Search query: Auger-process-induced charge separation in semiconductor nanocrystals

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abstract: Femtosecond nonlinear transmission techniques are applied to study mechanisms for optical nonlinearities and ultrafast carrier dynamics in CdS nanocrystals (NC{close_quote}s) The obtained data indicate the change in a dominant hole relaxation channel at high pump levels where nonlinear recombination effects start to play a significant role. This is manifested as a distinct difference in nonlinear optical responses measured at low and high pump intensities in quasiequilibrium at long times after excitation. The analysis of the wavelength and time dependence of the nonlinear transmission over a wide pump intensity range shows clearly that this difference is due to an Auger process assisted trapping of holes at surface/interface related states. This trapping leads to efficient charge separation and the generation of a dc electric field that modifies the nonlinear optical response in NC{close_quote}s at high pump intensities. {copyright} {ital 1997} {ital The American Physical Society}
author_names: Victor I Klimov

Duncan W Mcbranch
title: Auger process induced charge separation in semiconductor nanocrystals
year: 1997

abstract: We demonstrate novel methods for the study of multiple exciton generation from a single photon absorption event (carrier multiplication) in semiconductor nanocrystals (or nanocrystal quantum dots) that are complementary to our previously reported transient absorption method. By monitoring the time dependence of photoluminescence (PL) from CdSe nanocrystals via time correlated single photon counting, we find that carrier multiplication is observable due to the Auger decay of biexcitons. We compare these data with similar studies using transient absorption and find that the two methods give comparable results. In addition to the observation of dynamical signatures of carrier multiplication due to the Auger decay, we observe spectral signatures of multiple excitons produced from the absorption of a single photon. PL spectra at short times following excitation with high energy photons are red shifted compared to the single exciton emission band, which is consistent with previous observations of significant exciton exciton interactions in nanocrystals. We then show using a combination of transient absorption and time resolved PL studies that charge transfer between a nanocrystal and a Ru based catalyst model compound takes place on a time scale that is faster than Auger recombination time constants, which points toward a possible design of donor acceptor assemblies that can be utilized to take advantage of the carrier multiplication process.
author_names: Richard D Schaller

Milan Sykora

Sohee Jeong

Victor I Klimov
title: High efficiency carrier multiplication and ultrafast charge separation in semiconductor nanocrystals studied via time resolved photoluminescence.
venue: The journal of physical chemistry. B
year: 2006

abstract: Abstract Slow carrier relaxation and extent of charge separation in semiconductor nanocrystals (NCs) are the desired parameters, which control the power conversion efficiency in photovoltaic. Although several synthesis approaches have explored, the spectroscopic analysis of the fundamental understanding of charge carrier dynamics in the quantum confined states of cadmium zinc selenide (CdxZn1 xSe) graded alloy NCs remain relatively unexplored. Here, we study the charge carrier dynamics of a series of CdxZn1 xSe alloy NCs by changing the metal (Cd and Zn; M) to selenide ratio adopting one step synthesis. Our findings underline that both excitonic absorption and band edge photoluminescence (PL) of all the alloys are red shifted concerning the binary analogs (CdSe and ZnSe) which implies the formation of gradient alloy structure where the core is CdSe rich that leads to increasing light absorption cross section in the solar spectrum. Femtosecond transient absorption (fs TA) measurements of the alloy NCs reveal that the CdxZn1 xSe alloy show three distinct bleaches due to the different electronic transition like 1S (1Se 1S3/2) 2S (1Se 2S3/2) and 1P (1Pe 1P3/2) while only CdSe shows two bleaches due to 1S, and 1P. The distinctive band structure (quasi type II) of the alloy leads to slow down the intraband carrier cooling time and prolonging the charge separation due to specially decoupled photo excited electron and hole. This finding offers new insights to understand the photophysical behavior and suggest that CdxZn1 xSe graded NCs can be used as efficient light harvesting materials in quantum dot solar cell.
author_names: Partha Maity

Nandan Ghorai

Jayanta Dana

Hirendra N Ghosh
title: Impact of one step alloying on the carrier relaxation and charge separation dynamics of CdxZn1 xSe graded nanocrystals
year: 2020

abstract: Type II quantum dots (QDs) are capable of light driven charge separation between their core and the shell structures; however, their light absorption is limited in the longer wavelength range. Biological light harvesting complex II (LHCII) efficiently absorbs in the blue and red spectral domains. Therefore, hybrid complexes of these two structures may be promising candidates for photovoltaic applications. Previous measurements had shown that LHCII bound to QD can transfer its excitation energy to the latter, as indicated by the fluorescence emissions of LHCII and QD being quenched and sensitized, respectively. In the presence of methyl viologen (MV) both fluorescence emissions are quenched, indicating an additional electron transfer process from QDs to MV. Transient absorption spectroscopy confirmed this notion and showed that electron transfer from QDs to MV is much faster than fluorescence energy transfer between LHCII and QD. The action spectrum of MV reduction by LHCII QD complexes reflected the LHCII absorption spectrum, showing that light absorbed by LHCII and transferred to QDs increased the efficiency of MV reduction by QDs. Under continuous illumination, at least 28 turnovers were observed for the MV reduction. Presumably, the holes in QD cores were filled by a reducing agent in the reaction solution or by the dihydrolipoic acid coating of the QDs. The LHCII QD construct can be viewed as a simple model of a photosystem with the QD component acting as reaction center.
author_names: Mara Werwie

Lars Dworak

Anne Bottin

Lisa R Mayer

Thomas Basche

Josef Wachtveitl

Harald Paulsen
title: Light harvesting chlorophyll protein (LHCII) drives electron transfer in semiconductor nanocrystals.
venue: Biochimica et biophysica acta. Bioenergetics
year: 2018

abstract: Abstract Donor:acceptor systems are the core of many concepts for photovoltaic devices, because they provide a mean to separate photo generated electron hole pairs into extractable charge carriers. In organic photovoltaics, donor:acceptor systems consist of two different organic semiconductors. Alternatively, inorganic semiconductor nanocrystals are also suitable materials. Their use in combination with an organic material gives rise to hybrid systems. Charge separation being a key step in the energy conversion process, suitable methods are required to study this physical process. The present review article provides an overview on the use of light induced electron spin resonance as a powerful method to investigate charge separation in donor:acceptor blends. Thereby, organic and hybrid material systems are compared. Furthermore, detailed investigations of recombination processes are considered as well.
author_names: Christopher Krause

Holger Dr Borchert
title: Light induced electron spin resonance as a probe for charge separation and recombination in hybrid donor:acceptor systems
year: 2016

abstract: We synthesize colloidal CdSe@CdS octapod nanocrystals decorated with Pt domains, resulting in a metal semiconductor heterostructure. We devise a protocol to control the growth of Pt on the CdS surface, realizing both a selective tipping and a non selective coverage. Ultrafast optical spectroscopy, particularly femtosecond transient absorption, is employed to correlate the dynamics of optical excitations with the nanocrystal morphology. We find two regimes for capture of photoexcited electrons by Pt domains: a slow capture after energy relaxation in the semiconductor, occurring in tipped nanocrystals and resulting in large spatial separation of charges, and an ultrafast capture of hot electrons occurring in nanocrystals covered in Pt, where charge separation happens faster than energy relaxation and Auger recombination. Besides the relevance for fundamental materials science and control at the nanoscale, our nanocrystals may be employed in solar photocatalysis.
author_names: Erika Conca

Mauro Aresti

Michele Saba

Maria Francesca Casula

Francesco Quochi

Guido Mula

Danilo Loche

Mee Rahn Kim

Liberato Manna

Anna Corrias

A Mura

Giovanni Bongiovanni
title: Charge separation in Pt decorated CdSe@CdS octapod nanocrystals.
venue: Nanoscale
year: 2014

abstract: Surface functionalization and shape modifications are the key strategies being utilized to overcome the limitations of semiconductors in advanced oxidation processes (AOP) Herein, the uniform a Fe2O3 nanocrystals (a Fe2O3 NCs) were effectively synthesized via a simple solvothermal route. Meanwhile, the sulfonic acid functionalization (SAF) and the impregnation of a Fe2O3 NCs on g C3N4 (a Fe2O3 NCs@CN SAF) were achieved through complete solvent evaporation technique. The surface functionalization of the sulfonic acid group on g C3N4 accelerates the faster migration of electrons to the surface owing to robust electronegativity. The incorporation of a Fe2O3 NCs with CN SAF significantly enhances the optoelectronic properties, ultrafast spatial charge separation, and rapid charge transportation. The a Fe2O3 HPs@CN SAF and a Fe2O3 NPs@CN SAF nanocomposites attained 97.41% and 93.64% of Cr (VI) photoreduction in 10 min, respectively. The photocatalytic efficiency of a Fe2O3 NCs@CN SAF nanocomposite is 2.4 and 2.1 times higher than that of pure g C3N4 and a Fe2O3, respectively. Besides, the XPS, PEC and recycling experiments confirm the excellent photo induced charge separation via Z scheme heterostructure and cyclic stability of a Fe2O3 NCs@CN SAF nanocomposites.
author_names: Sridharan Balu

Yi-Lun Chen

Rei-Cheng Juang

Thomas Chung-Kuang Yang

Joon Ching Juan
title: Morphology Controlled Synthesis of a Fe2O3 Nanocrystals Impregnated on g C3N4 SO3H with Ultrafast Charge Separation for Photoreduction of Cr (VI) Under Visible Light.
venue: Environmental pollution
year: 2020

abstract: Semiconductor catalyst heterostructures have shown promising performances for light driven H2 generation, although further development of these materials is hindered by the lack of cost effective and efficient catalysts. In this paper, we adopt a colloidal method to prepare few layer WSe2 nanosheets without exfoliation and apply them as catalysts for forming heterostructures with a wide range of semiconductor absorbers (CdS nanorods, CdSe/CdS dot in rods, TiO2 nanoparticles, g C3N4 nanosheets) These WSe2 semiconductor heterostructures show enhanced solar to hydrogen conversion efficiencies compared with semiconductors without WSe2. The detailed mechanism of this enhancement has been investigated using WSe2 nanosheets decorated CdSe/CdS dot in rods as a model system, which display ~5.5 fold higher hydrogen generation apparent quantum efficiency compared with free CdSe/CdS dot in rods. Transient absorption spectroscopic studies reveal efficient charge separation in WSe2 decorated CdSe/CdS dot in rods, suggesting its key role in enhancing the H2 generation efficiency of WSe2 semiconductor heterostructures. This work demonstrates the great potentials of WSe2 nanosheets as catalysts for light driven hydrogen production and demonstrate the important effect of forming WSe2 semiconductor heterostructures in facilitating charge separation and photocatalysis.
author_names: Xu Guo

Qiuyang Li

Yawei Liu

Tao Jin

Yubin Chen

Liejin Guo

Tianquan Lian
title: Enhanced Light Driven Charge Separation and H2 Generation Efficiency in WSe2 Nanosheet Semiconductor Nanocrystal Heterostructures"
venue: ACS applied materials interfaces
year: 2020

author_names: Lihua Wang

Mujin Cai

Wei Sun

Le He

Xiaohong Zhang
title: Promoting Charge Separation in Semiconductor Nanocrystal Superstructures for Enhanced Photocatalytic Activity
year: 2018

abstract: Semiconductor colloidal metal chalcogenides (II VI) in the form of quantum dots (QDs) and different heterostructures (core/shell, alloys, etc. are of extensive interest in scientific research for both a fundamental understanding and technological applications because of their quantized size and different optical properties; however, due to their small size, the exciton (bound electron and hole) experiences a strong Coulombic attraction, which has a remarkable impact on the charge separation and photophysical properties of QDs. Thus, to achieve an efficient charge separation, numerous attempts have been made via the formation of different heterostructures, QD/molecular adsorbate (either organic or inorganic) assemblies, etc. These hybrid materials ameliorated the absorption of the incident light as well as charge separation. This article reviews the strategies for extending charge separation in these colloidal nanocrystals (NCs) which is one of the crucial steps to elevate the solar to electrical energy conversion efficiency in a quantum dot sensitized solar cell (QDSC) The article summarizes the benefits of co sensitization and experimental shreds of evidence for the multiple charge transfer processes involved in a QDSC. Studies have shown that in the co sensitization process, prolonged charge separation occurs via the dual behavior of the molecular adsorbate, sensitization (electron injection) and capture of holes from photoexcited QDs. This perspective emphases band edge engineering and control of charge carrier dynamics in various core/shell structures. The impact of colloidal alloy NCs on charge separation and interesting photophysical properties was recapitulated via the steady state and time resolved photoluminescence (PL) and femtosecond transient absorption spectroscopic techniques. Finally, the prolonged lifetime and extent of charge separation for these hybrid NCs (or the composites) assisted in the development of a better light harvester as compared to the case of their pure counterparts.
author_names: Partha Maity

Hirendra N Ghosh
title: Strategies for extending charge separation in colloidal nanostructured quantum dot materials.
venue: Physical chemistry chemical physics PCCP
year: 2019