Imaging Crystallization Preshit Dandekar and Michael F. Doherty Science 344, 705 (2014); DOI: 10.1126/science.1254259
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of May 15, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6185/705.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6185/705.full.html#related This article cites 10 articles, 6 of which can be accessed free: http://www.sciencemag.org/content/344/6185/705.full.html#ref-list-1 This article appears in the following subject collections: Materials Science http://www.sciencemag.org/cgi/collection/mat_sci
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on May 15, 2014
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PERSPECTIVES MATERIALS SCIENCE
Real-time atomic force microscopy provides insights into complex processes associated with crystal growth.
Imaging Crystallization Preshit Dandekar and Michael F. Doherty
T
he invention of the atomic force microscope (AFM) (1) created a revolution in the imaging of crystal growth processes. Surface features can now be visualized and measured in three dimensions so that step heights, terrace widths, and other surface features can be measured with subnanometerscale resolution. The current generation of commercial AFM instruments is robust and capable of angstrom-scale [and even single atom-scale (2)] resolution. During the 1990s, the AFM flow cell was introduced (3), which allows for real-time imaging as crystals grow, thereby providing insight into the time evolution of surface structure (4). On page 729 of this issue, Lupulescu and Rimer (5) report another exciting development in AFM imaging science—the ability to monitor the crystallization process in real time and under realistic growth conditions. Understanding the mechanism of crystal growth remains an active area of research in a diversity of fields, including geology, biomineralization, catalysis, and pharmaceutics. Classical growth mechanisms include the spiral growth model (6) and the birth-andspread model (7). Both mechanisms assume that the growth units are individual molecules or ions that attach on monomolecular steps present on a crystal surface. Aggregative growth of oriented nanoparticle precursors is a nonclassical crystallization pathway that has been widely observed since the late 1990s
CREDIT: ADAPTED WITH PERMISSION FROM DE YOREO AND DOVE (11).
Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106, USA. E-mail: [email protected]
A
(8). In situ transmission electron microscopy images have shown that these precursors may be crystalline and that the aggregation yields single crystals (9). Lupulescu and Rimer have developed an AFM instrument capable of imaging in situ, time-resolved crystal growth under extreme conditions of temperature (up to 300°C) and pH (from 10 to 12). Also, they have overcome the challenges associated with long imaging times (~40 hours) in these difficult experiments. Lupulescu and Rimer observed real-time crystal growth on the (010) face of silicalite-1 crystals and identified the attachment of two types of growth units: silica molecules and silica nanoparticle precursors on the zeolite crystal surface [see figure 1A of (5)]. The temporal profile of the measured height of the growth features on the surface is convincing evidence that both species are involved in the growth process. They have found that the deposition of silica molecules on the AFM tip is a major challenge associated with the in situ AFM imaging of silicalite-1 crystal growth. They verified that the error in the measurement of size of growth features due to this deposition is appreciable only for the lateral directions () and that the height of the features that attach on the (010) crystal surface is accurately measured. The agreement between the size of the precursor particles in the solution (from x-ray scattering) and on the (010) surface (from time-resolved AFM measurements) demonstrates that the precursor particles indeed attach on the crystal surface. The attachment of precursors on the terraces of a crystal surface could affect the
B
growth kinetics. These nanoparticles would provide more favorable sites for individual molecules to attach onto the surface, thereby promoting growth. However, structural rearrangement of the deposited nanoparticles to match the structure of the underlying surface (5) could impede the overall growth kinetics. A mechanistic growth model for zeolite crystals such as silicalite-1 must account for these competing effects of precursor deposition. In situ AFM imaging can elucidate the effect of impurity or additive molecules on crystal growth kinetics. These species may disrupt the crystal growth process in one or more ways (10). Time-resolved motion of the steps on crystal surfaces reveals the specific action of a particular additive species (see the figure) (11). Identification of the additive species that modify the crystal growth kinetics in the most desirable and efficient manner could lead to the rational design of crystalline materials with specific functionality. One of the remaining mysteries in crystal growth is the mechanism by which polar crystal faces stabilize and grow (12). Polar crystals have a nonzero net electrostatic Watching it grow. The effect of additive species on spiral growth and the shape of calcite crystals. Each panel shows an AFM image of spiral growth hillocks (main image) and a scanning electron microscope image of the crystal shape (inset), with dashed lines to indicate the glide plane symmetry. (A) Pure calcite. (B) Calcite plus Mg2+. The inset shows crystals grown at Mg2+:Ca2+ mole ratios of 1.5 (left) and 2.0 (right). (C) Calcite plus D-aspartic acid; a snapshot of a D-aspartic acid molecule binding to a particular step on calcite crystal surface.
C
www.sciencemag.org SCIENCE VOL 344 16 MAY 2014 Published by AAAS
705
PERSPECTIVES dipole moment in their unit cell (e.g., wurtzite structure) that results in asymmetric crystal shapes. Polar crystal faces find application in semiconductors (e.g., zinc oxide) and catalysis. It has been impossible to observe the growth of such crystals in real time under typical synthesis conditions (T ~ 100°C), but now there is reason for optimism that such important crystallizations can be tempted to reveal their secrets.
References 1. G. Binnig, C. F. Quate, Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986). 2. G. Meyer, N. M. Amer, Appl. Phys. Lett. 56, 2100 (1990). 3. S. Manne et al., Science 251, 183 (1991). 4. J. D. Rimer et al., Science 330, 337 (2010). 5. A. I. Lupulescu, J. D. Rimer, Science 344, 729 (2014). 6. W. K. Burton, N. Cabrera, F. C. Frank, Phil. Trans. Roy. Soc. A 243, 299 (1951). 7. M. Ohara, R. Reid, Modeling Crystal Growth Rates from Solution (Prentice-Hall, Upper Saddle River, NJ, 1973). 8. R. L. Penn, J. F. Banfield, Science 281, 969 (1998). 9. D. Li et al., Science 336, 1014 (2012).
10. N. Cabrera, D. Vermilyea, Growth and Perfection of Crystals, R. Doremus, B. Roberts, D. Turnbull, Eds. (Wiley, New York, 1958). 11. J. J. De Yoreo, P. M. Dove, Science 306, 1301 (2004). 12. J. Goniakowski et al., Rep. Prog. Phys. 71, 016501 (2008).
Acknowledgments: Financial support was provided by the Dow Chemical Company through a doctoral fellowship (P.D.) and by the National Science Foundation through CBET1159746 (M.F.D.). 10.1126/science.1254259
MATERIALS SCIENCE
Toward Recyclable Thermosets
Advances in synthesis are leading to thermoset plastics that can be converted to the starting monomers.
Timothy E. Long
706
that combine the desirable thermal and chemical stability of conventional thermosets with recyclability and reprocessability. For several decades, the microelectronics, aerospace, and automotive industries have used a relatively small number of thermally stable and mechanically ductile thermosets. A nitrogen-containing heterocycle exemplifies a desirable chemical linkage in polymeric thermosets due to thermal stability and restricted rotation that provides high glass transition temperatures (4, 5). However, many polymerization processes for thermosets require high temperatures and long reaction times. Researchers are therefore trying to develop chemical processes that require less energy. The microelectronics and automotive industries also demand higher performance, including increased modulus, improved
toughness, and flame resistance. Use of existing monomers in new synthetic methods can also lead to novel compositions with desirable properties. García et al.’s novel thermoset-forming reaction is based on the well-established reactivity of monofunctional aromatic and aliphatic amines with paraformaldehyde. A monofunctional amine in combination with paraformaldehyde readily forms a low–molar mass triazine. Use of a difunctional amine enabled the in situ formation of a triazine cross-link point that is the basis of the polymeric network of García et al.’s thermosets (see the figure). The use of a multifunctional amine for making high-performance triazine polymeric networks is unprecedented. The authors first created a family of lowtemperature, hemiaminal-based thermosets,
Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, VA 24061, USA. E-mail: [email protected]
Reversible thermoset plastics. García et al. show that the formation of a trifunctional cross-link point through reaction of a diamine and paraformaldehyde leads to a thermally stable and mechanically ductile thermoset. Low pH triggers the reversal of the thermoset architecture to the corresponding monomers.
pH < 2
+
Heat
=
N N
N O
=
NH2
H2N = O H
16 MAY 2014 VOL 344 SCIENCE www.sciencemag.org Published by AAAS
H
CREDIT: K. ZHANG/VIRGINIA TECH
R
ecycling codes on plastic food and beverage packaging serve to guide consumers’ daily decisions about the disposal of used packaging. However, technological obstacles remain for the recycling of more sophisticated polymeric packaging, ranging from multilayered food packaging to composite polymeric materials for electronic packaging. Moreover, many electronic devices contain heat-resistant, chemically stable polymers called thermosets that are not amenable to conventional collecting and recycling. On page 732 of this issue, García et al. (1) report a crucial step toward recyclable thermosets with the synthesis of ductile, insulating, temperature-resistant, and chemically inert thermosets that can be returned to their monomeric state through a pH trigger. Thermoplastics are polymers that become pliable or moldable at elevated temperatures, but return to a solid state when cooled. These polymers can thus be readily processed or reprocessed upon heating, and are therefore widely used in food and beverage packaging. In contrast, thermosets are chemically crosslinked polymers, networks, or gels with chemical bonds between chains that do not thermally dissociate, even at high temperature. They are ideal for high-temperature electronic or automotive applications, but cannot be reprocessed or recycled either by melting or by solution processing. Many researchers are now challenging this classical definition with concepts of recyclable networks, reworkable encapsulants, reversible gels, self-healing polymeric coatings, and stimuli-responsive polymeric structures (2, 3). The goal is to create reversible thermosets
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of May 15, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6185/705.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6185/705.full.html#related This article cites 10 articles, 6 of which can be accessed free: http://www.sciencemag.org/content/344/6185/705.full.html#ref-list-1 This article appears in the following subject collections: Materials Science http://www.sciencemag.org/cgi/collection/mat_sci
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on May 15, 2014
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
PERSPECTIVES MATERIALS SCIENCE
Real-time atomic force microscopy provides insights into complex processes associated with crystal growth.
Imaging Crystallization Preshit Dandekar and Michael F. Doherty
T
he invention of the atomic force microscope (AFM) (1) created a revolution in the imaging of crystal growth processes. Surface features can now be visualized and measured in three dimensions so that step heights, terrace widths, and other surface features can be measured with subnanometerscale resolution. The current generation of commercial AFM instruments is robust and capable of angstrom-scale [and even single atom-scale (2)] resolution. During the 1990s, the AFM flow cell was introduced (3), which allows for real-time imaging as crystals grow, thereby providing insight into the time evolution of surface structure (4). On page 729 of this issue, Lupulescu and Rimer (5) report another exciting development in AFM imaging science—the ability to monitor the crystallization process in real time and under realistic growth conditions. Understanding the mechanism of crystal growth remains an active area of research in a diversity of fields, including geology, biomineralization, catalysis, and pharmaceutics. Classical growth mechanisms include the spiral growth model (6) and the birth-andspread model (7). Both mechanisms assume that the growth units are individual molecules or ions that attach on monomolecular steps present on a crystal surface. Aggregative growth of oriented nanoparticle precursors is a nonclassical crystallization pathway that has been widely observed since the late 1990s
CREDIT: ADAPTED WITH PERMISSION FROM DE YOREO AND DOVE (11).
Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106, USA. E-mail: [email protected]
A
(8). In situ transmission electron microscopy images have shown that these precursors may be crystalline and that the aggregation yields single crystals (9). Lupulescu and Rimer have developed an AFM instrument capable of imaging in situ, time-resolved crystal growth under extreme conditions of temperature (up to 300°C) and pH (from 10 to 12). Also, they have overcome the challenges associated with long imaging times (~40 hours) in these difficult experiments. Lupulescu and Rimer observed real-time crystal growth on the (010) face of silicalite-1 crystals and identified the attachment of two types of growth units: silica molecules and silica nanoparticle precursors on the zeolite crystal surface [see figure 1A of (5)]. The temporal profile of the measured height of the growth features on the surface is convincing evidence that both species are involved in the growth process. They have found that the deposition of silica molecules on the AFM tip is a major challenge associated with the in situ AFM imaging of silicalite-1 crystal growth. They verified that the error in the measurement of size of growth features due to this deposition is appreciable only for the lateral directions () and that the height of the features that attach on the (010) crystal surface is accurately measured. The agreement between the size of the precursor particles in the solution (from x-ray scattering) and on the (010) surface (from time-resolved AFM measurements) demonstrates that the precursor particles indeed attach on the crystal surface. The attachment of precursors on the terraces of a crystal surface could affect the
B
growth kinetics. These nanoparticles would provide more favorable sites for individual molecules to attach onto the surface, thereby promoting growth. However, structural rearrangement of the deposited nanoparticles to match the structure of the underlying surface (5) could impede the overall growth kinetics. A mechanistic growth model for zeolite crystals such as silicalite-1 must account for these competing effects of precursor deposition. In situ AFM imaging can elucidate the effect of impurity or additive molecules on crystal growth kinetics. These species may disrupt the crystal growth process in one or more ways (10). Time-resolved motion of the steps on crystal surfaces reveals the specific action of a particular additive species (see the figure) (11). Identification of the additive species that modify the crystal growth kinetics in the most desirable and efficient manner could lead to the rational design of crystalline materials with specific functionality. One of the remaining mysteries in crystal growth is the mechanism by which polar crystal faces stabilize and grow (12). Polar crystals have a nonzero net electrostatic Watching it grow. The effect of additive species on spiral growth and the shape of calcite crystals. Each panel shows an AFM image of spiral growth hillocks (main image) and a scanning electron microscope image of the crystal shape (inset), with dashed lines to indicate the glide plane symmetry. (A) Pure calcite. (B) Calcite plus Mg2+. The inset shows crystals grown at Mg2+:Ca2+ mole ratios of 1.5 (left) and 2.0 (right). (C) Calcite plus D-aspartic acid; a snapshot of a D-aspartic acid molecule binding to a particular step on calcite crystal surface.
C
www.sciencemag.org SCIENCE VOL 344 16 MAY 2014 Published by AAAS
705
PERSPECTIVES dipole moment in their unit cell (e.g., wurtzite structure) that results in asymmetric crystal shapes. Polar crystal faces find application in semiconductors (e.g., zinc oxide) and catalysis. It has been impossible to observe the growth of such crystals in real time under typical synthesis conditions (T ~ 100°C), but now there is reason for optimism that such important crystallizations can be tempted to reveal their secrets.
References 1. G. Binnig, C. F. Quate, Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986). 2. G. Meyer, N. M. Amer, Appl. Phys. Lett. 56, 2100 (1990). 3. S. Manne et al., Science 251, 183 (1991). 4. J. D. Rimer et al., Science 330, 337 (2010). 5. A. I. Lupulescu, J. D. Rimer, Science 344, 729 (2014). 6. W. K. Burton, N. Cabrera, F. C. Frank, Phil. Trans. Roy. Soc. A 243, 299 (1951). 7. M. Ohara, R. Reid, Modeling Crystal Growth Rates from Solution (Prentice-Hall, Upper Saddle River, NJ, 1973). 8. R. L. Penn, J. F. Banfield, Science 281, 969 (1998). 9. D. Li et al., Science 336, 1014 (2012).
10. N. Cabrera, D. Vermilyea, Growth and Perfection of Crystals, R. Doremus, B. Roberts, D. Turnbull, Eds. (Wiley, New York, 1958). 11. J. J. De Yoreo, P. M. Dove, Science 306, 1301 (2004). 12. J. Goniakowski et al., Rep. Prog. Phys. 71, 016501 (2008).
Acknowledgments: Financial support was provided by the Dow Chemical Company through a doctoral fellowship (P.D.) and by the National Science Foundation through CBET1159746 (M.F.D.). 10.1126/science.1254259
MATERIALS SCIENCE
Toward Recyclable Thermosets
Advances in synthesis are leading to thermoset plastics that can be converted to the starting monomers.
Timothy E. Long
706
that combine the desirable thermal and chemical stability of conventional thermosets with recyclability and reprocessability. For several decades, the microelectronics, aerospace, and automotive industries have used a relatively small number of thermally stable and mechanically ductile thermosets. A nitrogen-containing heterocycle exemplifies a desirable chemical linkage in polymeric thermosets due to thermal stability and restricted rotation that provides high glass transition temperatures (4, 5). However, many polymerization processes for thermosets require high temperatures and long reaction times. Researchers are therefore trying to develop chemical processes that require less energy. The microelectronics and automotive industries also demand higher performance, including increased modulus, improved
toughness, and flame resistance. Use of existing monomers in new synthetic methods can also lead to novel compositions with desirable properties. García et al.’s novel thermoset-forming reaction is based on the well-established reactivity of monofunctional aromatic and aliphatic amines with paraformaldehyde. A monofunctional amine in combination with paraformaldehyde readily forms a low–molar mass triazine. Use of a difunctional amine enabled the in situ formation of a triazine cross-link point that is the basis of the polymeric network of García et al.’s thermosets (see the figure). The use of a multifunctional amine for making high-performance triazine polymeric networks is unprecedented. The authors first created a family of lowtemperature, hemiaminal-based thermosets,
Department of Chemistry, Macromolecules and Interfaces Institute, Virginia Tech, Blacksburg, VA 24061, USA. E-mail: [email protected]
Reversible thermoset plastics. García et al. show that the formation of a trifunctional cross-link point through reaction of a diamine and paraformaldehyde leads to a thermally stable and mechanically ductile thermoset. Low pH triggers the reversal of the thermoset architecture to the corresponding monomers.
pH < 2
+
Heat
=
N N
N O
=
NH2
H2N = O H
16 MAY 2014 VOL 344 SCIENCE www.sciencemag.org Published by AAAS
H
CREDIT: K. ZHANG/VIRGINIA TECH
R
ecycling codes on plastic food and beverage packaging serve to guide consumers’ daily decisions about the disposal of used packaging. However, technological obstacles remain for the recycling of more sophisticated polymeric packaging, ranging from multilayered food packaging to composite polymeric materials for electronic packaging. Moreover, many electronic devices contain heat-resistant, chemically stable polymers called thermosets that are not amenable to conventional collecting and recycling. On page 732 of this issue, García et al. (1) report a crucial step toward recyclable thermosets with the synthesis of ductile, insulating, temperature-resistant, and chemically inert thermosets that can be returned to their monomeric state through a pH trigger. Thermoplastics are polymers that become pliable or moldable at elevated temperatures, but return to a solid state when cooled. These polymers can thus be readily processed or reprocessed upon heating, and are therefore widely used in food and beverage packaging. In contrast, thermosets are chemically crosslinked polymers, networks, or gels with chemical bonds between chains that do not thermally dissociate, even at high temperature. They are ideal for high-temperature electronic or automotive applications, but cannot be reprocessed or recycled either by melting or by solution processing. Many researchers are now challenging this classical definition with concepts of recyclable networks, reworkable encapsulants, reversible gels, self-healing polymeric coatings, and stimuli-responsive polymeric structures (2, 3). The goal is to create reversible thermosets
