发表文章毕业论文201606-yin shaofeng-RSC Advances-Optical response of a quantum dotepoxy resin composite_ effect of tensile strain

Optical response of a quantum dotepoxy resin composite: eff ect of tensile strain Shaofeng Yin,aZiming Zhao,aWeiling Luan*aand Fuqian Yang*b The structural applications of quantum dots (QDs) can be artifi cially realized through the preparation of QDs-based structural materials, which have unique characteristics of photoluminescence (PL) in response to mechanical deformation, via the dispersion of quantum dots in various materials, including polymers andplastics. AQDs-basedcompositeconsisting ofCdSeZnS coreshellQDsand a bisphenol-A type epoxy resin has been prepared by mixing a solution of CdSeZnS coreshell QDs in chloroform with bisphenol-A type epoxy resin and modifi ed amine at room temperature. The tensile deformation of the tensile specimens made from the QDs-based composite causes a signifi cant change in the PL intensity for large engineering strains, and there is no observable strain-induced shift of the wavelength corresponding to the maximum PL intensity. Two major modes are observed for the temporal variation of the PL intensity of the QDs-based composite coated on AA 7009 tensile specimens under cyclic loading and unloading of small strains. One exhibits in-phase characteristics with strain, and the other exhibits antiphase characteristics with strain. The variation of the PL intensity of the QDs-based composite with tensile strain suggests that there exists strain-dependent photoluminescence, which determines the PL characteristics of the CdSeZnS coreshell QDs in the bisphenol-A type epoxy under the irradiation of ultraviolet light. The experimental results demonstrate the potential of developing sensitive opto-mechanical devices from QDs-based composites. 1.Introduction Quantum dots (QDs), which are light-emitting nanoparticles under the irradiation of ultraviolet (UV) light, have potential in the applications of bio-imaging, bio-diagnostics, solar cells and light emitting diodes due to unique optical and electronic properties.16Of importance for the characteristics of the pho- toluminescence (PL) of QDs is the exciton state, including exciton energies, polarization, and phase, which is determined by the composition and shape of the QDs, the composition of the surrounding material, and the strain state of the QDs. It is of paramount importance to understand the eff ect of strain on the PL characteristics of QDs for applications in photonics,sensors,quantuminformationandstrain- engineering of QDs. Gell et al.7 studied the eff ect of a surface- acoustic-wave (SAW) on the emission of a single InAs QD and observed a characteristic broadening of the time-averaged emission spectra due to the SAW-induced oscillation of the energy levels of the QD. Nakaoka et al.8used micromachined air-bridge to demonstrate the strain eff ect on the quantum states of single self-assembled InGaAs QDs and observed the variation of the PL peak energy and linewidth with the applied voltage for the bending of the air-bridge. Embedding an InAs QD in a GaAs nanobridge, Bryant et al.9showed the dependence of the optical response of the InAs QD on mechanical strain due to the strain-induced shi of the electron and hole levels. They suggested that an applied shearing strain breaks lateral symmetry of the QD. Controlling the thickness of a cap layer, Persson et al.10 studied the eff ect of strain on the photo- luminescence of single InP/GaInP QDs of diff erent sizes and observed the dependence of the strain-induced energy shis on the size and aspect ratio of the QDs. Ding et al.11used piezoelectric-induced biaxial stress to regulate the exciton energy state in self-assembled InGaAs/GaAs QDs and observed the increase of the emission blue shis and the binding energies of positive trion (X+) and biexciton (XX) relative to neutral exciton (X) with increasing compression. Fu et al.12found that the peak wavelength for the luminescence of InAs QDs capped with an In0.4Ga0.6As layer is red-shied to 1.33 mm due to the decrease of the eff ective energy barrier induced by strain. Note that the eff ects ofstrainandstraingradientontheredshiandbroadeningofthe near-edge emission of ZnO nanowires in cathodoluminescence spectra were observed via the bending of ZnO nanowires.1316Such behavior is size-dependent and can be attributed to the electro- mechanical interaction in ZnO nanowires.1316 Realizing the potential application of QDs in opto-mechanical devices, Choi et al.17dispersed CdSe/CdS core/shell QDs, aKey Laboratory of Pressure Systems and Safety (MOE), School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China. E-mail: luan bMaterials Program, Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40513, USA. E-mail: Cite this: RSC Adv., 2016, 6, 18126 Received 4th December 2015 Accepted 1st February 2016 DOI: 10.1039/c5ra25894d /advances 18126| RSC Adv., 2016, 6, 1812618133This journal is The Royal Society of Chemistry 2016 RSC Advances PAPER Published on 02 February 2016. Downloaded by East China University of Science (a) an as-prepared tensile bar and (b) an elongated tensile bar (engineering strain 0.3) (the QDs are circled in red circles. The amount of CdSeZnS coreshell QDs in 1 mL of chloroform is 25 mg). 18128| RSC Adv., 2016, 6, 1812618133This journal is The Royal Society of Chemistry 2016 RSC AdvancesPaper Published on 02 February 2016. Downloaded by East China University of Science the relative PL intensity is 70% for the engineering tensile strain of ?16%. Note that slight increase in the relative PL intensity also was observed for some QDs-based tensile bars shortly aer the start of the tensile test. In general, the overall trend shows that the relative PL intensity decreases with increasing tensile strain for the tensile deformation of the tensile bars made from the CdSeZnS coreshell QDs-based composite. As shown in Fig. 5 and 6, the numbers of QDs per unit area decrease with increasing tensile strain, and the tensile strain can cause the separation of QDs from the aggregates of QDs, leading to the increase of the numbers of QDs per unit area. The change of the numbers of QDs per unit area likely contributes partially to the change of the relative PL intensity during the tensile tests. In general, the PL intensity of a given amount of QDs will likely be related to their quantum yield, i.e. QDs with lower quantum yields will produce lower PL intensity. Biju et al.26 investigated the PL effi ciencies and lifetimes of CdSe QDs under photoactivation in various chemical environments including polymer solutions and solvent systems, and concluded that the static passivation of the surface defects of QDs by polymer chains was responsible for the increase of the PL intensity. Carrillo-Carrion et al.25suggested that the photoactivation process depends on several factors, including atmospheric conditions (oxygen, humidity), intensity of light, presence of water, polarity of the solvent, and there mainly exist four prin- cipal pathways for the photoactivation phenomenon. One might expect that the tensile deformation introduces the change of the factors associated with the photoactivation process of the CdSeZnS coreshell QDs, resulting in the decrease of the PL intensity. Chen et al.27used the Matthews-Blakeslee theory as a rst- order approximation to analyze the strain eff ects on the optical response of core/shell hetero-nanostructures. They considered the formation of strain-induced mist dislocations and obtained a critical thickness of 1 nm. It is known that the ideal strength of a crystal derived from sinusoidal shear resis- tance, si, is28 si mb 2ph (1) in which m is the shear modulus of the crystal, b is the magni- tude of Burgers vector, and h is the inter-planar spacing of the crystal. To calculate the critical stress for the nucleation of a defect (dislocation) in a core/shell hetero-nanostructure, one can approximate h 1 nm and b z 0.1 nm in eqn (1) and obtain the critical stress, si, as siz m 20p (2) Fig. 6TEM images of the distribution of CdSeZnS coreshell QDs in tensile bars; (a) an as-prepared tensile bar without the presence of aggregates, (b) an elongated tensile bar after subjected to a tensile force of 900 N, (c) as-prepared tensile bar with the presence of aggregates, and (d) an elongated tensile bar with the presence of aggregates after subjected to a tensile force of 900 N (the red rect- angular boxes show the locations where the tensile deformation induced the separation of aggregates QDs. The amount of CdSeZnS coreshell QDs in 1 mL of chloroform is 75 mg). Fig. 7Variation of the relative PL intensity of a QDs-based tensile bar with tensile strain (the amountof CdSeZnScoreshell QDs in 1 mL of chloroform is 25 mg) (I0is the PL intensity without any deformation). This journal is The Royal Society of Chemistry 2016RSC Adv., 2016, 6, 1812618133 |18129 PaperRSC Advances Published on 02 February 2016. Downloaded by East China University of Science the other exhibits antiphase change of the PL intensity, i.e. the PL intensity decreases with increasing engineering strain and increases with decreasing engineering strain (Fig. 11b). The fraction for each mode is ?45% out of more than 20 tests. The diff erence of the PL intensity between the maximum PL intensity and the minimum PL intensity for each cycle is less than 16% for the experimental conditions. Both modes demonstrate a certain degree of “memory” eff ect, i.e. the PL characteristics approxi- mately remain the same aer unloading (3 0). According to Fig. 11, the maximum relative PL intensity at the maximum load during the cyclic deformation is always larger than 1 for the in-phase mode and mostly less than 1 for the antiphase mode. Such behavior suggests that there exists interaction between the CdSeZnS coreshell QDs and the bisphenol-A type epoxy resin, which is strain-dependent. It needs to point out that the underlying mechanism for such behavior is unclear. It might be associated the processes of the tension-induced separation of the aggregates of QDs and the tension-induced decrease of the concentration of QDs during the cyclic loading of small strain. 4.Summary In summary, the composite consisting of CdSeZnS core shell QDs and bisphenol-A type epoxy resin was prepared by mixing the solution of CdSeZnS coreshell QDs in chloro- form with the solution of bisphenol-A type epoxy resin/ modied amine at room temperature. Tensile tests of the tensile specimens made from the QDs-based composite were performed under the irradiation of UV light. No shi of the wavelength corresponding to the maximum PL intensity was observed, in contrast to the reports in literature, which suggests that the mechanical strain used is not large enough. On the contrary, the tensile deformation led to signicant decrease of the PL intensity of the QDs-based composite for large engineering strains, which cannot simply be explained by the decrease of the concentration of QDs induced by tensile strain. The variation of the PL intensity with tensile strain suggests that the stretch and re-orientation of polymer coils due to plastic deformation can alter the optical behavior of the QDs-based composite and enhance the migration of quenchers, such as oxygen, into the material, which might lead to the change of the PL intensity of the tensile specimens under the irradiation of UV light. The PL characteristics of the CdSeZnS coreshell QDs in the bisphenol-A type epoxy under cyclic loading and unloading were also determined by performing the cyclic loading- unloading tests of AA 7009 tensile bars coated with the QDs- based composite. There are two major modes for the temporal variation of the PL intensity for the cyclic deformation of small strain. One exhibits in-phase characteristic with engineering strain, and the other exhibits antiphase characteristic with engineering strain. Both modes demonstrate a certain degree of “memory” eff ect. The maximum relative PL intensity at the maximum load during the cyclic deformation is always larger than 1 for the in-phase mode and mostly less than 1 for the antiphase mode. The experimental results demonstrate the strain-dependence of the PL characteristics of the CdSeZnS coreshell QDs embedded in the bisphenol-A type epoxy. The PL character- istics of the QDs-based composites can be optimized through the design and optimization of the interaction between QDs and epoxy, which likely will make it possible to prepare opto-mechanical systems sensitive to strains of a variety of magnitudes. Acknowledgements WLisgratefulforthenancial supportfromtheNationalNatural Science Fund of China (51172072, 51475166) and the National Basic Research Program of China (2013CB035505). FY is grateful for the nancial support from the KSEF (KSEF-148-502-15-341). Fig. 11Temporal variation of the PL intensity of the CdSeZnS core shell QDs-based composite coated on the surface of AA 7009 bars for fi ve loading-unloading cycles (the amount of CdSeZnS coreshell QDs in 1 mL of chloroform is 25 mg); (a) in-phase mode, and (b) antiphase mode. 18132| RSC Ad