Guest Commentary pubs.acs.org/JPCL
Tailoring the Inherent Optical and Electrical Properties of Nanostructures
O
these materials, which exploit their enhanced spin, luminescent, and electronic properties. Unfortunately, nanostructure doping has been hindered by the exclusion of impurities during growth.14 Notable advances that overcome this problem, however, have recently been ushered in with the successful Mn doping of high-quality ZnSe nanocrystals.15 Now, several strategies exist for introducing impurities into colloidal nanostructures. They include the use of single-source cluster precursors that already incorporate the impurity,16 the use of cation exchange to introduce impurities into premade nanostructures,17 and the embedding of impurities within core/shell structures once adsorbed onto nanocrystal surfaces.18 In this issue, Jin Zhang and co-workers (Zhang, J. Z.; Cooper, J. K.; Gul, S. Rational Codoping as a Strategy to Improve Optical Properties of Doped Semiconductor Quantum Dots. J. Phys. Chem. Lett. 2014, 5, 3694−3700. DOI: 10.1021/ jz501739v) describe a new strategy for introducing impurities into colloidal QDs. The approach that they outline entails introducing pairs of dopants (e.g., two cations, two anions, or a cation and an anion) into doping reactions instead of using a single impurity species. Advantages of this codoping scheme include the ability to charge compensate as well as reduce the overall lattice distortion experienced by hosts when incorporating impurities. This results in improved doping efficiencies. More intriguingly, codoping opens up a new parameter space whereby the intrinsic optical and electrical properties of QDs can be tuned through the introduction of new impurity-related donor and acceptor levels. Already, preliminary work has shown enhanced emission properties from such codoped nanostructures.19 Despite these initial successes, Jin and co-workers note that much work remains to be done. Questions exist about the actual oxidation states of the dopants. Furthermore, distortions of the local host, resulting from the incorporation of impurities, must be better understood because they ultimately influence the nanostructure’s optical and electrical properties. This, in turn, requires a synergistic effort of optical, structural, and computational studies, which will ultimately result in rational doping approaches that maximize the efficiency as well as tunability of nanostructure optical/electrical properties. Next in this issue, Agarwal and co-worker (Aspetti, C.; Agarwal, R. Tailoring the Spectroscopic Properties of Semiconductor Nanowires via Surface-Plasmon-Based Optical Engineering. J. Phys. Chem. Lett. 2014, 5, 3768−3780. DOI: 10.1021/jz501823d) describe analogous work to enhance the optical response of semiconductor nanowires (NWs). In this case, plasmonic interactions rather than dopants are used to influence and even alter the intrinsic optical response of the
ver 3 decades of work has been devoted to developing nanostructures. This has been motivated by their sizedependent optical and electrical properties, which transcend the inherent limitations of bulk material properties. Early efforts aimed at making high-quality nanomaterials using various growth techniques. The focus was on controlling the size as well as size distribution of resulting ensembles. In the case of colloidal quantum dots (QDs), this entailed using arrested precipitation among other techniques1 and was informed by prior work in the field of colloidal chemistry.2 A notable success from these early studies was the development of high-quality colloidal CdSe nanocrystals.3,4 Since then, CdSe QDs have become a model system for nanocrystal research.5 The establishment of successful procedures for making highquality nanostructures soon led to a second wave of research focused on understanding their size-dependent optical and electrical properties. In the case of colloidal CdSe QDs, this led to detailed optical measurements that revealed their exquisite size-dependent excited-state progressions as well as band edge fine structure.6,7 These efforts were aided by comparisons to theoretical predictions first established to explain the optical properties of corresponding QD glass samples.8 Analogous work on other systems (e.g., InAs nanocrystals9) has since expanded our knowledge about the size-dependent evolution of QD optical and electrical properties. We are now in the midst of a third era of nanoscale research wherein serious efforts are being made to use high-quality nanocrystals, their shape-controlled variants, and other like nanostructures in real world applications. In the case of colloidal QDs, notable success has come from using these materials in biological labeling applications.10,11 Consequently, functionalized and biocompatible QDs are now available from companies such as Life Technologies. Advantages of such QD labels include a superior resistance to photobleaching as well as absorption profiles that readily enable multicolor labeling applications. More recently, QDs have been used as elements in solid-state consumer lighting.12 They are now even in televisions under the Sony Triluminos label.13 All of these efforts have ultimately involved a delicate tailoring of nanomaterial chemical, optical, and electrical properties beyond the use of confinement-induced, size-tunable optical/electrical properties. In some cases, considerable surface functionalization chemistries have been developed to alter nanostructure chemical properties. Compositional changes have also been pursued to expand the parameter space by which nanostructure properties can be tuned. Chemical doping, the introduction of impurity elements into a host lattice, represents yet another approach for fine-tuning the optical and electrical response of nanostructures. In the electronics industry, doping is crucial and enables bulk silicon to be used in transistors, light-emitting diodes, and solar cells, among other technologies. Consequently, analogous doping of nanostructures is expected to engender new applications for © 2014 American Chemical Society
Received: September 25, 2014 Accepted: October 9, 2014 Published: November 6, 2014 3817
dx.doi.org/10.1021/jz502041a | J. Phys. Chem. Lett. 2014, 5, 3817−3818
The Journal of Physical Chemistry Letters
Guest Commentary
conductors with a Degenerate Valence Band: Dark and Bright Exciton States. Phys. Rev. B 1996, 54, 4843. (8) Ekimov, A. I.; Hache, F.; Schanne-Klein, M. C.; Ricard, D.; Flytzaniz, C.; Kudryavtsev, I. A.; Yazeva, T. V.; Rodina, A. V. Absorption and Intensity-Dependent Photoluminescence Measurements on CdSe Quantum Dots: Assignment of the First Electronic Transitions. J. Opt. Soc. Am. B 1993, 10, 100−107. (9) Banin, U.; Lee, C. J.; Guzelian, A. A.; Kadavanich, A. V.; Alivisatos, A. P.; Jaskolski, W.; Bryant, G. W.; Efros, AL. L.; Rosen, M. Size-Dependent Electronic Level Structure of InAs Nanocrystal Quantum Dots: Test of Multiband Effective Mass Theory. J. Chem. Phys. 1998, 109, 2306−2309. (10) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Semiconductor Nanocrystals as Fluorescent Biological Labels. Science 1998, 281, 2013−2016. (11) Chan, W.C. W.; Nie, S. Quantum Dot Bioconjugates for Sensitive Nonisotopic Detection. Science 1998, 281, 2016−2018. (12) Quantum Dots, A Quantum Leap for Lighting. The Economist, March 4, 2010. (13) Bourzac, K. Quantum Dots Go on Display. Nature 2013, 493, 283. (14) Mikulec, F. V.; Kuno, M.; Bennati, M.; Hall, D. A.; Griffin, R. G.; Bawendi, M. G. Organometallic Synthesis and Spectroscopic Characterization of Manganese-Doped CdSe Nanocrystals. J. Am. Chem. Soc. 2000, 122, 2532−2540. (15) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. HighQuality Manganese-Doped ZnSe Nanocrystals. Nano Lett. 2001, 1, 3− 7. (16) Meulenberg, R. W.; van Buuren, T.; Hanif, K. M.; Willey, T. M.; Strouse, G. F.; Terminello, L. J. Structure and Composition of CuDoped CdSe Nanocrystals Using Soft X-ray Absorption Spectroscopy. Nano Lett. 2004, 4, 2277−2285. (17) Mocatta, D.; Cohen, G.; Schattner, J.; Millo, O.; Rabani, E.; Banin, U. Heavily Doped Semiconductor Nanocrystal Quantum Dots. Science 2011, 332, 77−81. (18) Pradhan, N.; Goorskey, D.; Thessing, J.; Peng, X. G. An Alternative of CdSe Nanocrystal Emitters: Pure and Tunable Impurity Emissions in ZnSe Nanocrystals. J. Am. Chem. Soc. 2005, 127, 17586− 17587. (19) Cooper, J. K.; Gul, S.; Lindley, S. A.; Yano, J.; Zhang, J. Z. Tunable Visible Photoluminescence from Codoped ZnSe Quantum Dots. Unpublished. (20) Duan, X.; Huang, Y.; Agarwal, R.; Lieber, C. M. SingleNanowire Electrically Driven Lasers. Nature 2003, 421, 241−245. (21) Hayden, O.; Agarwal, R.; Lieber, C. M. Nanoscale Avalanche Photodiodes for Highly Sensitive and Spatially Resolved Photon Detection. Nat. Mater. 2006, 5, 352−356. (22) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nanowire Dye-Sensitized Solar Cells. Nat. Mater. 2005, 4, 455−459.
wires. Already, the unique optical properties and polarization sensitivities of semiconductor NWs have been used to demonstrate NW-based lasers,20 photodetectors,21 and even solar cells.22 By creating hybrid metal/semiconductor NW morphologies, Agarwal and co-workers demonstrate that these optical responses can be even further enhanced, opening the door to new NW-based applications. As specific examples, in the case of CdS NWs, metal-induced plasmonic interactions result in enhanced lasing action. In addition, when NW core/conformal metal coating geometries are used, Agarwal and co-workers demonstrate that NW spontaneous emission rates can be altered, making them competitive with picosecond time scale intraband relaxation. This ultimately leads to the observation of hot emission from CdS wires. Even Si, a critically important technological material but one that possesses an indirect band gap, can be made to “turn on” through this approach. This is a profound result given the ubiquity of Si electronics and of the possibility for eventually establishing Si-based optoelectronics. Finally, Agarwal and co-workers demonstrate that such plasmonic interactions can be employed to enhance NW absorption, with direct gap CdSe and indirect gap Ge NWs used as case studies. Both Agarwal and Jin therefore illustrate ongoing efforts being undertaken in this era of nanoscale research to enhance the optical and electrical response of existing high-quality nanostructures developed over the last 3 decades. Introducing dopants or engendering interactions between nanostructures and plasmonic species are just some of the many possible ways by which the inherent optical and electrical properties of nanostructures can be fine-tuned. In this way, resulting enhancements will improve the use of nanostructures in existing applications while corresponding emergent properties will likewise lead to new applications not yet foreseen.
Masaru Kuno*
■
Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
■
REFERENCES
(1) Kuno, M. Introductory Nanoscience, Physical and Chemical Concepts; Garland Science: New York, 2011. (2) LaMer, V. K.; Dinegar, R. H. Theory, Production and Mechanism of Formation of Monodispersed Hydrosols. J. Am. Chem. Soc. 1950, 72, 4847−4854. (3) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E = S, Se, Te) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706− 8715. (4) Katari, J. E. B.; Colvin, V. L.; Alivisatos, A. P. X-ray Photoelectron-Spectroscopy of CdSe Nanocrystals with Applications to Studies of the Nanocrystal Surface. J. Phys. Chem. 1994, 98, 4109− 4117. (5) Kuno, M. Colloidal Quantum Dots: A Model Nanoscience System. J. Phys. Chem. Lett. 2013, 4, 680. (6) Norris, D. J.; Bawendi, M. G. Measurement and Assignment of the Size-Dependent Optical Spectrum in CdSe Quantum Dots. Phys. Rev. B 1996, 53, 16338. (7) Efros, Al. L.; Rosen, M.; Kuno, M.; Nirmal, M.; Norris, D. J.; Bawendi, M. G. Band-Edge Exciton in Quantum Dots of Semi3818
dx.doi.org/10.1021/jz502041a | J. Phys. Chem. Lett. 2014, 5, 3817−3818
