Spectroscopic and Device Aspects of Nanocrystal Quantum Dots

Pietryga, Jeffrey M.,Park, Young-Shin,Lim, Jaehoon,Fidler, Andrew F.,Bae, Wan Ki,Brovelli, Sergio,Klimov, Victor I. American Chemical Society 2016 Chemical reviews Vol.116 No.18

<P>The field of nanocrystal quantum dots (QDs) is already more than 30 years old, and yet continuing interest in these structures is driven by both the fascinating physics emerging from strong quantum confinement of electronic excitations, as well as a large number of prospective applications that could benefit from the tunable properties and amenability toward solution-based processing of these materials. The focus of this review is on recent advances in nanocrystal research related to applications of QD materials in lasing, light-emitting diodes (LEDs), and solar energy conversion. A specific underlying theme is innovative concepts for tuning the properties of QDs beyond what is possible via traditional size manipulation, particularly through heterostructuring. Examples of such advanced control of nanocrystal functionalities include the following: interface engineering for suppressing Auger recombination in the context of QD LEDs and lasers; Stokes-shift engineering for applications in large-area luminescent solar concentrators; and control of intraband relaxation for enhanced carrier multiplication in advanced QD photovoltaics. We examine the considerable recent progress on these multiple fronts of nanocrystal research, which has resulted in the first commercialized QD technologies. These successes explain the continuing appeal of this field to a broad community of scientists and engineers, which in turn ensures even more exciting results to come from future exploration of this fascinating class of materials.</P>

Influence of Shell Thickness on the Performance of Light-Emitting Devices Based on Type-I Core/Shell Heterostructure Quantum Dots

임재훈,정병국,박명진,Jeffrey M. Pietryga,김재경,박영신,Victor I. Klimov,이창희,배완기,이도창 한국고분자학회 2014 한국고분자학회 학술대회 연구논문 초록집 Vol.2014 No.10

The Scaling of the Effective Band Gaps in Indium−Arsenide Quantum Dots and Wires

Wang, Fudong,Yu, Heng,Jeong, Sohee,Pietryga, Jeffrey M.,Hollingsworth, Jennifer A.,Gibbons, Patrick C.,Buhro, William E. American Chemical Society 2008 ACS NANO Vol.2 No.9

Auger Recombination of Biexcitons and Negative and Positive Trions in Individual Quantum Dots

Park, Young-Shin,Bae, Wan Ki,Pietryga, Jeffrey M.,Klimov, Victor I. American Chemical Society 2014 ACS NANO Vol.8 No.7

<P>Charged exciton states commonly occur both in spectroscopic studies of quantum dots (QDs) and during operation of QD-based devices. The extra charge added to the neutral exciton modifies its radiative decay rate and also opens an additional nonradiative pathway associated with an Auger process whereby the recombination energy of an exciton is transferred to the excess charge. Here we conduct single-dot spectroscopic studies of Auger recombination in thick-shell (“giant”) CdSe/CdS QDs with and without an interfacial alloy layer using time-tagged, time-correlated single-photon counting. In photoluminescence (PL) intensity trajectories of some of the dots, we resolve three distinct states of different emissivities (“bright”, “gray”, and “dark”) attributed, respectively, to the neutral exciton and negative and positive trions. Simultaneously acquired PL lifetime trajectories indicate that the positive trion is much shorter lived than the negative trion, which can be explained by a high density of valence band states and a small hole localization radius (defined by the QD core size), factors that favor an Auger process involving intraband excitation of a hole. A comparison of trion and biexciton lifetimes suggests that the biexciton Auger decay can be treated in terms of a superposition of two independent channels associated with positive- and negative-trion pathways. The resulting interdependence between Auger time constants might simplify the studies of multicarrier recombination by allowing one, for example, to infer Auger lifetimes of trions of one sign based on the measurements of biexciton decay and dynamics of the trions of the opposite sign or, alternatively, estimate the biexciton lifetime based on studies of trion dynamics.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2014/ancac3.2014.8.issue-7/nn5023473/production/images/medium/nn-2014-023473_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn5023473'>ACS Electronic Supporting Info</A></P>

Effect of the Core/Shell Interface on Auger Recombination Evaluated by Single-Quantum-Dot Spectroscopy

Park, Young-Shin,Bae, Wan Ki,Padilha, Lazaro A.,Pietryga, Jeffrey M.,Klimov, Victor I. American Chemical Society 2014 NANO LETTERS Vol.14 No.2

<P>Previous single-particle spectroscopic studies of colloidal quantum dots have indicated a significant spread in biexciton lifetimes across an ensemble of nominally identical nanocrystals. It has been speculated that in addition to dot-to-dot variation in physical dimensions, this spread is contributed to by variations in the structure of the quantum dot interface, which controls the shape of the confinement potential. Here, we directly evaluate the effect of the composition of the core–shell interface on single- and multiexciton dynamics via side-by-side measurements of individual core–shell CdSe/CdS nanocrystals with a sharp versus smooth (graded) interface. To realize the latter type of structures we incorporate a CdSe<SUB><I>x</I></SUB>S<SUB>1–<I>x</I></SUB> alloy layer of controlled composition and thickness between the CdSe core and the CdS shell. We observe that while having essentially no effect on single-exciton decay, the interfacial alloy layer leads to a systematic increase in biexciton lifetimes, which correlates with the increase in the biexciton emission efficiency, as inferred from two-photon correlation measurements. These observations provide direct experimental evidence that in addition to the size of the quantum dot, its interfacial properties also significantly affect the rate of Auger recombination, which governs biexciton decay. These findings help rationalize previous observations of a significant heterogeneity in the biexciton lifetimes across similarly sized quantum dots and should facilitate the development of “Auger-recombination-free” colloidal nanostructures for a range of applications from lasers and light-emitting diodes to photodetectors and solar cells.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2014/nalefd.2014.14.issue-2/nl403289w/production/images/medium/nl-2013-03289w_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl403289w'>ACS Electronic Supporting Info</A></P>

Temperature and Magnetic-Field Dependence of Radiative Decay in Colloidal Germanium Quantum Dots

Robel, Istvá,n,Shabaev, Andrew,Lee, Doh C.,Schaller, Richard D.,Pietryga, Jeffrey M.,Crooker, Scott A.,L. Efros, Alexander,Klimov, Victor I. American Chemical Society 2015 Nano letters Vol.15 No.4

<P>We conduct spectroscopic and theoretical studies of photoluminescence (PL) from Ge quantum dots (QDs) fabricated via colloidal synthesis. The dynamics of late-time PL exhibit a pronounced dependence on temperature and applied magnetic field, which can be explained by radiative decay involving two closely spaced, slowly emitting exciton states. In 3.5 nm QDs, these states are separated by ∼1 meV and are characterized by ∼82 μs and ∼18 μs lifetimes. By using a four-band formalism, we calculate the fine structure of the indirect band-edge exciton arising from the electron–hole exchange interaction and the Coulomb interaction of the Γ-point hole with the anisotropic charge density of the <I>L</I>-point electron. The calculations suggest that the observed PL dynamics can be explained by phonon-assisted recombination of excitons thermally distributed between the lower-energy “dark” state with the momentum projection <I>J</I> = ± 2 and a higher energy “bright” state with <I>J</I> = ± 1. A fairly small difference between lifetimes of these states is due to their mixing induced by the exchange term unique to crystals with a highly symmetric cubic lattice such as Ge.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2015/nalefd.2015.15.issue-4/acs.nanolett.5b00344/production/images/medium/nl-2015-003445_0005.gif'></P>