1
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Aschendorf CJ, Degbevi M, Prather KV, Tsui EY. EPR spin trapping of nucleophilic and radical reactions at colloidal metal chalcogenide quantum dot surfaces. Chem Sci 2023; 14:13080-13089. [PMID: 38023529 PMCID: PMC10664490 DOI: 10.1039/d3sc04724e] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2023] [Accepted: 10/31/2023] [Indexed: 12/01/2023] Open
Abstract
The participation of the surfaces of colloidal semiconductor nanocrystal quantum dots (QDs) in QD-mediated photocatalytic reactions is an important factor that distinguishes QDs from other photosensitizers (e.g. transition metal complexes or organic dyes). Here, we probe nucleophilic and radical reactivity of surface sulfides and selenides of metal chalcogenide (CdSe, CdS, ZnSe, and PbS) QDs using chemical reactions and NMR spectroscopy. Additionally, the high sensitivity of EPR spectroscopy is adapted to study these surface-centered reactions through the use of spin traps like 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) under photoexcitation and thermal conditions. We demonstrate that DMPO likely adds to CdSe QD surfaces under thermal conditions by a nucleophilic mechanism in which the surface chalcogenides add to the double bond, followed by further oxidation of the surface-bound product. In contrast, CdS QDs more readily form surface sulfur-centered radicals that can perform reactions including alkene isomerization. These results indicate that QD surfaces should be an important consideration for the design of photocatalysis beyond simply tuning QD semiconductor band gaps.
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Affiliation(s)
- Caroline J Aschendorf
- Department of Chemistry and Biochemistry, University of Notre Dame Notre Dame IN USA
| | - Mawuli Degbevi
- Department of Chemistry and Biochemistry, University of Notre Dame Notre Dame IN USA
| | - Keaton V Prather
- Department of Chemistry and Biochemistry, University of Notre Dame Notre Dame IN USA
| | - Emily Y Tsui
- Department of Chemistry and Biochemistry, University of Notre Dame Notre Dame IN USA
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2
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Stam M, du Fossé I, Infante I, Houtepen AJ. Guilty as Charged: The Role of Undercoordinated Indium in Electron-Charged Indium Phosphide Quantum Dots. ACS NANO 2023; 17:18576-18583. [PMID: 37712414 PMCID: PMC10540256 DOI: 10.1021/acsnano.3c07029] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/28/2023] [Accepted: 09/12/2023] [Indexed: 09/16/2023]
Abstract
Quantum dots (QDs) are known for their size-dependent optical properties, narrow emission bands, and high photoluminescence quantum yield (PLQY), which make them interesting candidates for optoelectronic applications. In particular, InP QDs are receiving a lot of attention since they are less toxic than other QD materials and are hence suitable for consumer applications. Most of these applications, such as LEDs, photovoltaics, and lasing, involve charging QDs with electrons and/or holes. However, charging of QDs is not easy nor innocent, and the effect of charging on the composition and properties of InP QDs is not yet well understood. This work provides theoretical insight into electron charging of the InP core and InP/ZnSe QDs. Density functional theory calculations are used to show that charging of InP-based QDs with electrons leads to the formation of trap states if the QD contains In atoms that are undercoordinated and thus have less than four bonds to neighboring atoms. InP core-only QDs have such atoms at the surface, which are responsible for the formation of trap states upon charging with electrons. We show that InP/ZnSe core-shell models with all In atoms fully coordinated can be charged with electrons without the formation of trap states. These results show that undercoordinated In atoms should be avoided at all times for QDs to be stably charged with electrons.
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Affiliation(s)
- Maarten Stam
- Optoelectronic
Materials Section, Faculty of Applied Sciences, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The
Netherlands
| | - Indy du Fossé
- Optoelectronic
Materials Section, Faculty of Applied Sciences, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The
Netherlands
| | - Ivan Infante
- BC
Materials, Basque Center for Materials, Applications, and Nanostructures, UPV/EHU Science Park, Leioa 48940, Spain
- Ikerbasque,
Basque Foundation for Science, Bilbao 48009, Spain
| | - Arjan J. Houtepen
- Optoelectronic
Materials Section, Faculty of Applied Sciences, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The
Netherlands
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3
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Homer MK, Kuo DY, Dou FY, Cossairt BM. Photoinduced Charge Transfer from Quantum Dots Measured by Cyclic Voltammetry. J Am Chem Soc 2022; 144:14226-14234. [PMID: 35897128 DOI: 10.1021/jacs.2c04991] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
Measuring and modulating charge-transfer processes at quantum dot interfaces are crucial steps in developing quantum dots as photocatalysts. In this work, cyclic voltammetry under illumination is demonstrated to measure the rate of photoinduced charge transfer from CdS quantum dots by directly probing the changing oxidation states of a library of molecular charge acceptors, including both hole and electron acceptors. The voltammetry data demonstrate the presence of long-lived charge donor states generated by native photodoping of the quantum dots as well as a positive correlation between driving force and rate of charge transfer. Changes to the voltammograms under illumination follow mechanistic predictions from the ErCi' zone diagram, and electrochemical modeling allows for measurement of the rate of productive electron transfer. Observed rates for photoinduced charge transfer are on the order of 0.1 s-1, which are distinct from the picosecond dynamics measured by conventional transient optical spectroscopy methods and are more closely connected to the quantum yield of light-mediated chemical transformations.
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Affiliation(s)
- Micaela K Homer
- Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
| | - Ding-Yuan Kuo
- Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
| | - Florence Y Dou
- Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
| | - Brandi M Cossairt
- Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States
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4
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Widness JK, Enny DG, McFarlane-Connelly KS, Miedenbauer MT, Krauss TD, Weix DJ. CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Processes. J Am Chem Soc 2022; 144:12229-12246. [PMID: 35772053 DOI: 10.1021/jacs.2c03235] [Citation(s) in RCA: 17] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Abstract
Strong reducing agents (<-2.0 V vs saturated calomel electrode (SCE)) enable a wide array of useful organic chemistry, but suffer from a variety of limitations. Stoichiometric metallic reductants such as alkali metals and SmI2 are commonly employed for these reactions; however, considerations including expense, ease of use, safety, and waste generation limit the practicality of these methods. Recent approaches utilizing energy from multiple photons or electron-primed photoredox catalysis have accessed reduction potentials equivalent to Li0 and shown how this enables selective transformations of aryl chlorides via aryl radicals. However, in some cases, low stability of catalytic intermediates can limit turnover numbers. Herein, we report the ability of CdS nanocrystal quantum dots (QDs) to function as strong photoreductants and present evidence that a highly reducing electron is generated from two consecutive photoexcitations of CdS QDs with intermediate reductive quenching. Mechanistic experiments suggest that Auger recombination, a photophysical phenomenon known to occur in photoexcited anionic QDs, generates transient thermally excited electrons to enable the observed reductions. Using blue light-emitting diodes (LEDs) and sacrificial amine reductants, aryl chlorides and phosphate esters with reduction potentials up to -3.4 V vs SCE are photoreductively cleaved to afford hydrodefunctionalized or functionalized products. In contrast to small-molecule catalysts, QDs are stable under these conditions and turnover numbers up to 47 500 have been achieved. These conditions can also effect other challenging reductions, such as tosylate protecting group removal from amines, debenzylation of benzyl-protected alcohols, and reductive ring opening of cyclopropane carboxylic acid derivatives.
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Affiliation(s)
- Jonas K Widness
- Department of Chemistry, UW─Madison, Madison, Wisconsin 53706, United States
| | - Daniel G Enny
- Department of Chemistry, UW─Madison, Madison, Wisconsin 53706, United States
| | | | - Mahilet T Miedenbauer
- Materials Science Program, University of Rochester, Rochester, New York 14627, United States
| | - Todd D Krauss
- Department of Chemistry, University of Rochester, Rochester, New York 14627, United States.,Materials Science Program, University of Rochester, Rochester, New York 14627, United States.,Institute of Optics, University of Rochester, Rochester, New York 14627, United States
| | - Daniel J Weix
- Department of Chemistry, UW─Madison, Madison, Wisconsin 53706, United States
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5
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Wu Z, Zhang Y, Hu R, Jiang M, Liang P, Yang Q, Deng L, Jia T, Sun Z, Feng D. Hole-Acceptor-Manipulated Electron Spin Dynamics in CdSe Colloidal Quantum Dots. J Phys Chem Lett 2021; 12:2126-2132. [PMID: 33625852 DOI: 10.1021/acs.jpclett.0c03669] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/12/2023]
Abstract
Electron spin dynamics in CdSe quantum dots with hole acceptors are investigated by time-resolved ellipticity spectroscopy. Two types of hole acceptors, Li[Et3BH] and 1-octanethiol, result in distinctly different electron spin dynamics. The differences include electron g factors, spin dephasing/relaxation times, and mechanisms. In CdSe quantum dots with Li[Et3BH], the electron spin dephasing and relaxation are dominated by electron-nuclear hyperfine interactions in zero and weak magnetic fields. In contrast, hyperfine interactions, electron carrier lifetimes, and exchange interactions between electrons and holes or surface dangling bond spins control the electron spin dynamics in CdSe quantum dots with 1-octanethiol. Inhomogeneous dephasing limits the spin coherence time in larger transverse magnetic fields for both hole acceptor cases, but with distinct different g-factor inhomogeneity. These findings manifest that surface conditions play an important role in the spin dynamics and that thereby the surface and its surroundings can be exploited to control the spin in colloidal nanostructures.
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Affiliation(s)
- Zhen Wu
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Yuanyuan Zhang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Rongrong Hu
- College of Sciences, Shanghai Institute of Technology, Shanghai 201418, China
| | - Meizhen Jiang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Pan Liang
- College of Arts and Sciences, Shanghai Dianji University, Shanghai 201306, China
| | - Qing Yang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Li Deng
- School of Physics & Electronic Science, East China Normal University, Shanghai 200241, China
| | - Tianqing Jia
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Zhenrong Sun
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
| | - Donghai Feng
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
- Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China
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6
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Peper JL, Gentry NE, Brezny AC, Field MJ, Green MT, Mayer JM. Different Kinetic Reactivity of Electrons in Distinct TiO 2 Nanoparticle Trap States. THE JOURNAL OF PHYSICAL CHEMISTRY. C, NANOMATERIALS AND INTERFACES 2021; 125:680-690. [PMID: 34178203 PMCID: PMC8232823 DOI: 10.1021/acs.jpcc.0c10633] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Electrons added to TiO2 and other semiconductors often occupy trap states, whose reactivity can determine the catalytic and stoichiometric chemistry of the material. We previously showed that reduced aqueous colloidal TiO2 nanoparticles have two distinct classes of thermally-equilibrated trapped electrons, termed Red/e - and Blue/e -. Presented here are parallel optical and electron paramagnetic resonance (EPR) kinetic studies of the reactivity of these electrons with solution-based oxidants. Optical stopped-flow measurements monitoring reactions of TiO2/e - with sub-stoichiometric oxidants showed a surprising pattern: an initial fast (seconds) decrease in TiO2/e - absorbance followed by a secondary, slow (minutes) increase in the broad TiO2/e - optical feature. Analysis revealed that the fast decrease is due to the preferential oxidation of the Red/e - trap states, and the slow increase results from re-equilibration of electrons from Blue to Red states. This kinetic model was confirmed by freeze-quench EPR measurements. Quantitative analysis of the kinetic data demonstrated that Red/e - react ~5 times faster than Blue/e - with the nitroxyl radical oxidant, 4-MeO-TEMPO. Similar reactivity patterns were also observed in oxidations of TiO2/e - by O2, which like 4-MeO-TEMPO is a proton-coupled electron transfer (PCET) oxidant, and by the pure electron transfer (ET) oxidant KI3. This suggests that the faster intrinsic reactivity of one trap state over another on the seconds-minutes timescale is likely a general feature of reduced TiO2 reactivity. This differential trap state reactivity is likely to influence the performance of TiO2 in photochemical/electrochemical devices, and it suggests an opportunity for tuning catalysis.
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Affiliation(s)
- Jennifer L. Peper
- Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States
| | - Noreen E. Gentry
- Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States
| | - Anna C. Brezny
- Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States
- Department of Chemistry, Skidmore College, Saratoga Springs, New York 12866, United States
| | - Mackenzie J. Field
- Department of Chemistry and Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697, United States
| | - Michael T. Green
- Department of Chemistry and Department of Molecular Biology and Biochemistry, University of California, Irvine, California 92697, United States
| | - James M. Mayer
- Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States
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7
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Schival KA, Gipson RR, Prather KV, Tsui EY. Photoinduced Surface Charging in Iron-Carbonyl-Functionalized Colloidal Semiconductor Nanocrystals. NANO LETTERS 2019; 19:7770-7774. [PMID: 31596596 DOI: 10.1021/acs.nanolett.9b02726] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
Organometallic surface functionalization of colloidal CdSe and CdS nanocrystals using iron tetracarbonyl moieties is demonstrated to enable study of in situ colloidal nanocrystal surface redox chemistry. Spectroscopic measurements of the surface-bound metal carbonyl C-O stretches were used to elucidate the coordination environments and local symmetry of surface sites. The C-O stretching frequencies of these fragments were correlated to the electric field induced by nanocrystal surface charges and shift in energy upon surface reduction or oxidation. These measurements revealed that CdSe nanocrystals can accumulate multiple surface electrons under supra-band gap photoexcitation, a process likely relevant to photoactivated nanocrystal processes such as photobrightening. These surface charges are stable for hours and decay extremely slowly under anaerobic conditions.
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Affiliation(s)
- Keith A Schival
- Department of Chemistry and Biochemistry , University of Notre Dame , Notre Dame , Indiana 46556 , United States
| | - Robert R Gipson
- Department of Chemistry and Biochemistry , University of Notre Dame , Notre Dame , Indiana 46556 , United States
| | - Keaton V Prather
- Department of Chemistry and Biochemistry , University of Notre Dame , Notre Dame , Indiana 46556 , United States
| | - Emily Y Tsui
- Department of Chemistry and Biochemistry , University of Notre Dame , Notre Dame , Indiana 46556 , United States
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8
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Gudjonsdottir S, Koopman C, Houtepen AJ. Enhancing the stability of the electron density in electrochemically doped ZnO quantum dots. J Chem Phys 2019; 151:144708. [DOI: 10.1063/1.5124534] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Affiliation(s)
- Solrun Gudjonsdottir
- Chemical Engineering, Optoelectronic Materials, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Christel Koopman
- Chemical Engineering, Optoelectronic Materials, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Arjan J. Houtepen
- Chemical Engineering, Optoelectronic Materials, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
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9
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Hu R, Wu Z, Zhang Y, Yakovlev DR, Liang P, Qiang G, Guo J, Jia T, Sun Z, Bayer M, Feng D. Long-Lived Negative Photocharging in Colloidal CdSe Quantum Dots Revealed by Coherent Electron Spin Precession. J Phys Chem Lett 2019; 10:4994-4999. [PMID: 31408346 DOI: 10.1021/acs.jpclett.9b02341] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Photoinduced charging in CdSe colloidal quantum dots (QDs) is investigated by time-resolved pump-probe spectroscopy that is sensitive to electron spin polarization. This technique monitors the coherent spin dynamics of optically oriented electrons precessing around an external magnetic field. By addition of 1-octanethiol to the CdSe QD solution in toluene, an extremely long-lived negative photocharging is detected that lives up to 1 month in an N2 atmosphere and hours in an air atmosphere at room temperature. 1-Octanethiol not only acts as a hole acceptor but also results in a reduction of the oxygen-induced photo-oxidation in CdSe QDs, allowing air-stable negative photocharging. Two types of negative photocharging states with different spin precession frequencies and very different lifetimes are identified. These findings have important implications for understanding the photophysical processes in colloidal nanostructures.
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Affiliation(s)
- Rongrong Hu
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
| | - Zhen Wu
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
| | - Yuanyuan Zhang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
| | - Dmitri R Yakovlev
- Experimentelle Physik 2, Technische Universität Dortmund, 44221 Dortmund, Germany
- Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
| | - Pan Liang
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
| | - Gang Qiang
- Experimentelle Physik 2, Technische Universität Dortmund, 44221 Dortmund, Germany
| | - Jiaxing Guo
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
| | - Tianqing Jia
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
| | - Zhenrong Sun
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
| | - Manfred Bayer
- Experimentelle Physik 2, Technische Universität Dortmund, 44221 Dortmund, Germany
- Ioffe Institute, Russian Academy of Sciences, 194021 St. Petersburg, Russia
| | - Donghai Feng
- State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200062, China
- Collaborative Innovation Center of Extreme Optics, Shanxi University, Shanxi 030006, China
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10
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Gudjonsdottir S, van der Stam W, Koopman C, Kwakkenbos B, Evers WH, Houtepen AJ. On the Stability of Permanent Electrochemical Doping of Quantum Dot, Fullerene, and Conductive Polymer Films in Frozen Electrolytes for Use in Semiconductor Devices. ACS APPLIED NANO MATERIALS 2019; 2:4900-4909. [PMID: 31475245 PMCID: PMC6711363 DOI: 10.1021/acsanm.9b00863] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/08/2019] [Accepted: 07/16/2019] [Indexed: 05/03/2023]
Abstract
Semiconductor films that allow facile ion transport can be electronically doped via electrochemistry, where the amount of injected charge can be controlled by the potential applied. To apply electrochemical doping to the design of semiconductor devices, the injected charge has to be stabilized to avoid unintentional relaxation back to the intrinsic state. Here, we investigate methods to increase the stability of electrochemically injected charges in thin films of a wide variety of semiconductor materials, namely inorganic semiconductors (ZnO NCs, CdSe NCs, and CdSe/CdS core/shell NCs) and organic semiconductors (P3DT, PCBM, and C60). We show that by charging the semiconductors at elevated temperatures in solvents with melting points above room temperature, the charge stability at room temperature increases greatly, from seconds to days. At reduced temperature (-75 °C when using succinonitrile as electrolyte solvent) the injected charge becomes entirely stable on the time scale of our experiments (up to several days). Other high melting point solvents such as dimethyl sulfone, ethylene carbonate, and poly(ethylene glycol) (PEG) also offer increased charge stability at room temperature. Especially the use of PEG increases the room temperature charge stability by several orders of magnitude compared to using acetonitrile. We discuss how this improvement of the charge stability is related to the immobilization of electrolyte ions and impurities. While the electrolyte ions are immobilized, conductivity measurements show that electrons in the semiconductor films remain mobile. These results highlight the potential of using solidified electrolytes to stabilize injected charges, which is a promising step toward making semiconductor devices based on electrochemically doped semiconductor thin films.
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Affiliation(s)
- Solrun Gudjonsdottir
- Chemical
Engineering, Optoelectronic Materials, and Kavli Institute of Nanoscience, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Ward van der Stam
- Chemical
Engineering, Optoelectronic Materials, and Kavli Institute of Nanoscience, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Christel Koopman
- Chemical
Engineering, Optoelectronic Materials, and Kavli Institute of Nanoscience, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Bob Kwakkenbos
- Chemical
Engineering, Optoelectronic Materials, and Kavli Institute of Nanoscience, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Wiel H. Evers
- Chemical
Engineering, Optoelectronic Materials, and Kavli Institute of Nanoscience, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Arjan J. Houtepen
- Chemical
Engineering, Optoelectronic Materials, and Kavli Institute of Nanoscience, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
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11
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du Fossé I, ten Brinck S, Infante I, Houtepen AJ. Role of Surface Reduction in the Formation of Traps in n-Doped II-VI Semiconductor Nanocrystals: How to Charge without Reducing the Surface. CHEMISTRY OF MATERIALS : A PUBLICATION OF THE AMERICAN CHEMICAL SOCIETY 2019; 31:4575-4583. [PMID: 31274957 PMCID: PMC6595709 DOI: 10.1021/acs.chemmater.9b01395] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/09/2019] [Revised: 05/25/2019] [Indexed: 05/19/2023]
Abstract
The efficiency of nanocrystal (NC)-based devices is often limited by the presence of surface states that lead to localized energy levels in the bandgap. Yet, a complete understanding of the nature of these traps remains challenging. Although theoretical modeling has greatly improved our comprehension of the NC surface, several experimental studies suggest the existence of metal-based traps that have not yet been found with theoretical methods. Since there are indications that these metal-based traps form in the presence of excess electrons, the present work uses density functional theory (DFT) calculations to study the effects of charging II-VI semiconductor NCs with either full or imperfect surface passivation. It is found that charge injection can lead to trap-formation via two pathways: metal atom ejection from perfectly passivated NCs or metal-metal dimer-formation in imperfectly passivated NCs. Fully passivated CdTe NCs are observed to be stable up to a charge of two electrons. Further reduction leads to charge localization on a surface Cd atom and the formation of in-gap states. The effects of suboptimal passivation are probed by charging NCs where an X-type ligand is removed from the (100) plane. In this case, injection of even one electron leads to Cd-dimerization and trap-formation. Addition of an L-type amine ligand prevents this dimer-formation and is suggested to also prevent trapping of photoexcited electrons in charge neutral NCs. The results presented in this work are generalized to NCs of different sizes and other II-VI semiconductors. This has clear implications for n-doping II-VI semiconductor NCs without introducing surface traps due to metal ion reduction. The possible effect of these metal ion localized traps on the photoluminescence efficiency of neutral NCs is also discussed.
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Affiliation(s)
- Indy du Fossé
- Optoelectronic
Materials Section, Faculty of Applied Sciences, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Stephanie ten Brinck
- Department
of Theoretical Chemistry, Faculty of Science, Vrije Universiteit Amsterdam, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
| | - Ivan Infante
- Department
of Theoretical Chemistry, Faculty of Science, Vrije Universiteit Amsterdam, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands
- Department
of Nanochemistry, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy
- E-mail:
| | - Arjan J. Houtepen
- Optoelectronic
Materials Section, Faculty of Applied Sciences, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands
- E-mail:
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12
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Utterback JK, Ruzicka JL, Hamby H, Eaves JD, Dukovic G. Temperature-Dependent Transient Absorption Spectroscopy Elucidates Trapped-Hole Dynamics in CdS and CdSe Nanorods. J Phys Chem Lett 2019; 10:2782-2787. [PMID: 31067408 DOI: 10.1021/acs.jpclett.9b00764] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Charge-carrier traps play a central role in the excited-state dynamics of semiconductor nanocrystals, but their influence is often difficult to measure directly. In CdS and CdSe nanorods of nonuniform width, spatially separated electrons and trapped holes display relaxation dynamics that follow a power-law function in time that is consistent with a recombination process limited by trapped-hole diffusion. However, power-law relaxation can originate from mechanisms other than diffusion. Here we report transient absorption spectroscopy measurements on CdS and CdSe nanorods recorded at temperatures ranging from 160 to 294 K. We find that the exponent of the power law is temperature-independent, which rules out several models based on stochastic activated processes and provides insights into the mechanism of diffusion-limited recombination in these structures. The data point to weak electronic coupling between trap states and suggest that surface-localized trapped holes couple strongly to phonons, leading to slow diffusion. Trap-to-trap hole hopping behaves classically near room temperature, while quantum aspects of phonon-assisted tunneling become observable at low temperatures.
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Affiliation(s)
- James K Utterback
- Department of Chemistry , University of Colorado Boulder , Boulder , Colorado 80309 , United States
| | - Jesse L Ruzicka
- Department of Chemistry , University of Colorado Boulder , Boulder , Colorado 80309 , United States
| | - Hayden Hamby
- Department of Chemistry , University of Colorado Boulder , Boulder , Colorado 80309 , United States
| | - Joel D Eaves
- Department of Chemistry , University of Colorado Boulder , Boulder , Colorado 80309 , United States
| | - Gordana Dukovic
- Department of Chemistry , University of Colorado Boulder , Boulder , Colorado 80309 , United States
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13
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Garoz‐Ruiz J, Perales‐Rondon JV, Heras A, Colina A. Spectroelectrochemistry of Quantum Dots. Isr J Chem 2019. [DOI: 10.1002/ijch.201900028] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Affiliation(s)
- Jesus Garoz‐Ruiz
- Department of ChemistryUniversidad de Burgos Pza. Misael Bañuelos s/n E-09001 Burgos Spain
| | - Juan V. Perales‐Rondon
- Department of ChemistryUniversidad de Burgos Pza. Misael Bañuelos s/n E-09001 Burgos Spain
| | - Aranzazu Heras
- Department of ChemistryUniversidad de Burgos Pza. Misael Bañuelos s/n E-09001 Burgos Spain
| | - Alvaro Colina
- Department of ChemistryUniversidad de Burgos Pza. Misael Bañuelos s/n E-09001 Burgos Spain
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14
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Hartley CL, Dempsey JL. Electron-Promoted X-Type Ligand Displacement at CdSe Quantum Dot Surfaces. NANO LETTERS 2019; 19:1151-1157. [PMID: 30640472 DOI: 10.1021/acs.nanolett.8b04544] [Citation(s) in RCA: 17] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/21/2023]
Abstract
Quantum dot surfaces are redox active and are known to influence the electronic properties of nanocrystals, yet the molecular-level changes in surface chemistry that occur upon addition of charge are not well understood. In this paper, we report a systematic study monitoring changes in surface coordination chemistry in 3.4 nm CdSe quantum dots upon remote chemical doping by the radical anion reductant sodium naphthalenide (Na[C10H8]). These studies reveal a new mechanism for charge-balancing the added electrons that localize on surface states through loss of up to ca. 5% of the native anionic carboxylate ligands, as quantified through a combination of UV-vis absorption, 1H NMR, and FTIR spectroscopies. A new method for distinguishing between reduction of surface metal and chalcogenide ions by monitoring ligand loss and optical changes upon doping is introduced. This work emphasizes the importance of studying changes in surface chemistry with remote chemical doping and is more broadly contextualized within the redox reactivity of the QD surface.
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Affiliation(s)
- Carolyn L Hartley
- Department of Chemistry , University of North Carolina , Chapel Hill , North Carolina 27599-3290 , United States
| | - Jillian L Dempsey
- Department of Chemistry , University of North Carolina , Chapel Hill , North Carolina 27599-3290 , United States
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15
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Abstract
Transient absorption (TA) and time-resolved photoluminescence (PL) spectroscopies have been used to provide direct spectroscopic evidence for the recently reported phenomenon of thermal "surface charging" in II-VI quantum dots (QDs). In these studies, zincblende CdSe cores are synthesized by standard methods, and a thin CdS shell deposited by the decomposition of Cd(DDTC)2, resulting in core/shell QDs with chalcogenide-rich surfaces. Following ligand exchange with oleylamine, these QDs have empty low-lying surface states that can be thermally populated from the valence band. At room temperature, the surface charging equilibrium results in some fraction of the particles having a hole in the valence band, i.e., the surface acceptor states make the particle p-type. Photoexcitation of the surface charged state results in what is essentially a positive trion, which can undergo a fast Auger recombination. Both PL and TA (bleach recovery) kinetics of the CdSe/CdS QDs show a 70 ps decay component, which is assigned to Auger recombination. The empty nonbonding surface orbitals are passivated by ligation with a trialkylphosphine, and the fast decay component is absent when tributylphosphine is present. The comparison of the TA and PL kinetics shows that the relative amplitude of the 70 ps component is a factor of about 1.5 greater in the TA than in the PL. They also show that the fast component in the PL spectrum is shifted about 6 nm to the blue of the exciton luminescence. The above observations can be understood in terms of the trion versus exciton spectroscopy and strongly support the assignment of the 70 ps transient to the decay of a trion formed from the surface charged state.
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Affiliation(s)
- David Morgan
- Chemistry and Chemical Biology , University of California Merced , 5200 North Lake Road , Merced , California 95343 , United States
| | - David F Kelley
- Chemistry and Chemical Biology , University of California Merced , 5200 North Lake Road , Merced , California 95343 , United States
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16
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Araujo JJ, Brozek CK, Kroupa DM, Gamelin DR. Degenerately n-Doped Colloidal PbSe Quantum Dots: Band Assignments and Electrostatic Effects. NANO LETTERS 2018; 18:3893-3900. [PMID: 29763319 DOI: 10.1021/acs.nanolett.8b01235] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
We present a spectroscopic study of colloidal PbSe quantum dots (QDs) that have been photodoped to introduce excess delocalized conduction-band (CB) electrons. High-quality absorption spectra are obtained for these degenerately doped QDs with excess electron concentrations up to ∼1020 cm-3. At the highest doping levels, electrons have completely filled the 1Se orbitals of the CB and partially populated the higher-energy 1Pe orbitals. Spectroscopic changes observed as a function of carrier concentration permit an unambiguous assignment of the second excitonic absorption maximum to 1Ph-1Pe transitions. At intermediate doping levels, a clear absorption feature appears between the first two excitonic maxima that is attributable to parity-forbidden 1Sh,e-1Pe,h excitations, which become observable because of electrostatic symmetry breaking. Redshifts of the main excitonic absorption features with increased carrier concentration are also analyzed. The Coulomb stabilization energies of both the 1Sh-1Se and 1Ph-1Pe excitons in n-doped PbSe QDs are remarkably similar to those observed for multiexcitons with the same electron count. The origins of these redshifts are discussed.
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Affiliation(s)
- Jose J Araujo
- Department of Chemistry , University of Washington , Seattle , Washington 98195-1700 , United States
| | - Carl K Brozek
- Department of Chemistry , University of Washington , Seattle , Washington 98195-1700 , United States
| | - Daniel M Kroupa
- Department of Chemistry , University of Washington , Seattle , Washington 98195-1700 , United States
| | - Daniel R Gamelin
- Department of Chemistry , University of Washington , Seattle , Washington 98195-1700 , United States
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17
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Orfield NJ, Majumder S, McBride JR, Yik-Ching Koh F, Singh A, Bouquin SJ, Casson JL, Johnson AD, Sun L, Li X, Shih CK, Rosenthal SJ, Hollingsworth JA, Htoon H. Photophysics of Thermally-Assisted Photobleaching in "Giant" Quantum Dots Revealed in Single Nanocrystals. ACS NANO 2018; 12:4206-4217. [PMID: 29709173 DOI: 10.1021/acsnano.7b07450] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Quantum dots (QDs) are steadily being implemented as down-conversion phosphors in market-ready display products to enhance color rendering, brightness, and energy efficiency. However, for adequate longevity, QDs must be encased in a protective barrier that separates them from ambient oxygen and humidity, and device architectures are designed to avoid significant heating of the QDs as well as direct contact between the QDs and the excitation source. In order to increase the utility of QDs in display technologies and to extend their usefulness to more demanding applications as, for example, alternative phosphors for solid-state lighting (SSL), QDs must retain their photoluminescence emission properties over extended periods of time under conditions of high temperature and high light flux. Doing so would simplify the fabrication costs for QD display technologies and enable QDs to be used as down-conversion materials in light-emitting diodes for SSL, where direct-on-chip configurations expose the emitters to temperatures approaching 100 °C and to photon fluxes from 0.1 W/mm2 to potentially 10 W/mm2. Here, we investigate the photobleaching processes of single QDs exposed to controlled temperature and photon flux. In particular, we investigate two types of room-temperature-stable core/thick-shell QDs, known as "giant" QDs for which shell growth is conducted using either a standard layer-by-layer technique or by a continuous injection method. We determine the mechanistic pathways responsible for thermally-assisted photodegradation, distinguishing effects of hot-carrier trapping and QD charging. The findings presented here will assist in the further development of advanced QD heterostructures for maximum device lifetime stability.
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Affiliation(s)
- Noah J Orfield
- Materials Physics and Applications Division: Center for Integrated Nanotechnologies , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States
| | - Somak Majumder
- Materials Physics and Applications Division: Center for Integrated Nanotechnologies , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States
| | - James R McBride
- Department of Chemistry , Vanderbilt University , Nashville , Tennessee 37235 , United States
| | - Faith Yik-Ching Koh
- Materials Physics and Applications Division: Center for Integrated Nanotechnologies , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States
| | - Ajay Singh
- Materials Physics and Applications Division: Center for Integrated Nanotechnologies , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States
| | - Sarah J Bouquin
- Materials Physics and Applications Division: Center for Integrated Nanotechnologies , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States
| | - Joanna L Casson
- Chemistry Division , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States
| | - Alex D Johnson
- Physics Department and Center for Complex Quantum Systems , University of Texas at Austin , Austin , Texas 78712 , United States
| | - Liuyang Sun
- Physics Department and Center for Complex Quantum Systems , University of Texas at Austin , Austin , Texas 78712 , United States
| | - Xiaoqin Li
- Physics Department and Center for Complex Quantum Systems , University of Texas at Austin , Austin , Texas 78712 , United States
| | - Chih-Kang Shih
- Physics Department and Center for Complex Quantum Systems , University of Texas at Austin , Austin , Texas 78712 , United States
| | - Sandra J Rosenthal
- Department of Chemistry , Vanderbilt University , Nashville , Tennessee 37235 , United States
| | - Jennifer A Hollingsworth
- Materials Physics and Applications Division: Center for Integrated Nanotechnologies , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States
| | - Han Htoon
- Materials Physics and Applications Division: Center for Integrated Nanotechnologies , Los Alamos National Laboratory , Los Alamos , New Mexico 87545 , United States
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18
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Saniepay M, Mi C, Liu Z, Abel EP, Beaulac R. Insights into the Structural Complexity of Colloidal CdSe Nanocrystal Surfaces: Correlating the Efficiency of Nonradiative Excited-State Processes to Specific Defects. J Am Chem Soc 2018; 140:1725-1736. [PMID: 29293359 DOI: 10.1021/jacs.7b10649] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
II-VI colloidal semiconductor nanocrystals (NCs), such as CdSe NCs, are often plagued by efficient nonradiative recombination processes that severely limit their use in energy-conversion schemes. While these processes are now well-known to occur at the surface, a full understanding of the exact nature of surface defects and of their role in deactivating the excited states of NCs has yet to be established, which is partly due to challenges associated with the direct probing of the complex and dynamic surface of colloidal NCs. Here, we report a detailed study of the surface of cadmium-rich zinc-blende CdSe NCs. The surfaces of these cadmium-rich species are characterized by the presence of cadmium carboxylate complexes (CdX2) that act as Lewis acid (Z-type) ligands that passivate undercoordinated selenide surface species. The systematic displacement of CdX2 from the surface by N,N,N',N'-tetramethylethylene-1,2-diamine (TMEDA) has been studied using a combination of 1H NMR and photoluminescence spectroscopies. We demonstrate the existence of two independent surface sites that differ strikingly in the binding affinity for CdX2 and that are under dynamic equilibrium with each other. A model involving coupled dual equilibria allows a full characterization of the thermodynamics of surface binding (free energy, as well as enthalpic and entropic terms), showing that entropic contributions are responsible for the difference between the two surface sites. Importantly, we demonstrate that cadmium vacancies only lead to important photoluminescence quenching when created on one of the two sites, allowing a complete picture of the surface composition to be drawn where each site is assigned to specific NC facet locale, with CdX2 binding affinity and nonradiative recombination efficiencies that differ by up to two orders of magnitude.
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Affiliation(s)
- Mersedeh Saniepay
- Department of Chemistry, Michigan State University , East Lansing, Michigan 48824-1322, United States
| | - Chenjia Mi
- Department of Chemistry, Michigan State University , East Lansing, Michigan 48824-1322, United States
| | - Zhihui Liu
- Department of Chemistry, Michigan State University , East Lansing, Michigan 48824-1322, United States
| | - E Paige Abel
- Department of Chemistry, Michigan State University , East Lansing, Michigan 48824-1322, United States
| | - Rémi Beaulac
- Department of Chemistry, Michigan State University , East Lansing, Michigan 48824-1322, United States
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19
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Hartstein KH, Erickson CS, Tsui EY, Marchioro A, Gamelin DR. Electron Stability and Negative-Tetron Luminescence in Free-Standing Colloidal n-Type CdSe/CdS Quantum Dots. ACS NANO 2017; 11:10430-10438. [PMID: 28915009 DOI: 10.1021/acsnano.7b05551] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
We examine the effects of CdS shell growth on photochemical reduction of colloidal CdSe quantum dots (QDs) and describe the spectroscopic properties of the resulting n-type CdSe/CdS QDs. CdS shell growth greatly slows electron trapping. Because of this improvement, complete two-electron occupancy of the 1Se conduction-band orbital is achieved in CdSe/CdS QDs and found to be much more stable than in past experiments. Simultaneous photoluminescence at two different energies is now observed from QDs possessing two excess conduction-band electrons, reflecting competing recombination of discretized 1Se and 1Pe conduction-band electrons within photogenerated four-carrier negative tetrons (three electrons and one hole). Stable occupancy of the 1Pe level is not achievable under these conditions, and possible reasons are discussed. The stability and accessibility of these multielectron configurations, and the facile spectroscopic detection of negative tetrons, both make photodoped core/shell QDs attractive for exploring the physical properties of free-standing heavily n-doped colloidal CdSe-based QDs.
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Affiliation(s)
- Kimberly H Hartstein
- Department of Chemistry, University of Washington , Seattle, Washington 98195-1700, United States
| | - Christian S Erickson
- Department of Chemistry, University of Washington , Seattle, Washington 98195-1700, United States
| | - Emily Y Tsui
- Department of Chemistry, University of Washington , Seattle, Washington 98195-1700, United States
| | - Arianna Marchioro
- Department of Chemistry, University of Washington , Seattle, Washington 98195-1700, United States
| | - Daniel R Gamelin
- Department of Chemistry, University of Washington , Seattle, Washington 98195-1700, United States
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20
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van der Stam W, Gudjonsdottir S, Evers WH, Houtepen AJ. Switching between Plasmonic and Fluorescent Copper Sulfide Nanocrystals. J Am Chem Soc 2017; 139:13208-13217. [PMID: 28841295 PMCID: PMC5609121 DOI: 10.1021/jacs.7b07788] [Citation(s) in RCA: 44] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Abstract
![]()
Control over the doping density in
copper sulfide nanocrystals
is of great importance and determines its use in optoelectronic applications
such as NIR optical switches and photovoltaic devices. Here, we demonstrate
that we can reversibly control the hole carrier density (varying from
>1022 cm–3 to intrinsic) in copper
sulfide
nanocrystals by electrochemical methods. We can control the type of
charge injection, i.e., capacitive charging or ion intercalation,
via the choice of the charge compensating cation (e.g., ammonium salts
vs Li+). Further, the type of intercalating ion determines
whether the charge injection is fully reversible (for Li+) or leads to permanent changes in doping density (for Cu+). Using fully reversible lithium intercalation allows us to switch
between thin films of covellite CuS NCs (Eg = 2.0 eV, hole density 1022 cm–3, strong
localized surface plasmon resonance) and low-chalcocite CuLiS NCs
(Eg = 1.2 eV, intrinsic, no localized
surface plasmon resonance), and back. Electrochemical Cu+ ion intercalation leads to a permanent phase transition to intrinsic
low-chalcocite Cu2S nanocrystals that display air stable
fluorescence, centered around 1050 nm (fwhm ∼145 meV, PLQY
ca. 1.8%), which is the first observation of narrow near-infrared
fluorescence for copper sulfide nanocrystals. The dynamic control
over the hole doping density and fluorescence of copper sulfide nanocrystals
presented in this work and the ability to switch between plasmonic
and fluorescent semiconductor nanocrystals might lead to their successful
implementation into photovoltaic devices, NIR optical switches and
smart windows.
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Affiliation(s)
- Ward van der Stam
- Optoelectronic Materials Section, Faculty of Applied Sciences, Delft University of Technology , van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Solrun Gudjonsdottir
- Optoelectronic Materials Section, Faculty of Applied Sciences, Delft University of Technology , van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Wiel H Evers
- Optoelectronic Materials Section, Faculty of Applied Sciences, Delft University of Technology , van der Maasweg 9, 2629 HZ Delft, The Netherlands.,Kavli Institute of Nanoscience, Delft University of Technology , van der Maasweg 9, 2629 HZ Delft, The Netherlands
| | - Arjan J Houtepen
- Optoelectronic Materials Section, Faculty of Applied Sciences, Delft University of Technology , van der Maasweg 9, 2629 HZ Delft, The Netherlands
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