news & views been able to measure the suppression of electronic friction when a metal undergoes a superconducting transition, a situation where many of those fluctuations become energetically forbidden as a result of the presence of a gap in the energy spectrum5. Kisiel and co-authors have used the pendulum-AFM set-up to determine the energy dissipation when the tip approaches the surface of a NbSe2 crystal in its lowtemperature CDW ground state (the CDW corresponds to a joint distortion of the lattice and the electron density, and has an incommensurate periodicity and a longrange coherent phase that extends over hundreds of lattice spacings). They detected three distinctive dissipation peaks at particular distances that seem to correspond to well-defined values for the tip–sample interaction (Fig. 1a), which is at odds with the expected monotonic increase in friction as the tip–sample distance is reduced. The authors also performed measurements with a tuning-fork tip operated in the conventional dynamic AFM mode (tip oscillating normal to the surface), which systematically showed the presence of the three dissipation maxima, thus confirming the general nature of these phenomena. Kisiel and colleagues connect the dissipation processes with tip-induced local changes in the electron density modulation associated with the CDW. This is supported by the temperature dependence of the dissipation peaks, which disappear above the CDW transition temperature, as well as by experiments on similar layered materials that do not present the CDW phase; these only show a smooth, monotonic increase in the dissipation energy when the tip–sample distance is reduced1. To understand the source of the dissipation peaks, one can resort to a concept that pervades the interpretation of friction: force hysteresis. When the tip
is laterally and cyclically moved on top of the surface — or, in the case of dynamic AFM, when the tip approaches and retracts from the surface — the different forces in opposite directions define a hysteresis cycle, whose area determines the energy dissipated per cycle. Force hysteresis arises because of the presence of energy barriers between the ground state and excited configurations of the system. As the tip– sample interaction can stabilize one of these excited configurations, energy barriers can prevent the relaxation back to the ground state during tip retraction. Hence, the system is in different states during tip approach and retraction, which provides a quantitative understanding of the energy dissipated in tip–sample adhesion at the level of a single atomic bond6. Following this idea, Kisiel and co-workers suggest that the observed dissipation peaks result from the hysteresis cycle formed by the crossing between CDW states with phases differing by 2π that are stabilized by the interaction with the tip (Fig. 1b). The ground state of the CDW can be visualized as a periodic charge-density modulation (Fig. 1b, black curve). The coherence of the state is reflected in the constant phase value (Fig. 1b, dashed horizontal black line), which is fixed by the boundary conditions. Other higher energy solutions with a non-uniform charge and phase distribution are possible (Fig. 1b, green and blue curves), provided that the phase changes by 2πN (with N being an integer ‘winding number’ that characterizes the different CDW states) to satisfy the boundary conditions. During approach, the attractive interaction between the tip and the sample can stabilize these excited CDW solutions through the energy balance between the gain in potential energy when extra charge density is brought under the tip and the cost of the spatial variation of the
phase needed to create this additional charge modulation. In the retraction stage, the orthogonality among these states prevents an immediate relaxation back to the ground state, defining a force-hysteresis cycle that leads to energy dissipation. Although the explanation of energy dissipation in terms of phase slips in the CDW state is appealing, we are still far from a complete understanding of the process. For example, it is not clear what effect the geometrical configuration and the chemical nature of the tip have on the dissipation peaks, which are of the order of a few meV per cycle for the pendulum atomic force microscope yet 100 times larger for the tuning-fork measurements. Also, although the spatial decay and strength of the electrostatic and vdW forces at play are quite different, their role seems almost equivalent. In fact, when changing the bias polarity, a solution with charge depletion close to the tip could be stabilized, yet the dissipation energy per cycle is symmetric with respect to the bias voltage (Fig. 1a). Nevertheless, it is clear from Kisiel and colleagues’ work that dynamic AFM is a powerful tool for exploring the collective electronic effects that are responsible for exotic phases like CDWs, superconductivity and spin density waves in real materials. ❐ Rubén Pérez is at the Department of Theoretical Physics of Condensed Matter and the Condensed Matter Physics Centre (IFIMAC), Universidad Autónoma de Madrid, E-28049 Madrid, Spain. e-mail: [email protected] References 1. Langer, M. et al. Nature Mater. 13, 173–177 (2014). 2. Urbakh, M., Klafter, J., Gourdon, D. & Israelachvili, J. Nature 430, 525–528 (2004). 3. Garcia, R. & Perez, R. Surf. Sci. Rep. 47, 197–301 (2002). 4. Volokitin, A. I. & Persson, B. N. J. Rev. Mod. Phys. 79, 1291–1329 (2007). 5. Kisiel, M. et al. Nature Mater. 10, 119–122 (2011). 6. Oyabu, N. et al. Phys. Rev. Lett. 96, 106101 (2006).
THERMOELECTRIC POLYMERS
Behind organics’ thermopower
Conjugated polymers with high electrical conductivity and high thermopower are now demonstrated. The electronic structure of these materials is that of a semi-metal, a previously unreported state for organic conductors.
Michael Chabinyc
S
ince the discovery that the electrical properties of polymers can be engineered to display insulating, semiconducting or near-metallic behaviour, these organic materials have been considered as model systems for the study
of one-dimensional charge transport 1. In their undoped and doped state, conjugated polymers have been successfully used in thin-film electronics as well as in optoelectronic applications — a striking example being organic light-emitting diodes,
NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
now commercialized in mobile phones and large-area displays2. The emerging interest for organic materials in thermoelectric applications has now brought more attention to the precise control of electrical doping. There has been recognition that conjugated 119
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Figure 1 | Charge carriers in PEDOT. a, In the neutral form, each repeat unit is best represented by the aromatic form with two double bonds. b, Removal of one electron leads to a singly charged polaron; a positive charge that induces a rearrangement of the double bonds in the repeat units. The plus symbol is the positive charge and the dot is a radical (unpaired electron). c, Removal of another electron leads to a bipolaron having two positive charges delocalized along the backbone. d, The bipolarons move in a thermal gradient to form an electrical potential in PEDOT:Tos.
polymers such as polyacetylene1 and, more recently, poly(3,4-ethylenedioxythiophene) (PEDOT)3,4 represent a unique opportunity in thermoelectrics, based on reports showing that their efficiency in converting temperature differences (ΔT) to electrical potentials (ΔV) — that is the thermopower, or Seebeck coefficient, S = –ΔV/ΔT — is similar to that of inorganic materials (such as Bi2Te3) near room temperature5. Now a team led by Xavier Crispin reports in Nature Materials that doped PEDOT exhibits an electronic structure and electrical transport properties characteristic of a semimetal, thus explaining its performance as a thermoelectric material6. To understand the basis of the work of these researchers, one must consider the unique properties of conjugated polymers. The structure of a semiconducting polymer comprises a backbone with unsaturated carbon–carbon bonds in electronic conjugation, along which charge carriers are transported with high mobility; in deposited films, molecular packing allows good intermolecular electronic coupling, so that charges can also move between chains. To obtain a high electrical conductivity, carriers must be introduced by doping, that is oxidation or reduction of the polymer chains. The carriers in organic polymers may bear either a single charge, and are termed polarons, or two charges, termed bipolarons (Fig. 1). It has long been known that bipolarons are the dominant charge carriers in polythiophenes at high oxidation states1. 120
In a commercial water-soluble version of PEDOT, used for the realization of electrostatic dissipation layers and holeinjection layers in organic diodes, it is complexed with a strong acid, polystyrene sulphonate (PSS). In this blend, as in many doped semiconducting polymers, polarons and bipolarons coexist and the ratio of their populations depends on the amount of PSS introduced in PEDOT. In contrast, Crispin and colleagues argue that bipolarons are the dominant species in vapour-deposited samples of PEDOT with a tosylate counterion (Tos). The key to their observation is the combination of the structural order of the PEDOT:Tos (observed by X-ray scattering) and its oxidation state, which is sufficiently high that the carriers in the backbone are only bipolarons. The latter conjecture is confirmed by means of electron spin resonance experiments showing a lack of spin centres — a hallmark of polaronic carriers that is present in comparative samples of PEDOT:PSS. The identity of the charge carriers provides important information on how the carriers fill the electronic states of the material and, as a consequence, on the behaviour of the material itself. In a semiconductor there is a relatively large gap between the energy of the highest filled (valence) states and the lowest unfilled (conduction) states. In a metal the carriers partially occupy a band of states such that there is no gap near the Fermi level. The researchers propose a model for the
electronic structure of films of PEDOT:Tos, where the bipolaron states form an empty band very close (or touching) in energy to the valence band. Materials where there is no gap between the valence and conduction states are referred to as semi-metals. The energetic distribution of the states in proximity of the Fermi level — which shows a steep rise in semi-metals — is related to the Seebeck coefficient: the steeper the distribution, the higher the coefficient. Consistently, Crispin and co-workers measure high values of S in PEDOT:Tos films as well as striking signatures of metallic conduction; therefore these authors propose, based on experimental evidence, that these organic materials behave as semi-metals. These results, supporting earlier discussions on the formation of bipolaron bands in conducting polymers7, are likely to reignite the study of the electronic structure of doped organic polymers. In view of the high thermopower and stability, PEDOT is the current champion organic thermoelectric material. Thermoelectrics are used in applications ranging from power supplies in deep space probes to seat coolers in automobiles8. This polymer demonstrates a thermoelectric conversion efficiency that is still behind the performance shown by inorganics used in these applications, but the potential to develop low-cost, solution-processable and simply deposited organic thermoelectrics has practical advantages that may prove useful. As an example, rugged thinfilm thermoelectric modules, which are
NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
news & views particularly interesting for waste heat capture in wearable systems and also in cooling devices for electronics, could be fabricated easily. Conjugated polymers continue to exhibit surprising behaviour. In addition to the work by Crispin and colleagues, the recent report of the inverse spin Hall effect in PEDOT:PSS demonstrates that this well-known material can still reveal new characteristics9.
The large number of unique organic semiconductors — a materials family whose variety far surpasses its inorganic equivalent — suggests the potential for many exciting properties yet undiscovered. ❐ Michael Chabinyc is at the Materials Department, University of California, Santa Barbara, California 93106-5050, USA. e-mail: [email protected]
References 1. 2. 3. 4. 5. 6. 7. 8. 9.
Heeger, A. J. J. Phys. Chem. B 105, 8475–8491 (2001). Yang, Y. & Wudl, F. Adv. Mater. 21, 1401–1403 (2009). Bubnova, O. et al. Nature Mater. 10, 429–433 (2011). Kim, G.-H., Shao, L., Zhang, K. & Pipe, K. P. Nature Mater. 12, 719–723 (2013). Snyder, G. J. & Toberer, E. S. Nature Mater. 7, 105–114 (2008). Bubnova, O. et. al. Nature Mater. 13, 190–194 (2014). Conwell, E. M. & Mizes, H. A. Phys. Rev. B 44, 937–942 (1991). Bell, L. E. Science 321, 1457–1461 (2008). Ando, K., Watanabe, S., Mooser, S., Sitaoh, E. & Sirringhaus, H. Nature Mater. 12, 622–627 (2013).
NANOPARTICLE CRYSTALLIZATION
DNA-bonded ‘atoms’
DNA-capped nanoparticles crystallize into uniform microcrystals of Wulff polyhedra when cooled slowly through the melting temperature of the DNA linkers.
Shogo Hamada, Shawn J. Tan and Dan Luo
F
rom a bottom-up perspective, the rational assembly of supramolecular architectures from designed building blocks should allow for the synthesis of a wide variety of materials with unique properties. In this respect, DNA-mediated self-assembly is a promising strategy for engineering the geometry of nanosized colloidal building blocks1–3 and for controlling their connectivity through Watson–Crick base-pairing rules. In fact, the precise design of both DNA length and sequence in DNA-capped nanoparticles has allowed the design of interparticle distances and interactions, resulting in the formation of two- and three-dimensional superlattices4–7. However, whereas considerable progress has been made in the synthesis of elementary building blocks with controlled size and morphology, programming their crystallization into specific symmetries at the microand macroscales remains a challenge.
Reporting in Nature, Chad Mirkin and colleagues now show that DNA-capped gold nanoparticles crystallize into Wulff polyhedra — that is, the equilibrium shapes that minimize the surface energy associated with the crystal facets — when cooled down slowly past the melting temperature of the DNA linkers, and that the polyhedra formed can be predicted from theoretical principles long used in atomic-scale crystallization8. The demonstration that the Wulff construction can be extended to DNA-mediated nanoparticle crystallization represents a significant step towards establishing a unified framework for the design of macroscopic crystals of DNA-capped nanoparticles. By cooling solutions of DNA-capped gold nanoparticles from above the melting temperature of the DNA ligands over a few days — a commonly used process in the assembly of DNA nanostructures as well as in conventional crystallization techniques
a
for atoms and molecules — Mirkin and co-authors were able to form body-centred cubic (bcc), binary CsCl and face-centred cubic superlattices, of which the bcc and CsCl lattices resulted in micrometrescale rhombic dodecahedron single crystals with the shape of the expected Wulff polyhedron. Interestingly, rhombic dodecahedrons were obtained across a range of nanoparticle sizes and DNA chain lengths (Fig. 1a), suggesting that this is likely to be the most thermodynamically favourable shape for DNA-capped nanoparticle systems. Furthermore, molecular dynamics simulations of a coarse-grained model of the system (Fig. 1b) and calculations of the surface energy ratio of facets using a brokenbond approximation predicted the Wulff polyhedra observed experimentally. Mirkin and colleagues’ findings imply that, because the surface energy of crystals of DNA-capped nanoparticles are b
Figure 1 | Microcrystals of DNA-capped nanoparticles8. a, Scanning electron micrographs of rhombic dodecahedron crystals of DNA-capped gold nanoparticles with varying unit-cell size and composition (left, a bcc lattice of 5-nm nanoparticles; middle, a bcc lattice of 10-nm nanoparticles; right, a CsCl lattice of 20-nm and 15-nm nanoparticles). Scale bars (left to right), 2 μm, 4 μm and 2 μm. b, Snapshot of a molecular dynamics simulation showing a rhombic dodecahedron crystal made of two types of bead interacting according to a coarse-grained model of DNA-capped nanoparticles that form a bcc lattice. NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
121
is laterally and cyclically moved on top of the surface — or, in the case of dynamic AFM, when the tip approaches and retracts from the surface — the different forces in opposite directions define a hysteresis cycle, whose area determines the energy dissipated per cycle. Force hysteresis arises because of the presence of energy barriers between the ground state and excited configurations of the system. As the tip– sample interaction can stabilize one of these excited configurations, energy barriers can prevent the relaxation back to the ground state during tip retraction. Hence, the system is in different states during tip approach and retraction, which provides a quantitative understanding of the energy dissipated in tip–sample adhesion at the level of a single atomic bond6. Following this idea, Kisiel and co-workers suggest that the observed dissipation peaks result from the hysteresis cycle formed by the crossing between CDW states with phases differing by 2π that are stabilized by the interaction with the tip (Fig. 1b). The ground state of the CDW can be visualized as a periodic charge-density modulation (Fig. 1b, black curve). The coherence of the state is reflected in the constant phase value (Fig. 1b, dashed horizontal black line), which is fixed by the boundary conditions. Other higher energy solutions with a non-uniform charge and phase distribution are possible (Fig. 1b, green and blue curves), provided that the phase changes by 2πN (with N being an integer ‘winding number’ that characterizes the different CDW states) to satisfy the boundary conditions. During approach, the attractive interaction between the tip and the sample can stabilize these excited CDW solutions through the energy balance between the gain in potential energy when extra charge density is brought under the tip and the cost of the spatial variation of the
phase needed to create this additional charge modulation. In the retraction stage, the orthogonality among these states prevents an immediate relaxation back to the ground state, defining a force-hysteresis cycle that leads to energy dissipation. Although the explanation of energy dissipation in terms of phase slips in the CDW state is appealing, we are still far from a complete understanding of the process. For example, it is not clear what effect the geometrical configuration and the chemical nature of the tip have on the dissipation peaks, which are of the order of a few meV per cycle for the pendulum atomic force microscope yet 100 times larger for the tuning-fork measurements. Also, although the spatial decay and strength of the electrostatic and vdW forces at play are quite different, their role seems almost equivalent. In fact, when changing the bias polarity, a solution with charge depletion close to the tip could be stabilized, yet the dissipation energy per cycle is symmetric with respect to the bias voltage (Fig. 1a). Nevertheless, it is clear from Kisiel and colleagues’ work that dynamic AFM is a powerful tool for exploring the collective electronic effects that are responsible for exotic phases like CDWs, superconductivity and spin density waves in real materials. ❐ Rubén Pérez is at the Department of Theoretical Physics of Condensed Matter and the Condensed Matter Physics Centre (IFIMAC), Universidad Autónoma de Madrid, E-28049 Madrid, Spain. e-mail: [email protected] References 1. Langer, M. et al. Nature Mater. 13, 173–177 (2014). 2. Urbakh, M., Klafter, J., Gourdon, D. & Israelachvili, J. Nature 430, 525–528 (2004). 3. Garcia, R. & Perez, R. Surf. Sci. Rep. 47, 197–301 (2002). 4. Volokitin, A. I. & Persson, B. N. J. Rev. Mod. Phys. 79, 1291–1329 (2007). 5. Kisiel, M. et al. Nature Mater. 10, 119–122 (2011). 6. Oyabu, N. et al. Phys. Rev. Lett. 96, 106101 (2006).
THERMOELECTRIC POLYMERS
Behind organics’ thermopower
Conjugated polymers with high electrical conductivity and high thermopower are now demonstrated. The electronic structure of these materials is that of a semi-metal, a previously unreported state for organic conductors.
Michael Chabinyc
S
ince the discovery that the electrical properties of polymers can be engineered to display insulating, semiconducting or near-metallic behaviour, these organic materials have been considered as model systems for the study
of one-dimensional charge transport 1. In their undoped and doped state, conjugated polymers have been successfully used in thin-film electronics as well as in optoelectronic applications — a striking example being organic light-emitting diodes,
NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
now commercialized in mobile phones and large-area displays2. The emerging interest for organic materials in thermoelectric applications has now brought more attention to the precise control of electrical doping. There has been recognition that conjugated 119
news & views
a
b O
O
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Hot
++ Charge flow
++
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Figure 1 | Charge carriers in PEDOT. a, In the neutral form, each repeat unit is best represented by the aromatic form with two double bonds. b, Removal of one electron leads to a singly charged polaron; a positive charge that induces a rearrangement of the double bonds in the repeat units. The plus symbol is the positive charge and the dot is a radical (unpaired electron). c, Removal of another electron leads to a bipolaron having two positive charges delocalized along the backbone. d, The bipolarons move in a thermal gradient to form an electrical potential in PEDOT:Tos.
polymers such as polyacetylene1 and, more recently, poly(3,4-ethylenedioxythiophene) (PEDOT)3,4 represent a unique opportunity in thermoelectrics, based on reports showing that their efficiency in converting temperature differences (ΔT) to electrical potentials (ΔV) — that is the thermopower, or Seebeck coefficient, S = –ΔV/ΔT — is similar to that of inorganic materials (such as Bi2Te3) near room temperature5. Now a team led by Xavier Crispin reports in Nature Materials that doped PEDOT exhibits an electronic structure and electrical transport properties characteristic of a semimetal, thus explaining its performance as a thermoelectric material6. To understand the basis of the work of these researchers, one must consider the unique properties of conjugated polymers. The structure of a semiconducting polymer comprises a backbone with unsaturated carbon–carbon bonds in electronic conjugation, along which charge carriers are transported with high mobility; in deposited films, molecular packing allows good intermolecular electronic coupling, so that charges can also move between chains. To obtain a high electrical conductivity, carriers must be introduced by doping, that is oxidation or reduction of the polymer chains. The carriers in organic polymers may bear either a single charge, and are termed polarons, or two charges, termed bipolarons (Fig. 1). It has long been known that bipolarons are the dominant charge carriers in polythiophenes at high oxidation states1. 120
In a commercial water-soluble version of PEDOT, used for the realization of electrostatic dissipation layers and holeinjection layers in organic diodes, it is complexed with a strong acid, polystyrene sulphonate (PSS). In this blend, as in many doped semiconducting polymers, polarons and bipolarons coexist and the ratio of their populations depends on the amount of PSS introduced in PEDOT. In contrast, Crispin and colleagues argue that bipolarons are the dominant species in vapour-deposited samples of PEDOT with a tosylate counterion (Tos). The key to their observation is the combination of the structural order of the PEDOT:Tos (observed by X-ray scattering) and its oxidation state, which is sufficiently high that the carriers in the backbone are only bipolarons. The latter conjecture is confirmed by means of electron spin resonance experiments showing a lack of spin centres — a hallmark of polaronic carriers that is present in comparative samples of PEDOT:PSS. The identity of the charge carriers provides important information on how the carriers fill the electronic states of the material and, as a consequence, on the behaviour of the material itself. In a semiconductor there is a relatively large gap between the energy of the highest filled (valence) states and the lowest unfilled (conduction) states. In a metal the carriers partially occupy a band of states such that there is no gap near the Fermi level. The researchers propose a model for the
electronic structure of films of PEDOT:Tos, where the bipolaron states form an empty band very close (or touching) in energy to the valence band. Materials where there is no gap between the valence and conduction states are referred to as semi-metals. The energetic distribution of the states in proximity of the Fermi level — which shows a steep rise in semi-metals — is related to the Seebeck coefficient: the steeper the distribution, the higher the coefficient. Consistently, Crispin and co-workers measure high values of S in PEDOT:Tos films as well as striking signatures of metallic conduction; therefore these authors propose, based on experimental evidence, that these organic materials behave as semi-metals. These results, supporting earlier discussions on the formation of bipolaron bands in conducting polymers7, are likely to reignite the study of the electronic structure of doped organic polymers. In view of the high thermopower and stability, PEDOT is the current champion organic thermoelectric material. Thermoelectrics are used in applications ranging from power supplies in deep space probes to seat coolers in automobiles8. This polymer demonstrates a thermoelectric conversion efficiency that is still behind the performance shown by inorganics used in these applications, but the potential to develop low-cost, solution-processable and simply deposited organic thermoelectrics has practical advantages that may prove useful. As an example, rugged thinfilm thermoelectric modules, which are
NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
news & views particularly interesting for waste heat capture in wearable systems and also in cooling devices for electronics, could be fabricated easily. Conjugated polymers continue to exhibit surprising behaviour. In addition to the work by Crispin and colleagues, the recent report of the inverse spin Hall effect in PEDOT:PSS demonstrates that this well-known material can still reveal new characteristics9.
The large number of unique organic semiconductors — a materials family whose variety far surpasses its inorganic equivalent — suggests the potential for many exciting properties yet undiscovered. ❐ Michael Chabinyc is at the Materials Department, University of California, Santa Barbara, California 93106-5050, USA. e-mail: [email protected]
References 1. 2. 3. 4. 5. 6. 7. 8. 9.
Heeger, A. J. J. Phys. Chem. B 105, 8475–8491 (2001). Yang, Y. & Wudl, F. Adv. Mater. 21, 1401–1403 (2009). Bubnova, O. et al. Nature Mater. 10, 429–433 (2011). Kim, G.-H., Shao, L., Zhang, K. & Pipe, K. P. Nature Mater. 12, 719–723 (2013). Snyder, G. J. & Toberer, E. S. Nature Mater. 7, 105–114 (2008). Bubnova, O. et. al. Nature Mater. 13, 190–194 (2014). Conwell, E. M. & Mizes, H. A. Phys. Rev. B 44, 937–942 (1991). Bell, L. E. Science 321, 1457–1461 (2008). Ando, K., Watanabe, S., Mooser, S., Sitaoh, E. & Sirringhaus, H. Nature Mater. 12, 622–627 (2013).
NANOPARTICLE CRYSTALLIZATION
DNA-bonded ‘atoms’
DNA-capped nanoparticles crystallize into uniform microcrystals of Wulff polyhedra when cooled slowly through the melting temperature of the DNA linkers.
Shogo Hamada, Shawn J. Tan and Dan Luo
F
rom a bottom-up perspective, the rational assembly of supramolecular architectures from designed building blocks should allow for the synthesis of a wide variety of materials with unique properties. In this respect, DNA-mediated self-assembly is a promising strategy for engineering the geometry of nanosized colloidal building blocks1–3 and for controlling their connectivity through Watson–Crick base-pairing rules. In fact, the precise design of both DNA length and sequence in DNA-capped nanoparticles has allowed the design of interparticle distances and interactions, resulting in the formation of two- and three-dimensional superlattices4–7. However, whereas considerable progress has been made in the synthesis of elementary building blocks with controlled size and morphology, programming their crystallization into specific symmetries at the microand macroscales remains a challenge.
Reporting in Nature, Chad Mirkin and colleagues now show that DNA-capped gold nanoparticles crystallize into Wulff polyhedra — that is, the equilibrium shapes that minimize the surface energy associated with the crystal facets — when cooled down slowly past the melting temperature of the DNA linkers, and that the polyhedra formed can be predicted from theoretical principles long used in atomic-scale crystallization8. The demonstration that the Wulff construction can be extended to DNA-mediated nanoparticle crystallization represents a significant step towards establishing a unified framework for the design of macroscopic crystals of DNA-capped nanoparticles. By cooling solutions of DNA-capped gold nanoparticles from above the melting temperature of the DNA ligands over a few days — a commonly used process in the assembly of DNA nanostructures as well as in conventional crystallization techniques
a
for atoms and molecules — Mirkin and co-authors were able to form body-centred cubic (bcc), binary CsCl and face-centred cubic superlattices, of which the bcc and CsCl lattices resulted in micrometrescale rhombic dodecahedron single crystals with the shape of the expected Wulff polyhedron. Interestingly, rhombic dodecahedrons were obtained across a range of nanoparticle sizes and DNA chain lengths (Fig. 1a), suggesting that this is likely to be the most thermodynamically favourable shape for DNA-capped nanoparticle systems. Furthermore, molecular dynamics simulations of a coarse-grained model of the system (Fig. 1b) and calculations of the surface energy ratio of facets using a brokenbond approximation predicted the Wulff polyhedra observed experimentally. Mirkin and colleagues’ findings imply that, because the surface energy of crystals of DNA-capped nanoparticles are b
Figure 1 | Microcrystals of DNA-capped nanoparticles8. a, Scanning electron micrographs of rhombic dodecahedron crystals of DNA-capped gold nanoparticles with varying unit-cell size and composition (left, a bcc lattice of 5-nm nanoparticles; middle, a bcc lattice of 10-nm nanoparticles; right, a CsCl lattice of 20-nm and 15-nm nanoparticles). Scale bars (left to right), 2 μm, 4 μm and 2 μm. b, Snapshot of a molecular dynamics simulation showing a rhombic dodecahedron crystal made of two types of bead interacting according to a coarse-grained model of DNA-capped nanoparticles that form a bcc lattice. NATURE MATERIALS | VOL 13 | FEBRUARY 2014 | www.nature.com/naturematerials
© 2014 Macmillan Publishers Limited. All rights reserved
121
