Article pubs.acs.org/JPCC
Ionic Bonding between Artificial Atoms Rekha Mahadevu and Anshu Pandey* Solid State and Structural Chemistry Unit, Indian Institute of Science Bangalore 560012, India S Supporting Information *
ABSTRACT: Conventional solids are prepared from building blocks that are conceptually no larger than a hundred atoms. While van der Waals and dipole−dipole interactions also influence the formation of these materials, stronger interactions, referred to as chemical bonds, play a more decisive role in determining the structures of most solids. Chemical bonds that hold such materials together are said to be ionic, covalent, metallic, dative, or otherwise a combination of these. Solids that utilize semiconductor nanocrystal quantum dots as building units have been demonstrated to exist; however, the interparticle forces in such materials are decidedly not chemical. Here we demonstrate the formation of charge transfer states in a binary quantum dot mixture. Charge is observed to reside in quantum confined states of one of the participating quantum dots. These interactions lead to materials that may be regarded as the nanoscale analog of an ionic solid. The process by which these materials form has interesting parallels to chemical reactions in conventional chemistry.
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transition metal series can be readily introduced into II−VI semiconductor lattices.14 A chemical peculiarity of these metals is the existence of variable valence. These ions can consequently oxidize or reduce other species in close proximity. We initially introduce these ions into redox-inert QDs. Subsequently a QD that is capable of accepting electrons or holes from these doped QDs is brought into close proximity (Figure 1a). Cu2+ doped II−VI materials have been studied
emiconductor nanocrystals with physical dimensions smaller than the size of the Bohr exciton exhibit strong quantum confinement of the carriers and are consequently are referred to as quantum dots (QDs). Due to their spheroidal confinement potential, electrons and holes confined in these materials occupy states with separable radial and angular components. Similar to atoms, the angular components of the electronic states are given by spherical harmonics.1 QDs are sometimes referred to as “artificial atoms” to highlight this similarity. Although QDs can indeed be assembled into solids in the same manner as atoms, the full realization of artificial materials based on these building blocks requires the demonstration of atomlike “chemical” interactions between individual QDs. Here we show the existence of charge transfer states2 between two formulations of QDs. These states act as nanoscale analogs of ionic bonds that are observed in inorganic compounds such as sodium chloride. Conventionally, ionic bonds have been known to exist in ionic solids as well as between atomic clusters as in Zintl Phases.3 More recently, ionic bonding has been demonstrated to exist in solids formed from fullerenes and alkali metal or organometallic donors.4−7 Chemical bonding in the above examples leads typically to structures that correspond to thermodynamic minima of the system. In contrast to these examples, the diffusion coefficients of QDs are several orders of magnitude smaller than those of atoms. While solids formed out of QDs with weak interparticle interactions8−13 do exhibit crystalline order, stronger interparticle interactions could in principle prevent the system from arriving at a thermodynamic minimum. Solids formed out of chemically interacting QDs are therefore expected to be kinetically frozen, unlike the other examples described above. In order to achieve electron transfer between QDs, we adopt a charge transfer doping approach. Divalent ions from the first © 2014 American Chemical Society
Figure 1. Schematic of emergence of ionic interactions between QDs. a. A charge neutral quantum dot pair undergo a redox process whereby a valence band electron from one QD is transferred into a dopant ion embedded in another QD. b. The resultant hole causes bleaching of interband transitions and also causes the occurrence of an intraband transition in the valence band. c. The two QDs are no longer neutral and interact through coulomb forces.
because of their luminescence and magnetic characteristics.15,16 In addition, Cu2+ ions can exhibit redox reactions whereby the ion is converted into a +I oxidation state by accepting an electron from a source. Nanocrystal materials such as ZnSe, CdSe, CdS, etc. are redox-inert relative to the ion.15,16 On the other hand, Received: September 23, 2014 Revised: November 26, 2014 Published: November 26, 2014 30101
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and Cu:CdS QDs. In each case, the concentration of the emitting QD is identical. Even though the doped QDs introduce no absorption in the vicinity of the emitter, a very significant quenching is observed. In this particular experiment, the two QDs were dissolved separately in hexane. The QD solutions were then mixed and the solvent was evaporated at room temperature, typically by evacuating the flask. The QD precipitate is redissolved in hexane for optical characterization. Further evidence of charge transfer states may be drawn from absorption spectra that exhibit a strong bleach of the QD band edge excitonic feature. Figure 2b shows the appearance of band edge bleach in PbSe magic sized clusters. In this example, hexane solutions of PbSe magic sized clusters21 and Cu:ZnSe QDs were mixed.22 Absorption spectra were collected for the QD mixture. The mixture was then dried by evaporating the solvent at room temperature. Subsequently the mixture was redissolved in hexane. The difference between the two absorption spectra was used to estimate the bleach. A similar bleach is observed in the case of ZnTe based QDs (SI, Figure S7). Yet another indicator of the existence of charge transfer states is the appearance of an intraband transition feature in QD mixtures.23−25 Figure 2c shows intraband spectra that correspond to a 1Sh-1Ph transition of a hole in the PbSe valence band. These data were collected in a QD film comprised of PbSe/CdSe and Cu:CdS QDs. The intraband spectrum in this case was inferred by subtracting IR spectra of individual QDs from the mixture. Charge transfer across QDs leads to the emergence of a novel interaction, hitherto unobserved in nanosystems. The charge transfer process causes the appearance of opposite charges in the donor and acceptor QDs, leading to the emergence of attractive coulomb interactions between the two QDs. Coulomb interactions arising from surface charges are well documented; however, in our case, the interaction only turns on following a QD−QD collision. This interaction is observed to be strong enough to offset the solvent−QD van der Waals type interactions and causes the materials to precipitate from colloidal solution at room temperature. In these experiments, we employ a variable ratio of the doped and undoped QDs. Both the QDs are initially dispersed in a hexane solution. Hexane is then removed by evaporation at room temperature to leave behind a precipitate consisting of QDs. The resulting precipitate is then redissolved in hexane. The
semiconductors such as PbSe and ZnTe have high lying valence bands17 that can donate electrons into the Cu2+ ion. A detailed description is presented in the Supporting Information (SI). In this work we observe charge transfer between QDs of various formulations utilizing the above principle; in each case the acceptor is a copper doped QD, while the donor is a QD with at least one layer of ZnTe or PbSe. Charge transfer between the two families of QDs is observed in the case of films as well as in solvent dispersion. As a consequence of charge transfer, a hole is injected into the valence band of the ZnTe or PbSe based QD (Figure 1b). The valence band hole manifests itself in several ways. The presence of a spectator hole first ensures that optical excitation of the QDs now produces a trion rather than an exciton.18 Trions are known to exhibit poor quantum yields due to rapid, nonradiative multicarrier recombination. Additionally, the presence of a hole in the valence band leads to optical bleaching of the interband absorption as well as the occurrence of a valence intraband transition in the infrared (Figure 1b).19 A final manifestation of this charge transfer is the emergence of coulomb attraction between QDs (Figure 1c). Briefly, electron transfer causes the two participating QDs to become oppositely charged. These QDs attract each other in the same manner as ions. Figure 2 lists some of the effects that originate from this inter-QD redox process. Figure 2a compares the photo-
Figure 2. a. PL quenching is observed upon mixing ZnTe/CdS QDs with Cu:CdS QDs. b. A mixture of PbSe magic sized clusters and Cu:ZnSe QDs shows an optical bleach of the PbSe band edge transitions. c. The occurrence of the band edge bleach is accompanied by the appearance of an intraband transition.
luminescence (PL) emission that is observed for pure ZnTe/ CdS20 QDs with the emission from a mixture of ZnTe/CdS
Figure 3. QD solids can be prepared by mixing two families of QDs in an organic solvent, followed by repeated cycles of drying and wetting. A fraction of the QDs is rendered insoluble in the solvent. Chemical analysis reveals that the composition of this fraction is independent of the starting ratios. 30102
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Figure 4. a. PL emission from QD mixtures continue to drop after several repeat cycles of wetting and drying (red, blue symbols). In contrast the emission from a pure sample of QDs is unchanged (green circles). b−e. Structures of the QD solid contain a variety of structural motifs, although typically a single motif predominates in a particular solid. The observed structures include b. dendrites, c. diskettes, d. thick dendrites (scale bar 20 nm), and e. seemingly unstructured loosely bound aggregates (scale bar 200 nm).
example shown in Figure 3 shows a mean stoichiometry of 1.8. We estimate that the QD labeled X contains 24 copper ions implying a +43 charge on QD A and a −24 charge on QD X. In this estimate, the number of copper ions has been inferred from TEM and ICP data. Briefly, TEM sizing is used to estimate the number of atoms in a QD. ICP results are then used to estimate the number of copper ions per QD. In the situation where both QDs are in contact, this implies an ionic interaction energy of 30 eV between the two particles. A dielectric screening of 10 has been assumed in this estimate. We note that this interaction is 1 order of magnitude stronger than the typical chemical bond (few eV). Nevertheless, the more significant parameter in judging the strength of such an interaction is the interaction energy per unit volume. Since the two QDs have radii of 3 and 2 nm respectively, a 30 eV bond strength implies a bond energy density of 0.2 eV/nm3. This is significantly lower than the bonding energy per unit volume of 104 eV/nm3 that is known to exist in a typical chemical bond. An estimate of the cohesion energy is also provided in the SI. We note that the number presented (somewhat weaker than a conventional ionic solid) assumes the existence crystalline order in our materials. As we see below, this is obviously lacking in these solids. The scope and validity of these estimates are also discussed in the SI. Finally, we note that the formation of this solid is in no way related to surface charges typically present in a QD, that are suspected to play a role in formation of other types of QD solids.8 For example, we see the appearance of these solids even by mixing QDs that exhibit like surface charges during zeta potential measurements. In such cases, the solid obtained still carries the identical sign of charge as the QDs. While the QD solids synthesized here are akin to ionic compounds familiar in inorganic chemistry, there are several fundamental differences as well. The diffusion coefficients of QDs used in this work are expected to be of the order of 10−7 cm2/s, in contrast to the diffusion coefficients of inorganic ions that are 2 orders of magnitude greater. This large difference ensures that QD−QD charge transfer is unlikely to be observed in on any practical time scale in a dilute QD mixture, simply because the probability of a QD−QD collision is extremely low. This necessitates the repeated drying and wetting cycles employed here. The QD−QD interaction of 30 eV generated in these solids is much greater than QD−solvent van der Waals interactions. These two properties together determine the thermodynamic properties of the QD solid. In particular, the process of finding the structural thermodynamic minimum would require successive QD−QD bond cleavage, QD diffusion, followed
wetting-drying process is repeated 5 times. It is observed that the solubility of the QDs is dramatically reduced after 2−4 cycles, and a significant fraction of the material is ultimately rendered insoluble in organic solvents that are typically used to disperse QDs. We further observed that optical excitation (as high as 10 W/cm2, 3.07 eV) had no qualitative effect on the formation or stability of this solid. Both the QDs have a ligand passivation layer that is quite involatile and is not expected to be removed along with hexane vapor. As evidence of this, we note that a similar treatment to either of the two QDs independently does not cause the appearance of an insoluble fraction of QDs. Evidence of retention of ligands may be drawn from infrared absorbance data (SI, Figure S18) as well as from High Resolution Transmission Electron Microscopy (HRTEM) (SI, Figure S15). In particular, we find that the ligands are retained entirely even after precipitate formation since the infrared absorbance of the QD solid is no different from the infrared absorbance of the original QDs. This procedure thus provides a straightforward route to bring both QDs into close proximity in order to facilitate charge transfer between the two species. The supernatant solution is removed and the precipitate is recovered by sonication in hexane. It is then washed several times by repeated suspension and centrifugation from hexane. The optical properties of the precipitate are consistent with a QD solid bound by coulomb interactions. We now turn to the rather unusual chemical properties of this solid. The composition of this QD solid was analyzed by using inductively coupled plasma (ICP) technique. We find that the composition of the QD solid is almost invariant of the ratios of the QDs in the starting solution. For example, as shown in Figure 3, when the ratio of two quantum dots is varied from 1:3.5 to 1:15, the stoichiometry of the solid changes from 1:2.2 to 1:1.5. This unusual property is reminiscent of the concept of stoichiometry in conventional chemistry. It is thus possible to regard this solid as an inter-QD “compound”, where the transferred charge resides in a delocalized, quantum confined state. In particular, the numerically deficient QD serves as a limiting reagent in this “reaction”. SI, Figure S16 shows the X-ray diffraction (XRD) patterns observed in the solid precipitate prepared from PbSe/CdSe and Cu:CdS QDs. We observe that the solid exhibits powder XRD patterns observed from the solid are a straightforward sum of the patterns exhibited by the pure materials. The solids are thus structurally identical to the parent materials however differ dramatically in their physical properties. The existence of stoichiometry in this material also enables us to estimate its bond strength as well as cohesive energy. The 30103
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by the eventual migration of the structure toward a more stable minimum. The tiny diffusion coefficients of QDs however ensure that QDs cannot migrate too far apart upon bond rupture. The strong interparticle interaction furthermore reduces the probability of solvent mediated QD separation. The system is therefore expected to be kinetically frozen with little possibility of attainment of the thermodynamic minimum at any temperature. This situation is rare in conventional chemical reactions that can in general be thermally driven toward the lowest energy structure. The evolution of sample emission due to repeated wettingdrying cycles provides evidence of remarkable kinetic stability of these materials. As shown in Figure 4a, the PL of a QD−QD mixture continues to drop even after 10 drying-wetting cycles.26 This behavior has similarities with jamming transitions that are known to prevent a granular system from attaining the global energy minimum.27,28 Transmission electron microscope images (Figure 4b−e) of these materials reveal a wide diversity of structures29 consistent with the glassiness observed above. The mechanisms and factors that lead to the formation of these shapes are unclear at this point. As these materials lack order, their comparison to more “standard” ionic solids like NaCl is obviously unjustified. Based on their disorder and ionicity alone, these materials are most similar to glassy ionic solids such as alkaline borosilicate or soda-lime glasses. While synergism has been reported in binary QD superlattices, the properties of the solids reported here are quite distinct. The most significant difference is that the solids observed in our case form due to a chemical interaction. In contrast, physical properties such as anomalous conductance only emerge in binary superlattice materials after postprocessing, e.g., thermal or chemical treatments.30 To conclude, we demonstrate the existence of charge transfer states between QDs. Charge on one of the participating QDs resides in a delocalized quantum confined state. These states are analogous to ionic bonding and give rise to QD solids that are bound by coulomb interactions. These solids are found to be held together by interactions that when volume averaged, are somewhat weaker than the interaction strengths of chemical bonds. An inherent absence of order prompts the comparison of these materials with alkali metal borosilicates and other similar ionic glasses. These solids exhibit physical properties (absorption, emission, solubility, cohesion, etc.) that are distinct from the properties of constitutive QDs. Finally, despite the involvement of quantum confined states in semiconductor bands, these materials also exhibit QD−QD stoichiometry in a manner similar to conventional chemical compounds. Demonstration of atom-like character in nanoscale building blocks paves the way for novel routes of synthesis and assembly that are distinct from the top-down and bottom-up approaches that are in vogue today. On a more fundamental level, this work represents the emergence of a route to increasing the length scale of chemical interactions. Conventional chemical bonds are no longer than a few angstroms in length. While ionic bonds are potentially long-ranged, screening by counterions ensures that that ionic interactions do not extend beyond a nanometer in practice. Ionic solids prepared from bulky, hindered building blocks thus exhibit counterintuitive properties partly due to the enhanced length scale of bonding interactions.
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ASSOCIATED CONTENT
S Supporting Information *
Synthetic details, structural and spectroscopic analysis, sample characterization, technical discussion of properties, estimates of cohesive energies, local stoichiometries, and charge transfer energetics. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS R.M. thanks the Council of Scientific and Industrial Research for financial support. A.P. thanks the Indian Institute of Science and the Department of Science and Technology for funding.
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