EDITORIAL pubs.acs.org/JPCL
Semiconductor Nanocrystals: To Dope or Not to Dope
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emiconductor nanocrystals and nanostructures continue to dominate nanoscience research. The size quantization effects, tunable emission, and capture of photogenerated charge carriers at the interface have made them attractive candidates for biological detection and light-harvesting applications. In addition, their emission response to surface-bound molecules or the surrounding chemical atmosphere makes them useful as sensors and optoelectronics. The high-temperature chemical synthesis of CdSe (developed in 1993) revolutionized nanoscience activities, with the possibility of designing semiconductor nanocrystallites of various sizes and shapes.1 The chemist’s quest to synthesize high-quality semiconductor materials quickly attracted the attention of physicists and engineers who otherwise relied on more expensive and elaborate techniques, such as molecular beam epitaxy (MBE) or atomic-layer deposition (ALD), to obtain high-quality semiconductor nanostructures on desired substrates. Today, semiconductor nanocrystals and nanostructures are major players across different disciplines of science and engineering. Physical chemistry, in particular, plays a dominant role in disseminating a fundamental understanding of excited-state dynamics and photoinduced charge-transfer processes. Recent efforts to utilize sizequantized semiconductor nanocrystals in quantum dot solar cells have opened up the discussion on tapping hot electron transfer and multiple exciton generation.2 5 Doping of single-crystal materials is an important and crucial factor in achieving desired semiconductor properties (for example, p- and n-type behavior) or carrier densities. However, relatively little attention is given to exploiting the doping of semiconductor nanocrystals. Recent efforts have focused on doping semiconductor nanocrystals with metals such as Cu, Mn, In, and Ga and tuning their optical and electronic properties.6 12 The Perspectives in this issue focus on controlled doping of metal chalcogenides and other semiconductor nanocrystals or quantum dots. The Perspective by Pradhan and Sarma focuses on recent advances in the synthetic approaches for controlled doping of semiconductor nanocrystals.13 Dopant ions such as Mn2+, when introduced in large-band-gap semiconductor nanocrystals such as ZnSe, allow one to obtain orange emission arising from a d d transition within the Mn 3d multiplet. Because this transition is spin-forbidden, the observed emission lifetimes are long compared to undoped semiconductor nanocrystals, often in the range of several microseconds. The doped systems have an advantage of exhibiting narrow emission bands even in partially passivated semiconductor nanocrystals. Viktor Chikan in his Perspective discusses challenges in achieving uniform size and composition of doped semiconductor nanocrystals and potential strategies involving both the thermodynamics and kinetics of the growth of CdSe quantum dots.14 In addition, CdSe quantum dots doped with tin and indium show a strong temperature-dependent photoluminescence quenching relative to the undoped quantum dots. Specific advantages exist for using doped semiconductor nanocrystals in solar cells. For example, the excess charge carriers introduced through dopants r 2011 American Chemical Society
can improve overall conductivity and fill up the trap sites in quantum dot solar cells. However, such beneficial aspects of doped semiconductors in solar cells have yet to be realized. Another approach to modifying the electronic property of semiconductor nanostructures is through heterostructure design. The Perspective by Shim et al. discusses recent developments in synthesizing nanorod heterostructures with controlled shape anisotropy.15 Type-II semiconductor heterostructures with a favorable band offset, which can improve charge separation, have potential applications in photonics, electronics, and photovoltaics. Controlled doping with various metals offers new opportunities to tailor the properties of semiconductor nanocrystals. However, utilization of semiconductor nanocrystals in practical devices is yet to be fully exploited. Doping of semiconductor nanocrystals is a useful way of obtaining high-efficiency visible luminescence from large-band-gap semiconductors such as ZnSe. However, the long emission lifetimes of doped semiconductor nanocrystals, as compared to undoped systems, may limit their application in many practical devices. Future research efforts need to focus on establishing excited-state dynamics and photocatalytic aspects of doped semiconductor nanocrystals and identifying areas in which these systems will have distinct advantages over undoped systems. Hence, the question remains, to dope or not to dope! Prashant V. Kamat Deputy Editor Department of Chemistry and Biochemistry and Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States
’ REFERENCES (1) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E = Sulfur, Selenium, Tellurium) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706–8715. (2) Mora-Sero, I.; Bisquert, J. Breakthroughs in the Development of Semiconductor Sensitized Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3046–3052. (3) Hetsch, F.; Xu, X.; Wang, H.; Kershaw, S. V.; Rogach, A. L. Semiconductor Nanocrystal Quantum Dots as Solar Cell Components and Photosensitizers: Material, Charge Transfer, and Separation Aspects of Some Device Topologies. J. Phys. Chem. Lett. 2011, 2, 1879–1887. (4) Buhbut, S.; Itzhakov, S.; Oron, D.; Zaban, A. Quantum Dot Antennas for Photoelectrochemical Solar Cells. J. Phys. Chem. Lett. 2011, 2, 1917–1924. (5) Beard, M. C. Multiple Exciton Generation in Semiconductor Quantum Dots. J. Phys. Chem. Lett. 2011, 2, 1282–1288. (6) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. HighQuality Manganese-Doped ZnSe Nanocrystals. Nano Lett. 2001, 1, 3–7. (7) Zu, L.; Norris, D. J.; Kennedy, T. A.; Erwin, S. C.; Efros, A. L. Impact of Ripening on Manganese-Doped ZnSe Nanocrystals. Nano Lett. 2006, 6, 334–340. Published: November 03, 2011 2832
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(8) Thakar, R.; Chen, Y.; Snee, P. T. Efficient Emission from Core/ (Doped) Shell Nanoparticles: Applications for Chemical Sensing. Nano Lett. 2007, 7, 3429–3432. (9) Karan, N. S.; Sarma, D. D.; Kadam, R. M.; Pradhan, N. Doping Transition Metal (Mn or Cu) Ions in Semiconductor Nanocrystals. J. Phys. Chem. Lett. 2010, 2863–2866. (10) Zeng, R.; Rutherford, M.; Xie, R.; Zou, B.; Peng, X. Synthesis of Highly Emissive Mn-Doped ZnSe Nanocrystals without Pyrophoric Reagents. Chem. Mater. 2010, 22, 2107–2113. (11) Zu, L.; Wills, A. W.; Kennedy, T. A.; Glaser, E. R.; Norris, D. J. Effect of Different Manganese Precursors on the Doping Efficiency in ZnSe Nanocrystals. J. Phys. Chem. C 2010, 114, 21969–21975. (12) Vlaskin, V. A.; Janssen, N.; van Rijssel, J.; Beaulac, R.; Gamelin, D. R. Tunable Dual Emission in Doped Semiconductor Nanocrystals. Nano Lett. 2010, 10, 3670–3674. (13) Pradhan, N.; Sarma, D. D. Advances in Light-Emitting Doped Semiconductor Nanocrystals. J. Phys. Chem. Lett. 2011, 2, 2818–2826. (14) Chikan, V. Challenges and Prospects of Electronic Doping of Colloidal Quantum Dots: Case Study of CdSe. J. Phys. Chem. Lett. 2011, 2, 2783–2789. (15) Shim, M.; McDaniel, H.; Oh, N. Prospects for Strained Type-II Nanorod Heterostructures. J. Phys. Chem. Lett. 2011, 2, 2722–2727.
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