visible absorption spectroscopy. The amount of Cd2+ in the CdS nanorod solution in each reaction was between 1×10−6 to 1×10−5 moles. The salt, tetrakis(acetonitrile)copper(I) hexafluorophosphate ([MeCN]4Cu(I)PF6), was used in the reactions as the weak binding affinity of the anion makes the salt readily soluble in methanol such that the Cu+ solution is miscible with the colloidal solution of nanorods dispersed in toluene. In a typical reaction, 12 mg of [MeCN]4Cu(I)PF6 was dissolved in 2.5 mL of methanol (MeOH). This solution was used for full conversion or was further diluted five or ten-fold for partial conversion. For full conversion, the [MeCN]4Cu(I)PF6 solution (˜0.6 to 1 mL) was added to a stirring solution of CdS nanorods in toluene (˜2 mL). For partial conversion a concentrated solution of CdS nanorods in toluene (˜50-500 μL) was added to a stirring [MeCN]4Cu(I)PF6 solution (˜0.1-1 mL) diluted in toluene (˜2 mL). The color of the nanocrystal solution changes rapidly (<1 second) from yellow to golden brown after mixing of the Cu+ and CdS solutions, and the nanorods were washed by the addition of MeOH followed by centrifugation and removal of the supernatant. To examine the effect of slow addition of Cu+ ions, the [MeCN]4Cu(I)PF6 solution was loaded in a syringe pump and added at a rate of 0.15 mL/min via a capillary needle to a stirring solution of CdS nanorods in toluene.
Further details regarding suitable processing conditions used to synthesize CdS—Cu2S binary nanorods and Cu2S nanorods by Cu+ cation exchange of CdS nanorods can be found in the Table below.
III. Characterization. Bright field TEM images were obtained using a Tecnai G2 S-Twin electron microscope operating at 200 kV. TEM samples were prepared by placing a drop of the nanocrystal solution onto a carbon-coated copper grid in ambient atmosphere. The elemental distribution of the nanocrystals was characterized by energy-filtered transmission electron microscopy (EFTEM). The EFTEM experiments were performed using a Philips CM200 microscope or a monochromated F20 UT Tecnai microscope. Both microscopes were equipped with a field emission gun, an electron energy loss spectrometer and a Gatan Image Filter (GIF) and were operated at 200 kV. The elemental maps were obtained by using the three-window method (Brydson, R., Electron Energy Loss Spectroscopy; BIOS Scientific: Oxford, (2001)). The Cd M-edge (404 eV) and Cu L-edge (931 eV) were used to make the color composite images. The color composites of Cd and Cu-EFTEM images were made using Image-Pro Plus software. The Cu M-edge (120 eV, minor) was used for the Cu energy-filtered images.
Statistics for the length and diameter of the initial CdS nanorods and fully converted Cu2S nanorods were gathered from bright field TEM images using Image-Pro Plus software, and at least 150 measurements were made for each sample. Statistics for the segment lengths of the CdS and Cu2S regions in the binary nanorods were determined from EFTEM images by making at least 150 measurements. The degree of asymmetry for each CdS—Cu2S binary nanorod was taken to be one minus the ratio of the length of the short Cu2S segment over the length of the long Cu2S segment. Using this definition, a nanorod possessing two Cu2S segments of equal lengths has an asymmetry value of 0, and a nanorod with Cu2S on only one side of the nanorod has an asymmetry value of 1. The length fraction of the nanorod converted to Cu2S was measured as the ratio of the combined length of Cu2S segments over the total length of the nanorod. Thus, a nanorod that is entirely composed of CdS will have a length conversion of 0, and a nanorod fully converted to Cu2S will have a value of 1. The CdS—Cu2S interfaces were grouped into three categories: flat and parallel to the nanorod cross-section, flat and at an angle to the cross-section, and multifaceted (which appear curved in low-magnification TEM images). As TEM imaging provides a two-dimensional projection of the nanorod heterostructures, the apparent angle and curvature of an interface depends on its relative orientation on the TEM substrate. Therefore, the fraction of each of these types of interfaces (measured from a population of over 200 nanorods) is approximate.
The crystal structures of the samples were determined from powder X-ray diffraction (XRD) obtained on a Bruker AXS diffractometer using Co Kα radiation (1.790 Å) and a general area detector. The instrument resolution was 0.05° in 2θ, and the acquisition time for each sample was one hour. XRD samples were prepared by dissolving the precipitated nanocrystals in a minimal amount of toluene or chloroform and using a capillary tube to drop the solution onto a glass sample plate.
IV. Ab initio calculations. Supercell geometries for CdS—Cu2S epitaxial attachments were studied using the Vienna ab-initio simulation package (VASP), a density functional theory (DFT) code using planewaves and pseudopotentials (Kresse, G.; Furthmuller, J. Computational Materials Science, 6:15-50 (1996); Kresse, G.; Furthmuller, J. Physical Review, 54:11169-11186 (B 1996)). The generalized gradient approximation (GGA) was used for the exchange-correlation part, along with projector augmented wave (PAW) pseudopotentials, and planewave energy cutoffs of 280 eV. The inventors used F-point only eigenenergies in the Brillouin zone as the supercells are sufficiently large to ensure weak dispersion of energy bands. All geometries were relaxed to have the forces on atoms reduced to 0.01 eV/Å or less. The lattice parameters of the relaxed cells were used in all calculations. CdS—Cu2S interface formation energies for epitaxial attachments between different facets of the two crystals were computed analogously to our previous calculations for the CdS—Ag2S system where the interface formation energy is defined as the ab initio total energy difference of the supercell containing the interface and its bulk constituents (Demchenko et al., ACS Nano, 2:627-636 (2008)). Total formation energies containing both chemical and elastic contributions were obtained by using the difference in energy between the supercell and natural bulk structures. To calculate the chemical energy alone, the bulk lattices were strained similar to the lattice in the supercell. The elastic contributions were computed assuming the distortions occurred in the Cu2S or Ag2S cell only to match to the lattice of the CdS cell. The cell thicknesses for Cu2S were 13.5 Å for the end-on and angled attachments to CdS and 27.3 Å for the side attachment. CdS—Ag2S interface formation energies for similar end-on connections to the CdS nanorods were previously calculated (Demchenko et al., ACS Nano, 2:627-636 (2008)), and an additional side attachment was modeled for this work. The cell thicknesses for Ag2S were 13.7 Å in all cases.
X-ray diffraction (XRD) patterns of the CdS nanorods before and after the addition of increasing amounts of Cu+ cations are shown in
For partial Cu+ exchange, energy-filtered TEM (EFTEM) was used to obtain elemental mappings of the Cu- and Cd-containing regions of binary nanorods. The composite energy-filtered images in
The high-resolution TEM (HRTEM) image of a Cu2S—CdS heterostructure in
While Cu+ cation exchange occurs at both ends of the CdS nanorods, the relative lengths of the two Cu2S end segments within a given nanorod can vary. As the CdS wurtzite lattice lacks inversion symmetry about the c-axis, the (0001) and (000
Sample 3 used the same initial nanorods as sample 2, but the Cu+ solution was added drop wise via a syringe pump to the CdS solution. Slowing the rate of addition of Cu+ cations to the CdS nanorods has several significant effects on the morphology of the CdS—Cu2S heterostructures. First, it greatly enhances the asymmetry of the heterostructures leading to a majority of nanorods with Cu2S only on one end as shown in
The heterostructure morphologies for different conversion fractions of the CdS nanorods to Cu2S or Ag2S aid in elucidating the movement of the reaction fronts during cation exchange within the nanocrystals (Demchenko et al., ACS Nano, 2:627-636 (2008)).
In the CdS—Cu2S system, where the elastic contributions to the interface formation energies are small, the relative values of the chemical formation energies determine the stability of the different CdS—Cu2S attachments. The end-on Cu2S attachments, parallel to the nanorod cross-section, possess the lowest chemical formation energies and are the interfaces observed most often by TEM in the heterostructures. The angled attachment connecting the basal facets of the monoclinic Cu2S lattice to CdS has both a higher chemical formation energy per interfacial unit and produces a greater interfacial area. Correspondingly, angled interfaces occur at a significantly lower frequency, particularly in the case where the Cu+ ions are slowly added to the CdS solution. Finally, growth of Cu2S on the sides of the CdS nanorods is rarely observed, which correlates with the calculated chemical formation energy that is approximately seven times greater than that of end-on connection to the (000
The present inventors previously reported that when relatively low amounts of Ag+ are added to CdS nanorods (Ag+/Cd2+<0.5), small Ag2S regions are found dispersed randomly over the surface of the nanocrystals (Robinson et al., A. P. Science, 317:355-358 (2007); Demchenko et al., ACS Nano, 2:627-636 (2008)). At higher conversion fractions of Ag+ exchange (0.5<Ag+/Cd2+<0.9), the Ag2S regions coalesce such that they form segments that span the diameter of the nanorod and possess flat interfaces parallel to the nanorod cross-section. The negative chemical formation energies for each of the CdS—Ag2S attachments favor the creation of Cd—S—Ag interfacial bonds on both the ends and sides of the CdS nanorods, leading to non-selective nucleation. However, as the Ag2S regions grow into the nanorods, the elastic strain becomes a more notable contribution to the total formation energy, driving ripening of the Ag2S regions to reduce the interfacial area. When the Ag2S regions grow to span the diameter of the nanorod, the interfaces parallel to the length of the nanorod disappear, which possess the greatest elastic energy. At this point the ripening process becomes kinetically hindered, as further exchange of cations between the flat interfaces of the Ag2S and CdS segments would increase the interfacial area until two like segments fully merge. While full phase segregation of the CdS and Ag2S regions to opposite ends of the nanorod would produce to the lowest energy structure, the Ag2S segments are stabilized by the large interfacial strain leading to a repulsive elastic interaction between like segments that decreases with increasing separation between them (Robinson et al., A. P. Science, 317:355-358 (2007); Demchenko et al., ACS Nano, 2:627-636 (2008)). Both the size and the spacing of the Ag2S segments tend to be uniform as this minimizes the repulsive elastic interaction. Thus, non-selective nucleation followed by partial phase-segregation leads to a metastable configuration consisting of alternating CdS and Ag2S segments. This is very different from the CdS—Cu2S case, where once the Cu2S regions nucleate at the ends of the nanorods, they grow until they meet in the middle.
The relative activation barriers for nucleation at each end of the nanorod control the asymmetry of the Cu2S segments. In principle, disparate rates of diffusion of cations in opposite directions along the nanorod could also contribute to asymmetric growth. However, previous kinetic studies of cation exchange suggest that interface nucleation provides the main kinetic barrier for transformation of the nanocrystal (Chan et al., Journal of Physical Chemisty A, 111:12210-12215 (2007)). The chemical formation energy for the Cu2S attachment to the CdS (000
The increased asymmetry of Cu2S segments in sample 2 over sample 1 as shown in
Maintaining a low concentration of Cu+ ions present in solution during the exchange reaction enhances the formation of a single interface in each binary nanorod. This can be seen as the asymmetry of Cu2S segments greatly increases for slow (sample 3,
Embodiments of the invention have demonstrated that the crystallographic selectivity for cation exchange to occur at different facets of ionic nanocrystals plays a notable role in determining the morphology of the resulting nanocrystal heterostructures. The preferential nucleation and growth of Cu2S at the ends of CdS nanorods during Cu+ exchange is attributed to the high stability of CdS—Cu2S interfaces formed at these facets. In comparison, non-selective nucleation in Ag+ exchange leads to the formation of multiple Ag2S regions within the nanorod. The differences between these two systems lie in both the chemical favorability for creating interfacial bonds as well as the elastic distortions between attachments connecting various facets of the two materials. The relative stabilities of the interfaces that were modeled correspond well with the frequency that the corresponding morphologies are observed. In the future, similar modeling of the epitaxy in nanoscale heterostructures may be applied to other material pairs to predict which interfaces will be the most stable. As both the shape and size of the nanocrystals determine the crystallographic facets exposed at the surface, these parameters can be used to control the nanocrystal's reactivity. Selective facet reactivity can in turn provide tunability of the physical properties of nanocrystal heterostructures through control of the spatial arrangement of their components.
Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, one of skill in the art will appreciate that certain changes and modifications may be practiced within the scope of the appended claims. In addition, each reference provided herein is incorporated by reference in its entirety to the same extent as if each reference was individually incorporated by reference.
This application is a non-provisional of and claims the benefit of the filing date of U.S. Patent Application No. 60/039,054, filed on Mar. 24, 2008, which is herein incorporated by reference in its entirety for all purposes. This application is also related to PCT/US2008/069384, filed on Jul. 8, 2008, and U.S. Provisional Application Nos. 60/948,971, filed on Jul. 10, 2007, and 60/987,547, filed on Nov. 13, 2007, which are all incorporated by reference in their entirety for all purposes.
The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract DE-AC02-05CH11231. The government has certain rights in this invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US09/37952 | 3/23/2009 | WO | 00 | 9/21/2010 |
| Number | Date | Country | |
|---|---|---|---|
| 61039054 | Mar 2008 | US |
