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Green Chemical Synthesis of II-VI Semiconductor Quantum Dots
KTH, School of Information and Communication Technology (ICT), Material Physics, Functional Materials, FNM.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nanotechnology is the science and technology of manipulating materials at atomic and molecular scale with properties different from bulk. Semiconductor QDs are important class of nanomaterials with unique physical and chemical properties owing to the quantum confinement effect. Size dependent optical properties make research on semiconductor QDs more attractive in the field of nanotechnology. Semiconductor QDs are usually composed of combination of elements from groups II–VI, III–V, or IV–VI of the periodic table. Group II-VI semiconductor QDs (ZnS, ZnSe, ZnO, CdSe, CdS) are most extensively studied systems, having bandgap which can be engineered through the variation of the material composition and size. Most common QDs are made of CdE (E=S, Se, Te) which are toxic. Recent environmental regulations restrict the use of toxic metals and therefore QDs containing nontoxic metals such as Zn are of great importance.

The chemical synthesis of QDs involves different methods. Usually high temperature thermal decomposition of organometallic compounds in high boiling point organic solvents is used which needs long reaction time and involves complex synthesis procedures. New simpler and efficient synthetic routes with alternative solvents are required. Recently the synthesis of non-toxic QDs using green chemical routes is a promising approach receiving increasing attention.

The aim of this Thesis is to develop novel routes for synthesis of semiconductor QDs employing green nanomaterial synthesis techniques. Therefore, in this work, we developed different green chemical routes mainly for the synthesis Zn-based QDs. Low temperature synthesis routes were developed for the synthesis of ZnS and ZnO QDs. Microwave irradiation was also used as efficient heating source which creates numerous nucleation sites in the solution, leading to the formation of homogeneous nanoparticles with small size and narrow size distribution. Different polar solvents with high MW absorption were used for synthesis of ZnS QDs. We also introduced ionic liquids as solvents in the synthesis of ZnS QDs using microwave heating. ILs are excellent reaction media for absorbing microwaves and are recognized as ‘green’ alternative to volatile and toxic organic solvents.

For ZnS systems, the QDs produced by different methods were less than 5 nm in size as characterized by high-resolution transmission electron microscopy (HR-TEM). Selected area electron diffraction (SAED) patterns revealed that ZnS QDs synthesized by low temperature synthesis technique using conventional heating are of cubic crystalline phase while the QDs synthesized by using MW heating are of wurtzite phase. The optical properties were investigated by UV-Vis absorption spectrum and show a blue shift in absorption as compared to bulk due to quantum confinement effect. The photoluminescence (PL) spectra of ZnS QDs show different defect states related emission peaks and depend on different synthesis methods, high bandedge related emission is observed for ZnS QDs synthesized by using ionic liquids. ZnO QDs synthesized by low temperature route were found to be less than 4 nm in size and also show a blue shift in their absorption. The PL spectrum show bandedge related emission which is blue shifted compared with bulk with no emission originating from surface defect levels. The results show that QDs are of high crystalline quality with narrow size distribution. A comparative study of using conventional and MW heating in the synthesis of CdSe QDs was performed. The reactions involving microwave heating showed enhanced rates and higher yields.

The developed methods involve all principles for green nanomaterials synthesis i.e. design of safer nanomaterials, reduced environmental impact, waste reduction, process safety, materials and energy efficiency.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , vii, 39 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2012:20
Keyword [en]
Semiconductor, Quantum Dots, Microwave, Ionic Liquids, Green Chemical Syntheis, Quantum Confinement, Optical Properties
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:kth:diva-104980ISBN: 978-91-7501-550-7 (print)OAI: oai:DiVA.org:kth-104980DiVA: diva2:567902
Public defence
2012-11-29, Sal C-1 Electrum, KTH-ICT, Isafjordsgatan 26, Kista, 10:00 (English)
Opponent
Supervisors
Note

QC 20121115

Available from: 2012-11-15 Created: 2012-11-14 Last updated: 2012-11-15Bibliographically approved
List of papers
1. Low temperature synthesis of cubic phase zinc sulfide quantum dots
Open this publication in new window or tab >>Low temperature synthesis of cubic phase zinc sulfide quantum dots
2012 (English)In: Central European Journal of Chemistry, ISSN 1895-1066, E-ISSN 1644-3624, Vol. 10, no 1, 54-58 p.Article in journal (Refereed) Published
Abstract [en]

In this study, we report on a new method for the synthesis of ZnS quantum dots (QDs). The synthesis was carried out at low temperature by a chemical reaction between zinc ions and freshly reduced sulfide ions in ethanol as reaction medium. Zinc chloride and elemental sulfur were used as zinc and sulfur sources, respectively and hydrazine hydrate was used as a strong reducing agent to convert elemental sulfur (S(8)) into highly reactive sulfide ions (S(2-)) which react spontaneously with zinc ions. This facile, less toxic, inexpensive route has a high yield for the synthesis of high quality metal sulfide QDs. Transmission electron microscopy (TEM) image analysis and selected area electron diffraction (SAED) reveal that ZnS QDs are less than 3 nm in diameter and are of cubic crystalline phase. The UV-Vis absorption spectrum shows an absorption peak at 253 nm corresponding to a band gap of 4.9 eV, which is high when compared to the bulk value of 3.68 eV revealing strong quantum confinement. PL emission transitions are observed at 314 nm and 439 nm and related to point defects in ZnS QDs.

Keyword
ZnS, Quantum Dots, Hydrazine Hydrate, Photoluminescence, Microwave
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-53386 (URN)10.2478/s11532-011-0108-5 (DOI)000297362800005 ()2-s2.0-83555174703 (Scopus ID)
Note
QC 20111229Available from: 2011-12-29 Created: 2011-12-28 Last updated: 2017-12-08Bibliographically approved
2. Novel lowtemperature route for large scale synthesis of ZnO QDs
Open this publication in new window or tab >>Novel lowtemperature route for large scale synthesis of ZnO QDs
(English)Article in journal (Other academic) Submitted
Identifiers
urn:nbn:se:kth:diva-104990 (URN)
Note

QS 2012

Available from: 2012-11-15 Created: 2012-11-15 Last updated: 2012-11-15Bibliographically approved
3. Microwave-assisted low temperature synthesis of wurtzite ZnS quantum dots
Open this publication in new window or tab >>Microwave-assisted low temperature synthesis of wurtzite ZnS quantum dots
2012 (English)In: Journal of Solid State Chemistry, ISSN 0022-4596, E-ISSN 1095-726X, Vol. 187, 130-133 p.Article in journal (Refereed) Published
Abstract [en]

In this work we report, for the first time, on microwave assisted synthesis of wurtzite ZnS quantum dots (QDs) in controlled reaction at temperature as low as 150 degrees C. The synthesis can be done in different microwave absorbing solvents with multisource or single source precursors. The QDs are less than 3 nm in size as characterized by transmission electron microscopy (TEM) using selected area electron diffraction (SAED) patterns to confirm the wurtzite phase of ZnS QDs. The optical properties were investigated by UV-Vis absorption which shows blue shift in absorption compared to bulk wurtzite ZnS due to quantum confinement effects. The photoluminescence (PL) spectra of QDs reveal point defects related emission of ZnS QDs.

Keyword
Nanostructure, Semiconductors, Quantum dots, Microwave, Chemical synthesis, Optical properties
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-92339 (URN)10.1016/j.jssc.2012.01.007 (DOI)000301011800020 ()2-s2.0-84857237346 (Scopus ID)
Note
QC 20120418Available from: 2012-04-18 Created: 2012-04-02 Last updated: 2017-12-07Bibliographically approved
4. Microwave mediated synthesis of semiconductor quantum dots
Open this publication in new window or tab >>Microwave mediated synthesis of semiconductor quantum dots
Show others...
2012 (English)In: Physica Status Solidi. C, Current topics in solid state physics, ISSN 1610-1634, E-ISSN 1610-1642, Vol. 9, no 7, 1551-1556 p.Article in journal (Refereed) Published
Abstract [en]

Colloidal quantum dots (QD) have tuneable optoelectronic properties and can be easily handled by simple solution processing techniques, making them very attractive for a wide range of applications. Over the past decade synthesis of morphology controlled high quality (crystalline, monodisperse) colloidal QDs by thermal decomposition of organometallic precursors has matured and is well studied. Recently, synthesis of colloidal QDs by microwave irradiation as heating source is being studied due to the inherently different mechanisms of heat transfer, when compared to solvent convection based heating. Under microwave irradiation, polar precursor molecules directly absorb the microwave energy and heat up more efficiently. Here we report synthesis of colloidal II-VI semiconductor QDs (CdS, CdSe, CdTe) by microwave irradiation and compare it with conventional synthesis based on convection heating. Our findings show that QD synthesis by microwave heating is more efficient and the chalcogenide precursor strongly absorbs the microwave radiation shortening the reaction time and giving a high reaction yield.

Keyword
CdSe, Microwave, Quantum dots, Reaction rate
National Category
Nano Technology
Identifiers
urn:nbn:se:kth:diva-95406 (URN)10.1002/pssc.201100545 (DOI)000306479300007 ()2-s2.0-84864000771 (Scopus ID)
Conference
16th International Semiconducting and Insulating Materials Conference (SIMC-XVI) Location: Royal Inst Technol (KTH), Stockholm, Sweden Date: JUN 19-23, 2011
Note
QC 20120810Available from: 2012-05-24 Created: 2012-05-24 Last updated: 2017-12-07Bibliographically approved
5. Microwave assisted synthesis of ZnS quantum dots using ionic liquids
Open this publication in new window or tab >>Microwave assisted synthesis of ZnS quantum dots using ionic liquids
Show others...
2012 (English)In: Materials letters (General ed.), ISSN 0167-577X, E-ISSN 1873-4979, Vol. 89, 316-319 p.Article in journal (Refereed) Published
Abstract [en]

In this work we report results from microwave (MW) assisted synthesis of highly crystalline ZnS quantum dots (QDs) using ionic liquid (ILs) as MW absorbing medium. Two types of ionic liquids, imidazolium and phosphonium based, were used. The QDs are less than 5 nm in size and of wurtzite ZnS type, as characterized by high-resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) pattern. The optical properties were investigated by UV-vis absorption and show a blue shift in absorption as compared to bulk wurtzite ZnS due to quantum confinement effects. The photoluminescence (PL) spectra of the QDs show different trap state emissions.

Keyword
Nanocrystalline materials, Semiconductors, Chemical synthesis, Electron microscopy, Crystal structure, Optical properties
National Category
Other Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-104989 (URN)10.1016/j.matlet.2012.08.143 (DOI)000311327200089 ()2-s2.0-84867537888 (Scopus ID)
Funder
Swedish Research CouncilStandUp
Note

QC 20121115

Available from: 2012-11-15 Created: 2012-11-15 Last updated: 2017-12-07Bibliographically approved

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