PerspectiveBIOMINERALIZATION

Naturally Aligned Nanocrystals

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Science  04 Aug 2000:
Vol. 289, Issue 5480, pp. 736-737
DOI: 10.1126/science.289.5480.736

By what sequence of events does a mineral form? In traditional models, individual atoms or molecules are added or subtracted as crystals grow [1]. But a different view is emerging. In a series of laboratory experiments, Penn and Banfield have previously shown that inorganic nanocrystals, made up of hundreds or even thousands of atoms, can be the fundamental building blocks for the creation of highly ordered extended solids [2]. On page 751 of this issue, Banfield et al.[3] (1) provide strong evidence that some natural minerals also grow through such a mechanism of “oriented attachment” of nanocrystals. The results will be important not only for geochemistry but also for the creation of advanced artificial materials.

A nanocrystal typically has a diameter of between 1 and 10 nm and may contain as few as a hundred or as many as tens of thousands of atoms. Many fundamental properties of nanocrystals depend strongly on their size in smooth and predictable ways. Examples include the external field required to switch a magnetized particle [of great importance in magnetotactic bacteria [4] (2) and in hard disk drives [5] (3)] and the color of light emission from a semiconductor [6] [used for the fluorescent labeling of cells [7] (4) and in lasers (5)]. This facile tuning of properties by size variation is one reason why nanocrystals are widely viewed as promising components for new artificial optical and electrical materials.

But there is another reason why nanocrystals are particularly attractive as a building block for larger structures. Extended solids always contain a certain number of defects [8], which must be controlled to achieve desirable properties [9] (6). If the number of atoms is large and the free energy of defect formation is finite, then a certain density of defects, such as vacancies, is inevitable even at equilibrium. However, when this same number of atoms is partitioned into nanometer-sized crystals, then each nanocrystal on average need not contain any interior defects. It is possible—even easy—to prepare nanocrystals that are highly perfect, because the time required to anneal a nonequilibrium defect is short when a crystal is small. (That small perfect crystals are easier to make than large ones is confirmed by a quick check of the size dependence of prices for diamonds at a jewelry store!)

Can small but highly perfect nanocrystals be assembled into pure extended solids? Until recently, it seemed that aggregation of nanocrystals would always lead to disordered (albeit very interesting) solids. For example, Matijevic [10] and co-workers demonstrated exquisite shape control of polycrystalline micrometer-sized particles prepared by nanocrystal aggregation (7). Gleiter [11] showed that ceramics derived from the random consolidation of nanoparticles are particularly resistant to fracture (8). The discovery that aggregation of nanocrystals can yield an ordered crystal thus comes as a surprise.

Penn and Banfield previously showed that long chains of highly ordered titania can emerge from a solution of primary titania nanoparticles [12] (9) (see the figure). This chain can only arise if each nanocrystal “docks” with the next, such that the two crystals are fully aligned, a process the authors call “oriented attachment.” There is certainly a strong thermodynamic driving force for oriented attachment, because the surface energy is reduced substantially when the interface is eliminated. The surprise is that two crystals can align, dock, and fuse without first getting stuck in any one of an almost innumerable set of misoriented arrangements.

An example of oriented attachment.

Transmission electron micrograph [21] of TiO2 nanocrystal aggregates (9). Primary particles align, dock, and fuse to form these oriented chains.

CREDIT: FROM (9)

One possible explanation for this behavior comes from Averback [13] and co-workers (10), who independently and simultaneously discovered the same type of nanocrystal addition process. They studied the deposition of a silver nanoparticle onto a copper substrate and observed a process they called “contact epitaxy.” The Ag nanocrystals were initially randomly oriented but subsequently aligned epitaxially with the substrate. Once aligned with the substrate, the nanocrystals are also aligned with each other. In a series of simulations, Averback et al. showed that this could arise if, after the nanocrystal lands on the substrate, the stress between the nanocrystal and the substrate creates a dislocation [HN14] within the nanoparticle. As this dislocation moves toward the nanocrystal surface, the particle “rotates,” resulting in full alignment with the substrate. Similar phenomena occur in simulations studying the docking of two nanocrystals (11).

Three aspects of oriented attachment are of particular interest: (i) it occurs naturally, (ii) it provides a convenient explanation for certain types of defects in solids, and (iii) it may be a useful method for creating advanced artificial materials.

Banfield et al. present a strong case for some naturally occurring materials arising through oriented attachment (1). They examine iron oxyhydroxide minerals recovered from a flooded mine and find both primary particles with diameters of 2 to 3 nm and oriented nanoparticle chains and sheets. They conclude that the latter have arisen during natural biomineralization [HN15] as a consequence of oriented attachment. It remains to be seen, however, how common this type of mineral growth is in nature.

Oriented attachment provides a simple set of explanations for how particular types of defects may arise in extended solids [HN16] (12). Nanocrystals, if well formed, have facets, separated by regions where step faces are exposed. If the face of one nanocrystal docks against the steps of another, the two crystallites will naturally be oriented at a particular angle. Depending on the orientation, further oriented attachment will yield various types of dislocations or screw axes [17] (13). Impurity incorporation may also be more facile under conditions where crystals grow by nanocrystal attachment rather than atom by atom.

Oriented attachment may enable the preparation of interesting artificial materials. One of the most advanced methods of crystal growth, molecular beam epitaxy [18], involves deposition of atomic or molecular precursors onto a hot substrate. Manipulation of the stress and strain at the interface between two solids can be used to control particle size and shape, as in the growth of quantum dots [19], or to create metastable materials. The use of preformed nanoparticles as precursors in molecular beam epitaxy may generate new and interesting artificial materials. Likewise, the growth of nanorods and other specially shaped nanostructures [20] with controlled defects may be possible by exploiting and controlling oriented attachment.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

The Academic Press Dictionary of Science and Technology provides definitions of over 130,000 scientific terms.

Geochemistry on the World Wide Web is a collection of Internet resources maintained by the Department of Geological Sciences of Cornell University.

SciCentral is a gateway to Internet science resources. A section of resources about nanotechnology is included.

The About.com Guide to Nanotechnology offers articles and Internet resources.

The Nanotechnology Database is maintained by the International Technology Research Institute at Loyola College in Maryland.

The Materials Research Society provides the Materials Gateway of links to materials resources on the Internet.

S. Heyes, Department of Chemistry, University of Oxford, UK, provides lecture notes on the structure of inorganic solids.

The Department of Materials Science and Engineering, University of Illinois, offers lecture notes by J. Bullard for a materials science course. A presentation on defects in crystals is included.

P. Howell, Materials Science and Engineering Department, Pennsylvania State University, provides lecture notes for a course in materials science.

Purdue University's General Chemistry Help Homepage offers a topic review on materials science.

Materials Theory is a Web publication from the Materialia Project, which is supported by a consortium of Swedish universities.

V. Colvin of the Colvin Group, Department of Chemistry, Rice University, presents an illustrated introduction to nanoscience.

P. Ajayan and R. Siegel, Materials Science and Engineering Department, Rensselaer Polytechnic Institute, offer presentations for a graduate course on nanostructured materials.

The Foresight Institute is a nonprofit educational organization with a primary focus on molecular nanotechnology. A collection of information resources on nanotechnology is provided.

R. Merkle of Zyvex provides a nanotechnology Web site.

The Virtual Journal of Nanoscience & Technology, developed by the American Institute of Physics and the American Physical Society, is a weekly compilation of articles that have appeared in one of the participating source journals.

The October 1997 issue of Scientific American had an article by E. Corcoran and G. Zorpette titled “Diminishing dimensions.”

The National Nanotechnology Initiative Web site provides Nanotechnology Research Directions, a workshop report available in Adobe Acrobat format.

The UK Institute of Nanotechnology provides Internet links and a glossary as well as a report titled “Opportunities for industry in the application of nanotechnology.”

The 24 December 1999 issue of Science had a meeting news article by Robert Service titled “Building the small world of the future.”

Numbered Hypernotes

1. S. Nelson, Department of Geology, Tulane University, offers lecture notes for a course on mineralogy. Britannica.com offers an Encyclopædia Britannica article on crystals that includes a section on crystal growth. D. Barthelmy's Mineralogy Database provides a section on crystallography. J. Banfield, Department of Chemistry, University of Wisconsin, provides a presentation on crystals for a course on gems and precious stones. J. Rice, Department of Geological Sciences, University of Oregon, provides lecture notes on crystal growth for a mineralogy course. P. Schroeder, Department of Geology, University of Georgia, provides lecture notes on crystal morphology for a course on Earth materials. The Crystallography Laboratory at the Department of Chemistry, University of Wisconsin, provides an introduction to crystal growth.

2. A tutorial on the structures of simple inorganic solids, provided by the Chemistry Online Web site of the University of Oxford, defines extended solid.

3. J. Banfield, S. Welch, and H. Zhang are in the Department of Geology and Geophysics, University of Wisconsin. R. L. Penn (and here) is in the Department of Earth and Planetary Sciences, Johns Hopkins University. An overview of the Banfield group's research on nanocrystalline materials is provided.

4. The Digital Learning Center for Microbial Ecology, a science education project developed at Michigan State University, includes an introduction to magnetotactic bacteria. A section on magnetotactic bacteria is included in a student project by P. Pham titled “Magnetic sensors in living systems” prepared for a physics course taught by G. Swartzlander, Physics Department, Worcester Polytechnic Institute. R. Frankel, Department of Physics, California Polytechnic State University, San Luis Obispo, provides information about his research on magnetotactic bacteria. The U.S. National Report to IUGG, 1991-1994 includes a contribution by B. Moskowitz on biomineralization of magnetic minerals that includes a section on magnetotactic bacteria.

5. The 17 March 2000 issue of Science had a news article by Robert Service titled “Nanocrystals may give boost to data storage” about the report in that issue by S. Sun et al. titled “Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices.” The Oak Ridge National Laboratory provides a press release titled “ORNL's colorful nanocrystals could lead to faster computers.”

6. The Microsystems Science, Technology, and Components Center of Sandia National Laboratories presents an introduction to semiconductor materials. Chemistry Online from the Oxford Information Technology Centre provides a tutorial on semiconductor synthesis. The World Technology Division of the International Technology Research Institute makes available the proceedings of a 1997 workshop titled “R&D status and trends in nanoparticles, nanostructured materials, and nanodevices in the United States” that includes a section by L. Brus on semiconductor nanocrystals. The Materials Research Science and Engineering Center at the University of Chicago offers a presentation on size-tuned light from single nanocrystal.

7. Lawrence Berkeley National Laboratory provides a 28 September 1998 research news article titled “Semiconductor nanocrystals: The next thing in fluorescent probes.” The 25 September 1998 issue of Science had a report of this research (“Semiconductor nanocrystals as fluorescent biological labels” by M. Bruchez, M. Moronne, P. Gin, S. Weiss, and A. P. Alivisatos), as well as a report by W. Chan and S. Nie titled “Quantum dot bioconjugates for ultrasensitive nonisotopic detection.” The Nie Research Group, Department of Chemistry, Indiana University, makes available an article by D. Leff titled “Color-coding: Quantum dots debut with promising careers in clinical diagnostics field” that appeared in the 25 September 1998 issue of Bioworld Today. An article about Nie's research titled “Nanoparticles and ultrasensitive microscopy: ‘The Next Generation' of chemical analysis” appeared in the September 1998 issue of Indiana University's Research & Creative Activity.

8. The Encyclopædia Britannica article on crystals includes a section on crystal defects. The General Chemistry Help Homepage offers a presentation on defects in solids. E. Goo, Department of Materials Science, University of Southern California, presents a series of summary slides about crystal defects for a course on the structure of matter. M. Darwish, Department of Mechanical Engineering, American University of Beirut, Lebanon, offers review notes on defects for a course on engineering materials. Materials Theory includes a section on the investigation of defects in crystals; vacancies are described and illustrated.

9. The 14 August 1998 issue of Science was a special issue titled “Frontiers in Materials Science: Control and use of defects in materials.” Included in the issue was a review by H. Queisser and E. Haller titled “Defects in semiconductors: Some fatal, some vital.”

10. E. Matijevic is at the Center for Advanced Materials Processing, Department of Chemistry, Clarkson University, Potsdam, NY.

11. H. Gleiter is at the Institut für Nanotechnologie, Forschungszentrums Karlsruhe, Germany. Gleiter's research is mentioned in an article by R. Seigel titled “Creating nanophase materials,” originally published in the December 1996 issue of Scientific American, which is made available in the NanoNews section of the Nanoworld Web site.

12. The Academic Press Dictionary of Science and Technology defines titania and titanium dioxide. The Nanophase Technologies Corporation offers information about their production of titanium dioxide (titania) particles.

13. R. Averback is in the Department of Materials Science and Engineering, University of Illinois.

14. The Academic Press Dictionary of Science and Technology defines dislocation.

15. The Banfield group's Web site offers a presentation about the biomineralization of Fe oxides at the Tennyson mine. An introduction to biomineralization is provided by the Biomineralization Web page offered by the Département des Sciences de la Terre, Université Paris-Sud, France. Wat on Earth, a publication of the Department of Earth Science, University of Waterloo, Canada, makes available an article by G. Ferris on biomineralization. The Deep Biosphere Laboratory at Göteborg University, Sweden, presents information about microorganisms and metals. Bacteria and Groundwater, a student project made available by D. Gallagher, Environmental Engineering and Sciences Program, Virginia Tech, includes a section on iron bacteria. Photomicrographs of bacteria that precipitate iron are included in a U.S. Geological Survey educational presentation on how to collect and see the microbes that fix iron and manganese in the natural environment.

16. The 14 August 1998 issue of Science had a report by R. L. Penn and J. Banfield titled “Imperfect oriented attachment: Dislocation generation in defect-free nanocrystals.”

17. Screw axis and screw dislocation are defined in the Academic Press Dictionary of Science and Technology.

18. The Surface Science Techniques Web site offers a definition of molecular beam epitaxy and provides Internet links. The MBE Group at the University of Illinois offers an introduction to molecular beam epitaxy. The Microelectronics Research Center, University of Texas, Austin, provides information about molecular beam epitaxy.

19. P. Kinsler's Information Maze describes quantum dots. The About.com Guide to Nanotechnology presents an article about quantum dots. The Quantum Dot Corporation offers a presentation on the technology and applications of quantum dot particles. The Nanostructures Section of the Naval Research Laboratory's Electronics Science and Technology Division presents an overview of their research on quantum dots and nanoelectronic devices. Sandia National Laboratories provides a news release titled “Quantum dots repel each other, researchers find.” The 20 November 1998 issue of Science had an Enhanced Perspective about quantum dots by F. Julien and A. Alexandrou.

20. Lawrence Berkeley National Laboratory provides a research news article titled “Researchers make first rod-shaped semiconductor nanocrystals.” The 8 June 1998 issue of Chemical & Engineering News had an article by R. Dagani titled “Materials symposium spotlights progress in the creation of supramolecular assemblies, nanostructures, and devices.” The January 1998 issue of Physics World had an article by M. Dresselhaus et al. titled “Carbon nanotubes.” The Nanotube Site is presented by D. Tomanek, Physics and Astronomy Department, Michigan State University. R. Smalley, Center for Nanoscale Science and Technology, Rice University, makes available papers and presentations on nanostructures, as well as an image gallery. The Integrated Product Team on Devices and Nanotechnology at NASA Ames Research Center provides a nanotechnology gallery.

21. The Nanoworld, a Web site provided by the Centre for Microscopy and Microanalysis, University of Queensland, Australia, offers an introduction to transmission electron microscopy. J. Chen, Department of Materials Science and Engineering, McMaster University, Canada, offers a section on the transmission electron microscope in a presentation on the microstructures of materials.

22. A. P. Alivisatos (and here) is in the Department of Chemistry, University of California, Berkeley. The Alivisatos Group has a Web page.

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