EDITORIAL pubs.acs.org/JPCL
Structure Determines Function in Nanoparticles, Their Interfaces, and Their Assemblies
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hemists’ abilities to design and tailor material properties continue to be crucial to technological advancements. For molecular materials, synthetic, theoretical, and spectroscopic methods all contribute to guide material design, but functional composite creation and assembly are too often limited by heterogeneities, solubilities, and interfacial interactions. Unfortunately, such spatial heterogeneities are difficult to observe and decode with bulk spectroscopies, leading to significant deviations from idealized or expected behaviors. Although often challenging to implement, probing and unraveling such environmental heterogeneity among individual members of the ensemble has been one focus of single molecule research for more than 20 years. Room-temperature single particle studies and their recent extension to absorption1,2 continue to enhance our collective understanding of how heterogeneity in molecular behavior manifests itself in both function and in measured response. With potential advantages over molecular systems, inorganic nanoparticle building blocks offer the potential of greatly improved utility, functionality, and stability. Combining the best of bulk behavior with the size, confinement, and tunability of molecular scale systems, new doors to improved performance are likely to be opened. An advantage and a liability, nanoparticle building blocks exhibit not only molecule-like environmental heterogeneities but also often a lack of atomic-level precision in their creation. Therefore, unless defects or sizes are intentionally utilized to dominate overall behavior,3,4 heterogeneities in size, aspect ratio, composition, surface energy, and atomic irregularities result in particle-to-particle variability such that the properties of any given nanoparticle differ from those of the ensemble.5,6 Because the desired nanoparticle properties often arise from confinement to dimensions smaller than characteristic length scales, even single atom defects in quantum-confined materials can yield large changes in optoelectronic behavior. Such effective “expansions” of the periodic table7 seem promising for tailoring nanoparticle functionality, but this exquisite size tunability may also limit which species can be made with good purity. In this issue of JPC Letters, excellent Perspectives on semiconductor quantum dots8 and on plasmon-supporting metal nanostructures,9 detail the manifestations of nanoparticle size, geometry, and interfacial heterogeneity on optical and electronic properties. In both localized and extended systems, atomic and nanoparticle interfacial structures strongly influence measured optical and electronic behavior. Only through fundamental understandings of how structure affects energies can the nanoparticle states and interparticle interfaces be controlled to design larger functional assemblies. Correlated structure and optical/ electronic properties of the same individual particles must be measured to generate this knowledge and to position nanoparticles as truly useful tools in the chemists’ toolbox. As cogently argued in these Perspectives, only by understanding the atomiclevel details of electron, exciton, and polariton confinement can we hope to control assemblies for improved nanocomposite materials and devices. r 2011 American Chemical Society
With the goal of tailoring nanoparticle assemblies for optical and electronic applications, the Perspective by Vanmaekelbergh et al.8 details the challenges associated with size and structural heterogeneity within semiconductor quantum dots. Although size dispersity can be quite low (∼5%) and spectra can be very narrow, the majority of observed spectral, dynamic, and electronic variability often arises from particle-to-particle heterogeneity. The sensitivity of exciton confinement to dimensions smaller than its Bohr radius makes the exciton wave function exquisitely sensitive to capping material, interfacial energies, dual excitations, defects, and nanoparticle size. Each differing among members of the ensemble, an argument is presented for the necessity of correlated optical, electrical, and high-resolution TEM studies on the same individual nanoparticles. Changing creation conditions can alter or nearly eliminate many nonideal behaviors, but the goal here is to understand the optical and electrical manifestations arising even from single defect or single atom differences in quantum dot structures. Only through understanding the influence of atomic-level and interfacial irregularities on nanoparticle energies can one fully tailor nanoparticle assemblies for specific purposes. The Perspective by Link et al.9 similarly advocates the need for correlated structural and optical studies to control plasmonic nanoparticles and their assemblies. Focusing on building up multiparticle couplings from shape-dependent single nanoparticle plasmons, the authors detail the importance of interfacial structure and geometry in determining plasmon energies. Resulting from optical excitation of the metallic free electrons, plasmons are confined to nanometer dimensions as they move at optical frequencies along metal nanostructures. In nanoparticles larger than a few nanometers, plasmons are relatively insensitive to individual atom defects, but their interparticle couplings depend crucially on each particle’s shape-dependent plasmon absorptions and therefore the interparticle geometry.5 Therefore, whereas individual particle shape and optical spectra are important for understanding fundamentals of plasmon absorption, characterizing the interfaces between nanoparticles is crucial to electromagnetic field enhancement, long distance light transmission, and sensing in subwavelength structures.5,10 As nanoparticle and junction geometries determine interparticle couplings, correlated structural and optical studies of individual particles are crucial to controlling the spectral properties in any larger assembly. In essence, one needs to enforce atomic-level accuracy in nanoparticle synthesis such that material heterogeneity is minimized, leaving only the environmental heterogeneity to influence behavior. Whereas confinement gives rise to new, desirable properties of bulk materials, the interface provides additional functionality or complexity, depending on one’s perspective. Although nanoparticle heterogeneity currently limits the utility of extended nanoparticle assemblies, such variability provides an
Published: August 18, 2011 2044
dx.doi.org/10.1021/jz200981k | J. Phys. Chem. Lett. 2011, 2, 2044–2045
The Journal of Physical Chemistry Letters
EDITORIAL
opportunity to more fully understand how structural properties of inter- and intraparticle interfaces control optical and electronic responses. Only through control of these interfaces can one begin to design and combine nanomaterials into functional assemblies accurately. Vanmaekelbergh states that “A final dream should then be the combination of single-dot optical and electrical spectroscopy on a given nanocrystal, whose complete atomic structure is also revealed....”8 I would suggest that this is not a final dream but rather a new and necessary beginning in the evolution of nanomaterials tailored for any application. Robert M. Dickson Senior Editor Georgia Institute of Technology, Atlanta, Georgia, United States
’ REFERENCES (1) Chong, S.; Min, W.; Xie, X. S. Ground-State Depletion Microscopy: Detection Sensitivity of Single-Molecule Optical Absorption at Room Temperature. J. Phys. Chem. Lett. 2010, 1, 3316–3322. (2) Kukura, P.; Celebrano, M.; Renn, A.; Sandoghdar, V. SingleMolecule Sensitivity in Optical Absorption at Room Temperature. J. Phys. Chem. Lett. 2010, 1, 3323–3327. (3) Dukes, A. D.; Samson, P. C.; Keene, J. D.; Davis, L. M.; Wikswo, J. P.; Rosenthal, S. J. Single-Nanocrystal Spectroscopy of White-LightEmitting CdSe Nanocrystals. J. Phys. Chem. A 2011, 115, 4076–4081. (4) Winterhalder, M. J.; Zumbusch, A.; Lippitz, M.; Orrit, M. Toward Far-field Vibrational Spectroscopy of Single Molecules at Room Temperature. J. Phys. Chem. B 2011, 115, 5425–5430. (5) Henry, A.; Bingham, J.; Ringe, E.; Marks, L. D.; Schatz, G. C.; Duyne, R. P. V. Correlated Structure and Optical Property Studies of Plasmonic Nanoparticles. J. Phys. Chem. C 2011, 115, 9291–9305. (6) Hui, Y. Y.; Chang, Y.-R.; Mohan, N; Lim, T.-S.; Chen, Y.-Y.; Chang, H.-C. Polarization Modulation Spectroscopy of Single Fluorescent Nanodiamonds with Multiple Nitrogen Vacancy Centers. J. Phys. Chem. A 2011, 115, 1878–1884. (7) Castleman, A. W.; Khanna, S. N. Clusters, Superatoms, and Building Blocks of New Materials. J. Phys. Chem. C 2009, 113, 2664–2675. (8) Vanmaekelbergh, D.; Casavola, M. Single-Dot Microscopy and Spectroscopy for Comprehensive Study of Colloidal Nanocrystals. J. Phys. Chem. Lett. 2011, 2, 2024–2031. (9) Slaughter, L.; Chang, W.-S.; Link, S. Characterizing Plasmons in Nanoparticles and Their Assemblies with Single Particle Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 2015–2023. (10) Dickson, R. M.; Lyon, L. A. Unidirectional Plasmon Propagation in Metallic Nanowires. J. Phys. Chem. B 2000, 104, 6095–6098.
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dx.doi.org/10.1021/jz200981k |J. Phys. Chem. Lett. 2011, 2, 2044–2045
