McGuire John A, Joo Jin, Pietryga Jeffrey M, Schaller Richard D, Klimov Victor I
Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Acc Chem Res. 2008 Dec;41(12):1810-9. doi: 10.1021/ar800112v.
One consequence of strong spatial confinement of electronic wave functions in semiconductor nanocrystals (NCs) is a significant enhancement in carrier-carrier Coulomb interactions. This effect leads to a number of novel physical phenomena including ultrafast decay of multiple electron-hole pairs (multiexcitons) by Auger recombination and high-efficiency generation of mutiexcitons by single photons via carrier multiplication (CM). Significant recent interest in multiexciton phenomena in NCs has been stimulated by studies of NC lasing, as well as potential applications of CM in solar-energy conversion. The focus of this Account is on CM. In this process, the kinetic energy of a "hot" electron (or a "hot" hole) does not dissipate as heat but is, instead, transferred via the Coulomb interaction to the valence-band electron, exciting it across the energy gap. Because of restrictions imposed by energy and translational-momentum conservation, as well as rapid energy loss due to phonon emission, CM is inefficient in bulk semiconductors, particularly at energies relevant to solar energy conversion. On the other hand, the CM efficiency can potentially be enhanced in zero-dimensional NCs because of factors such as a wide separation between discrete electronic states, which inhibits phonon emission ("phonon bottleneck"), enhanced Coulomb interactions, and relaxation in translational-momentum conservation. Here, we investigate CM in PbSe NCs by applying time-resolved photoluminescence and transient absorption. Both techniques show clear signatures of CM with efficiencies that are in good agreement with each other. NCs of the same energy gap show moderate batch-to-batch variations (within approximately 30%) in apparent multiexciton yields and larger variations (more than a factor of 3) due to differences in sample conditions (stirred vs static solutions). These results indicate that NC surface properties may affect the CM process. They also point toward potential interference from extraneous effects such as NC photoionization that can distort the results of CM studies. CM yields measured under conditions when extraneous effects are suppressed via intense sample stirring and the use of extremely low pump levels (0.02-0.03 photons absorbed per NC per pulse) reveal that both the electron-hole creation energy and the CM threshold are reduced compared with those in bulk solids. These results indicate a confinement-induced enhancement in the CM process in NC materials. Further optimization of CM performance should be possible by utilizing more complex (for example, shaped-controlled or heterostructured) NCs that allow for facile manipulation of carrier-carrier interactions, as well as single and multiexciton energies and dynamics.
电子波函数在半导体纳米晶体（NCs）中的强空间限制所带来的一个结果是载流子 - 载流子库仑相互作用显著增强。这种效应导致了许多新颖的物理现象，包括通过俄歇复合实现多电子 - 空穴对（多激子）的超快衰减，以及通过载流子倍增（CM）由单光子高效产生多激子。近期对NCs中多激子现象的大量关注受到了NC激光研究以及CM在太阳能转换中的潜在应用的推动。本综述的重点是CM。在这个过程中，一个“热”电子（或一个“热”空穴）的动能不会以热的形式耗散，而是通过库仑相互作用转移到价带电子上，使其跨越能隙被激发。由于能量和动量守恒的限制，以及声子发射导致的快速能量损失，CM在体半导体中效率较低，特别是在与太阳能转换相关的能量下。另一方面，由于离散电子态之间的大间距等因素，零维NCs中的CM效率可能会提高，这些因素抑制了声子发射（“声子瓶颈”）、增强了库仑相互作用以及动量守恒的弛豫。在这里，我们通过应用时间分辨光致发光和瞬态吸收来研究PbSe NCs中的CM。这两种技术都显示出清晰的CM特征，其效率彼此吻合良好。具有相同能隙的NCs在表观多激子产率上表现出适度的批次间变化（约30%以内），并且由于样品条件（搅拌与静态溶液）的差异而有更大的变化（超过3倍）。这些结果表明NC表面性质可能会影响CM过程。它们还指出了诸如NC光电离等外来效应可能产生的潜在干扰，这些干扰会扭曲CM研究的结果。在通过强烈搅拌样品和使用极低泵浦水平（每个NC每个脉冲吸收0.02 - 0.03个光子）抑制外来效应的条件下测量的CM产率表明，与体材料相比，电子 - 空穴产生能量和CM阈值都降低了。这些结果表明在NC材料中CM过程存在限制诱导增强。通过利用更复杂的（例如，形状可控或异质结构的）NCs，允许轻松操纵载流子 - 载流子相互作用以及单激子和多激子的能量与动力学，应该有可能进一步优化CM性能。
