Real-Time Observation of Ultrafast Intraband Relaxation and Exciton Multiplication in PbS Quantum Dots

Handle URI:
http://hdl.handle.net/10754/575596
Title:
Real-Time Observation of Ultrafast Intraband Relaxation and Exciton Multiplication in PbS Quantum Dots
Authors:
El-Ballouli, Ala’a O.; Alarousu, Erkki; Usman, Anwar; Pan, Jun; Bakr, Osman M. ( 0000-0002-3428-1002 ) ; Mohammed, Omar F. ( 0000-0001-8500-1130 )
Abstract:
We examine ultrafast intraconduction band relaxation and multiple-exciton generation (MEG) in PbS quantum dots (QDs) using transient absorption spectroscopy with 120 fs temporal resolution. The intraconduction band relaxation can be directly and excellently resolved spectrally and temporally by applying broadband pump-probe spectroscopy to excite and detect the wavelengths around the exciton absorption peak, which is located in the near-infrared region. The time-resolved data unambiguously demonstrate that the intraband relaxation time progressively increases as the pump-photon energy increases. Moreover, the relaxation time becomes much shorter as the size of the QDs decreases, indicating the crucial role of spatial confinement in the intraband relaxation process. Additionally, our results reveal the systematic scaling of the intraband relaxation time with both excess energy above the effective energy band gap and QD size. We also assess MEG in different sizes of the QDs. Under the condition of high-energy photon excitation, which is well above the MEG energy threshold, ultrafast bleach recovery due to the nonradiative Auger recombination of the multiple electron-hole pairs provides conclusive experimental evidence for the presence of MEG. For instance, we achieved quantum efficiencies of 159, 129 and 106% per single-absorbed photon at pump photoexcition of three times the band gap for QDs with band gaps of 880 nm (1.41 eV), 1000 nm (1.24 eV) and 1210 nm (1.0 eV), respectively. These findings demonstrate clearly that the efficiency of transferring excess photon energy to carrier multiplication is significantly increased in smaller QDs compared with larger ones. Finally, we discuss the Auger recombination dynamics of the multiple electron-hole pairs as a function of QD size.
KAUST Department:
Solar and Photovoltaic Engineering Research Center (SPERC); Solar and Photovoltaic Engineering Research Center (SPERC); Physical Sciences and Engineering (PSE) Division; Materials Science and Engineering Program; Solar and Photovoltaic Engineering Research Center (SPERC); Materials Science and Engineering Program; Functional Nanomaterials Lab (FuNL)
Publisher:
American Chemical Society (ACS)
Journal:
ACS Photonics
Issue Date:
19-Mar-2014
DOI:
10.1021/ph500016t
Type:
Article
ISSN:
2330-4022; 2330-4022
Appears in Collections:
Articles; Physical Sciences and Engineering (PSE) Division; Materials Science and Engineering Program; Solar and Photovoltaic Engineering Research Center (SPERC)

Full metadata record

DC FieldValue Language
dc.contributor.authorEl-Ballouli, Ala’a O.en
dc.contributor.authorAlarousu, Erkkien
dc.contributor.authorUsman, Anwaren
dc.contributor.authorPan, Junen
dc.contributor.authorBakr, Osman M.en
dc.contributor.authorMohammed, Omar F.en
dc.date.accessioned2015-08-24T08:33:44Zen
dc.date.available2015-08-24T08:33:44Zen
dc.date.issued2014-03-19en
dc.identifier.issn2330-4022en
dc.identifier.issn2330-4022en
dc.identifier.doi10.1021/ph500016ten
dc.identifier.urihttp://hdl.handle.net/10754/575596en
dc.description.abstractWe examine ultrafast intraconduction band relaxation and multiple-exciton generation (MEG) in PbS quantum dots (QDs) using transient absorption spectroscopy with 120 fs temporal resolution. The intraconduction band relaxation can be directly and excellently resolved spectrally and temporally by applying broadband pump-probe spectroscopy to excite and detect the wavelengths around the exciton absorption peak, which is located in the near-infrared region. The time-resolved data unambiguously demonstrate that the intraband relaxation time progressively increases as the pump-photon energy increases. Moreover, the relaxation time becomes much shorter as the size of the QDs decreases, indicating the crucial role of spatial confinement in the intraband relaxation process. Additionally, our results reveal the systematic scaling of the intraband relaxation time with both excess energy above the effective energy band gap and QD size. We also assess MEG in different sizes of the QDs. Under the condition of high-energy photon excitation, which is well above the MEG energy threshold, ultrafast bleach recovery due to the nonradiative Auger recombination of the multiple electron-hole pairs provides conclusive experimental evidence for the presence of MEG. For instance, we achieved quantum efficiencies of 159, 129 and 106% per single-absorbed photon at pump photoexcition of three times the band gap for QDs with band gaps of 880 nm (1.41 eV), 1000 nm (1.24 eV) and 1210 nm (1.0 eV), respectively. These findings demonstrate clearly that the efficiency of transferring excess photon energy to carrier multiplication is significantly increased in smaller QDs compared with larger ones. Finally, we discuss the Auger recombination dynamics of the multiple electron-hole pairs as a function of QD size.en
dc.publisherAmerican Chemical Society (ACS)en
dc.titleReal-Time Observation of Ultrafast Intraband Relaxation and Exciton Multiplication in PbS Quantum Dotsen
dc.typeArticleen
dc.contributor.departmentSolar and Photovoltaic Engineering Research Center (SPERC)en
dc.contributor.departmentSolar and Photovoltaic Engineering Research Center (SPERC)en
dc.contributor.departmentPhysical Sciences and Engineering (PSE) Divisionen
dc.contributor.departmentMaterials Science and Engineering Programen
dc.contributor.departmentSolar and Photovoltaic Engineering Research Center (SPERC)en
dc.contributor.departmentMaterials Science and Engineering Programen
dc.contributor.departmentFunctional Nanomaterials Lab (FuNL)en
dc.identifier.journalACS Photonicsen
kaust.authorUsman, Anwaren
kaust.authorPan, Junen
kaust.authorBakr, Osman M.en
kaust.authorEl-Ballouli, Ala'a O.en
kaust.authorAlarousu, Erkkien
kaust.authorMohammed, Omar F.en
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