{"id":378,"date":"2018-12-07T11:45:57","date_gmt":"2018-12-07T11:45:57","guid":{"rendered":"http:\/\/msp08.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=378"},"modified":"2018-12-07T11:57:42","modified_gmt":"2018-12-07T11:57:42","slug":"core-shell-semiconductor-nanocrystals-cssncs","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/chapter\/core-shell-semiconductor-nanocrystals-cssncs\/","title":{"rendered":"Core-Shell Semiconductor Nanocrystals (CSSNCs)"},"content":{"raw":"
\r\n
\"epgp<\/a>\r\n<\/span><\/div>\r\n \r\n\r\n \r\n\r\n \r\n\r\nIntroduction<\/strong>\r\n\r\n \r\n

Core-shell semiconducting nanocrystals are the materials whose properties are intermediary to those of individual atoms\/molecules and bulk, semiconductor crystals. They have novel properties which dependent upon their size. A core-shell nanocrystal comprises a semiconducting QD core and a shell of different semiconductor. Core and shell usually comprise: type-II-VI, IV-IV, and III-V semiconductors. Typical core\/shell examples include CdS\/ZnS, CdSe\/ZnS, etc. Organically passivated QDs exhibit low quantum fluorescence yield because of surface related trap states. The shell protects the nanocrystal against environmental variations, degradation caused by photo-oxidation, and is another means to control the properties of the crystal. The emission wavelength can be varied over a broad range by precisely controlling the size, shape, and composition of both the core and shell. This is more effective way to control emission properties than individually controlling the core or shell. The material has extensive applications in biological systems as well as optics.<\/p>\r\n \r\n\r\n\"\"\r\n\r\n \r\n

Figure 1 TEM image of NaYF<\/strong>4<\/strong>:Yb,Tm nanoparticles coated with ZnO (top left), and the associated chemical maps confirming their chemical composition.<\/strong><\/p>\r\n \r\n\r\n1. Background<\/strong>\r\n\r\n \r\n

Colloidal semiconductor nanocrystals or QDs, are ~1-10 nm in diameter and have organic ligands attached to their surfaces. Owing to the size dependent electrical and optical properties, these nanocrystals have extensive applications in nanoscaled photonics, photovoltaics, LEDs, etc. QDs are replacing organic dyes as fluorescent markers in biological imaging as well as biosensing because of their tiny size, adjustable emissions, and photostability.<\/p>\r\n \r\n

Luminescence in QDs results from exciton decay (electron hole recombination), which might be radiative or nonradiative. In radiative decay, electrons relax from conduction band to valence band via emitting photons. The wavelength of the emitted photons is equal to the bandgap of the semiconductor. In nonradiative recombination, the electrons relax by releasing their energy by emitting phonon or Auger recombination. At nanoscale, quantum effects result in size dependent increase in the bandgap, and\u00a0quantisation of energy levels. This <\/span>quantisation of energy levels in QDs renders their electronic structure intermediary to individual atoms\/molecules (having single HOMO-LUMO gap) and bulk semiconductors (having continuous energy bands).<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\"\"\r\n\r\n \r\n

Figure 2 Electronic structure of Bulk, QD, and single molecule.<\/strong><\/p>\r\n \r\n

The crystal structure of semiconductor nanocrystals is usually identical to their bulk counterparts, however, the lattice periodicity abruptly stops at the surface of the nanocrystals. Thus, surface atoms have lower coordination numbers than the atoms present in the interior of the nanocrystal. Due to this incomplete bonding (in comparison to the interior atoms), atomic orbitals point away from the surface, resulting in \u2018dangling orbitals\u2019, also known as unpassivated orbitals. The dangling orbitals present at the surface of the nanocrystal are localised and are slightly charged (positive or negative). A hypothetical band structure is assumed to be formed owing to the weak interactions among inhomogeneously charged energy states present at the surface. Electrons and holes can be trapped at the surface, when the energy of dangling orbital is within the bandgap of the semiconductor, e.g., in CdSe QDs, Cd dangling orbitals trap electrons whereas Se dangling orbitals trap holes. Moreover, charge carriers can also be trapped by any surface defects in the nanocrystal.<\/p>\r\n \r\n

Due to the trapping of charge carriers of the surface of QDs, the probability of nonradiative decay increases, this results in a corresponding decrease in fluorescence quantum yield. To passivate these surface traps, organic ligands are used to coordinate with the surface atoms, e.g.,trioctylphosphine (TOP) and tri-n-octylphosphine oxide (TOPO) are generally used to passivate the surface traps and for growth condition control in high quality CdSe QDs. Even though, this technique produces high quality QDs with uniform size, and good crystallinity, their quantum yield is ~5-15%.<\/p>\r\n \r\n\r\n2. Classification of Core-Shell nanocrystals<\/strong>\r\n\r\n \r\n

The properties of core-shell nanocrystals arise from relative alignment of conduction and valence band edges of core and shell materials. In the case of type-I semiconductor heterostructures, both the charge carriers (electron and hole) remain localised within the core, while for type-II, one carrier is localised in shell and the other in core.<\/p>\r\n\r\n<\/div>\r\n\"\"\r\n

\r\n

Figure 3 Three types of core-shell nanocrystals. Upper and lower edges are the upper and lower energy edges of core (blue) and shell (red).<\/strong><\/p>\r\n \r\n\r\nThe core-shell structures can be classified in the following types:\r\n\r\n \r\n\r\na. Type-I<\/strong>\r\n\r\n \r\n

In type-I, core has smaller bandgap than the shell, and the conduction and valence band edges of core lie within the bandgap of shell. This results in confinement of both electrons and holes within the core. This is shown in Figure 4<\/strong>, where an exciton at CdSe\/CdS interface occupies energy states in the CdSe core, corresponding to the lowest energy separation. Radiative recombination within the core emits the light of wavelength red-shifted relative to the uncoated CdSe. Typical examples include CdSe\/CdS, CdSe\/ZnS, InAs\/CdSe, etc.<\/p>\r\n \r\n\r\n\"\"\r\n\r\n \r\n

Figure 4 Energy level diagram of type-I core-shell heterostructure.<\/strong><\/p>\r\n \r\n\r\nb. Reverse Type-I<\/strong>\r\n\r\n \r\n

In this case, the bandgap of core is larger than that of shell, and the edges of conduction and valence bands of shell lie within the bandgap of core. Lowest available exciton energy separation occurs when charge carries are localised in shell. The wavelength of the emitted light can be tuned by varying the thickness of the shell. Typical examples include CdS\/HgS, ZnSe\/CdSe, etc.<\/p>\r\n \r\n\r\nc. Type-II<\/strong>\r\n\r\n \r\n

In this structure, both conduction and valence band edges of <\/span>core are both either higher or lower than those of the shell. <\/span>Figure 5<\/strong> shows a typical example of this case, where the band edges of the core (ZnTe) are higher than the shell (CdSe) band edges. Lowest energy separation of exciton occurs when <\/span>hole is confined in the valence band of <\/span>core (ZnTe), and <\/span>electron is confined in <\/span>conduction band of <\/span>shell (CdSe). The wavelength of the emitted light is determined by the energy difference between these occupied states (indicated by <\/span>red arrow in <\/span>Figure 5<\/strong>). Thus, the wavelength of emitted light is lower than either of the individual bandgaps, resulting in considerable red-shift in the emission wavelength in compatriot to the unpassivated core. Typical examples include ZnTe\/CdSe, CdS\/ZnSe, etc.<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\"\"\r\n\r\n \r\n

Figure 5 Type-II core-shell ZnTe (2.26 eV)\/CdSe (1.74 eV) structure.<\/strong><\/p>\r\n \r\n\r\n \r\n\r\nd. Doped core-shell Semiconductor Nanocrystals<\/strong>\r\n\r\n \r\n

Doping can be used to effectively alter the optical properties of QDs. The impurity content in QDs produced via colloidal synthesis is lower than their bulk counterparts. The most popular applications of QDs which involve doping include magnetic memory and spin based electronics. Doping has been found to strongly influence the magnetic properties of core-shell nanocrystals. For example, Doping the shell with Mn in CdSe\/ZnS made the nanocrystal paramagnetic.<\/p>\r\n \r\n

3. Synthesis<\/strong><\/p>\r\n \r\n

Core-shell nanostructures have been prepared via various wet chemical methods including chemical precipitation, sol-gel, micro-emulsion, and inverse micelle formation. Chalcogenide core-shell nanoparticles have been prepared from these techniques with <\/span>emphasising better control over size, shape, and size distribution. To control the optical properties of these nanoparticles, various support matrices have been used, e.g., glass, zeolite, polymer, fatty acids, etc.<\/span><\/p>\r\n \r\n

Additionally, nanoparticles of sulfide, selenide, and telluride have been prepared via Langmuir-Blodgett film method. <\/span>Electrochemical technique is preferred over wet chemical methods, due to the possibility of using aqueous solvents instead of toxic organic solvents, <\/span>formation of conformal deposits, deposition at room temperature, inexpensive, precise control over composition and thickness of the semiconductor coating on metal nanoparticles. Nonetheless, electrochemical synthesis of core-shell structures is rather difficult because of the challenges involved in producing electrically addressable arrays of nanoparticles. There have been recent reports about the electrochemical synthesis of core-shell CdS and CuI\u00a0<\/span>nanoparticles on <\/span>a three-dimensional nanoelectrodes via layer-by-layer deposition of alternate layers of nanoparticles and polyoxometalate (POM).<\/span><\/p>\r\n \r\n

Colloidal synthesis of core-shell semiconductor nanocrystals can be performed with proper control of the reaction kinetics. This method provides great control over size and shape of the semiconductor nanoparticles and various types of nanostructures have been grown with this technique, such as dots, tubes, wires, etc. which demonstrate exciting electrical and optical size dependent properties. Owing to the synergistic effects of close contact between core and shell, core-shell nanoparticles offer novel properties and improved functions, not observed in individual particles.<\/span><\/p>\r\n \r\n

It is possible to control the size of core and thickness of shell during <\/span>synthesis process, e.g., <\/span>volume of H2S gas influences the size of CdSe core nanoparticles during synthesis, such that the size of the core decreases with an increase in the volume of H2S. Also, the size can be reduced by rapid cooling of the products. The thickness of shell depends upon the quantity of shell material added during <\/span>coating process.<\/span><\/p>\r\n \r\n

4. Characterization<\/strong><\/p>\r\n \r\n

The wavelength of the emitted light increase with an increase in either the size of <\/span>core, or thickness of <\/span>shell. To passivate the relaxation pathways and increase radiative states, the interface between the core and shell can be modified. Since the nanoparticles exhibit size dependent bandgap arising from quantum confinement, the photoluminescence color from the nanoparticles, can be tuned from blue to red by altering the size of the nanoparticles. Thus, the luminescence <\/span>colour as well as purity can be tuned by adjusting the size and shape of the nanoparticles. Nonetheless, the surface traps limit the quantum yield and luminescence brightness of the core-shell nanoparticles.<\/span><\/p>\r\n \r\n

Core-shell semiconductor nanocrystals can be <\/span>characterised by UV-Vis absorption spectra, X-ray diffraction, transmission electron microscope, and X-ray photoelectron spectroscopy.<\/span><\/p>\r\n \r\n

5. Applications<\/strong><\/p>\r\n \r\n

The key property of core-shell nanostructures is that the quantum dot cores exhibit strong fluorescence, which is essential property in their optical and biomedical applications. Further, the shells can also be manipulated to control the bulk properties, e.g. solubility, reactivity, etc.<\/span><\/p>\r\n \r\n

Biomedical Applications<\/strong><\/p>\r\n \r\n

The desirable properties of core-shell nanoscrystals for their biomedical applications <\/span>are: high quantum yield, narrow fluorescence emission, broad absorption spectrum, photostability, moderate fluorescent lifetime (~<\/span>20s), and high brightness.<\/span><\/p>\r\n \r\n

High quantum yield implies <\/span>minimum energy required to induce fluorescence in the QD. Narrow fluorescence emission is important to image multiple <\/span>colours simultaneously with no <\/span>colour overlap between different type of core-shell QDs. Broad absorption spectrum allows multiple QDs to be simultaneously excited at <\/span>same wavelength. This allows simultaneous imaging of multiple QDs. Time-resolved imaging requires around 20 s fluorescent lifetime. Additionally, the core-shell QDs are compatible with organic fluorophores. These QDs are stable against photo-bleaching, however, their performance vis-a-vis fluorophores needs further investigations. Besides, two-photon fluorescence efficiency of core-shell QDs is around 100 to 1000 times than that of the organic dyes. For biological applications, the quantum dot core can be covered by a biocompatible shell (such as DNA) for specific targeting. In addition to this, an organic molecule can also be used as the shell, with which another\u00a0<\/span>biomolecule may be conjugated at a later stage. Typical examples of core-shell materials used are: CdSe\/ZnS, CdSe\/CdS, etc. The addition of shell protects the core from photobleaching, as compared to using core material alone. <\/span>Fluorescence color can be tuned by controlling the particle size. Nonetheless, further study on the effects of shell molecules, pH, temperature, etc. in the properties of the core-shell nanoparticles is required.<\/span><\/p>\r\n \r\n

In-vitro Labeling<\/strong><\/p>\r\n \r\n

Owing to the possibility of imaging multiple <\/span>colours, core-shell QDs are important for cell <\/span>labelling. An important challenge to use core-shell nanocrystals in cell <\/span>labelling is the difficulties involved in passing them through the cell membrane. Various techniques have been adopted to achieve this, such as endocytosis, direct microinjection, and electroporation. After entering the cell, these nanoparticles aggregate in the nuclei and remain there for prolonged time spans. Since core-shell nanoparticles stay in the cells even after the cell division, both mother and daughter cells can be easily imaged using them. This has been demonstrated in the case of Xenopus embryos.<\/span><\/p>\r\n\r\n<\/div>\r\n

\r\n\r\n\"\"\r\n\r\n \r\n

Core-shell nanocrystals can also be used for tacking. This is achieved by growing cells on a 2D matrix, which is pre-embedded with core-shell nanocrystals. As cells move, they uptake these nanoparticles, thereby leaving a trail which can be seen in the form of the absence of the nanocrystals. In this way, the mobility of the cells can be observed, which is important since the metastatic potential of breast tissue cells has been shown to increase with mobility. Furthermore, five different core-shell nanoparticles have been shown to simultaneously detect five different toxins.<\/p>\r\n \r\n

Environmentally friendly and less toxic core-shell structures have been prepared by using silicon QDs coated with different shells. Silicon is ten times safer than cadmium and present work is focused to make silicon more hydrophilic and biocompatible. Particularly, silicon QDs with poly(acrylic acid) and allylamine shells have been used in cell labelling. Other in vitro uses are flow cyclometry, pathogen detection, and genomic and proteomic detection.<\/p>\r\n \r\n

In vivo and Deep Tissue Imaging<\/strong><\/p>\r\n \r\n

Core-shell QDs exhibit emission in NIR (700 to 900 nm) region of the electromagnetic spectrum, thus their imaging is not affected by the autofluorescence from the tissues (400 to 600 nm), and scattering effects. This has been used to map sentinel lymph-nodes in cancer surgery in animals. In a study, 1 cm deep lymph nodes were imaged and the excised nodes with nanoparticles accumulation were found to have the\u00a0highest probability for containing metastatic cells. Core-shell QDs have been demonstrated to remain fluorescent within the cells in vivo for up to 4 months. To track and diagnose cancer cells, labeled squamous carminoma cell-line U14 cells were used and fluorescent images were observed after 6h. Core-shell QDs conjugated to doxorubicin have been also used to target, image, as well as detect prostate cancer cells which express the prostate-specific membrane antigen proteins. Conjugating nanoparticles (having polymer shells) to a cancer-specific antibody is the most frequently used tumor targeted imaging.<\/span><\/p>\r\n \r\n

An important concern for use of core-shell nanoparticles to image in vivo is regarding their excretion and toxicity. There have been reports showing DNA damage and toxicity towards liver cells. Besides, the use of shells <\/span>have shown to reduce these ill-effects. Presently, <\/span>core made from rare-earth elements as well as silicon are being studied for reducing toxicity. Other concerns are limited commercial availability, variations in surface chemistries, non-specific binding, and instrument limitations.<\/span><\/p>\r\n \r\n

Optics<\/strong><\/p>\r\n \r\n

The optical properties of the core-shell QDs are decided by their bandgap, which can be tuned by modifying their size, shape <\/span>and composition. Thus, by tuning these attributes of the core-shell QDs, they can be <\/span>optimised for use in optical devices like LEDs, photodetectors, lasers, photodiodes, photovoltaics, etc.<\/span><\/p>\r\n \r\n\r\n\r\n\r\n
you can view video on Core-Shell Semiconductor Nanocrystals (CSSNCs)<\/strong><\/td>\r\n\"\"<\/a><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n \r\n\r\nReferences<\/strong>\r\n
    \r\n \t
  1. Dou, Qing Qing; Rengaramchandran, Adith; Selvan, Subramanian Tamil; Paulmurugan, Ramasamy; Zhang, Yong (2015). <\/span>\"Core \u2013 shell upconversion nanoparticle \u2013 semiconductor heterostructures for photodynamic therapy\". Scientific Reports. 5: 8252.<\/span><\/li>\r\n \t
  2. Loukanov, Alexandre R.; Dushkin, Ceco D.; Papazova, Karolina I.; Kirov, Andrey V.; Abrashev, Miroslav V.; Adachi, Eiki (1 September 2004). \"Photoluminescence depending on the ZnS shell thickness of CdS\/ZnS core\u2013shell semiconductor nanoparticles\". Colloids and Surfaces A: Physicochemical and Engineering Aspects. 245 (1\u20133): 9\u201314.<\/li>\r\n \t
  3. Reiss, Peter; Proti\u00e8re, Myriam; Li, Liang (20 January 2009). \"Core\/Shell Semiconductor Nanocrystals\". Small. 5 (2): 154\u2013168.<\/li>\r\n \t
  4. Peng, Xiaogang; Schlamp, Michael C.; Kadavanich, Andreas V.; Alivisatos, A. P. (1 July 1997). \"Epitaxial Growth of Highly Luminescent CdSe\/CdS Core\/Shell Nanocrystals with Photostability and Electronic Accessibility\". Journal of the American Chemical Society. 119 (30): 7019\u20137029.<\/li>\r\n \t
  5. Bruchez Jr., M.; Moronne, M; Gin, P; Weiss, S; Alivisatos, AP (25 September 1998). \"Semiconductor Nanocrystals as Fluorescent Biological Labels\". Science. 281 (5385): 2013\u2013 2016.<\/li>\r\n \t
  6. Makhal, Abhinandan; Yan, Hongdan; Lemmens, Peter; Pal, Samir Kumar (14 January 2010). \"Light Harvesting Semiconductor Core\u2212Shell Nanocrystals: Ultrafast Charge Transport Dynamics of CdSe\u2212ZnS Quantum Dots\". The Journal of Physical Chemistry C. 114 (1): 627\u2013632.<\/li>\r\n \t
  7. Murphy, C.J. Coffer, J.L. Quantum Dots: A Primer. Appl. Spectrosc. 2002, 56, 16A-27A.<\/li>\r\n \t
  8. Smith, Andrew M.; Nie, Shuming (16 February 2010). \"Semiconductor Nanocrystals: Structure, Properties, and Band Gap Engineering\". Accounts of Chemical Research. 43 (2): 190\u2013200.<\/span><\/li>\r\n \t
  9. Pokrant, S.; Whaley, K.B. (1 May 1999). \"Tight-binding studies of surface effects on electronic structure of CdSe nanocrystals: the role of organic ligands, surface reconstruction, and inorganic capping shells\". European Physical Journal D. 6 (2): 255\u2013267.<\/span><\/li>\r\n \t
  10. Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. (1 November 1997). \"(CdSe)ZnS Core\u2212Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites\". The Journal of Physical Chemistry B. 101 (46): 9463\u20139475.<\/li>\r\n \t
  11. Talapin, Dmitri V.; Rogach, Andrey L.; Kornowski, Andreas; Haase, Markus; Weller, Horst (1 April 2001). \"Highly Luminescent Monodisperse CdSe and CdSe\/ZnS Nanocrystals Synthesized in a Hexadecylamine\u2212Trioctylphosphine Oxide\u2212Trioctylphospine Mixture\". Nano Letters. 1 (4): 207\u2013211.<\/li>\r\n \t
  12. Mekis, Ivo; Talapin, Dmitri V.; Kornowski, Andreas; Haase, Markus; Weller, Horst (1 July 2003). \"One-Pot Synthesis of Highly Luminescent CdSe\/CdS Core\u2212Shell Nanocrystals via Organometallic and \"Greener\" Chemical Approaches\". The Journal of Physical Chemistry B. 107 (30): 7454\u20137462.<\/li>\r\n \t
  13. Ivanov, Sergei A.; Nanda, Jagjit; Piryatinski, Andrei; Achermann, Marc; Balet, Laurent P.; Bezel, Ilia V.; Anikeeva, Polina O.; Tretiak, Sergei; Klimov, Victor I. (1 July 2004). \"Light Amplification Using Inverted Core\/Shell Nanocrystals: Towards Lasing in the Single-Exciton Regime\". The Journal of Physical Chemistry B. 108 (30): 10625\u201310630.<\/li>\r\n \t
  14. Xie, R.; Zhong, X.; Basch\u00e9, T. (18 November 2005). \"Synthesis, Characterization, and Spectroscopy of Type-II Core\/Shell Semiconductor Nanocrystals with ZnTe Cores\". Advanced Materials. 17 (22): 2741\u20132745.<\/li>\r\n \t
  15. Kim, Sungjee; Fisher, Brent; Eisler, Hans-J\u00fcrgen; Bawendi, Moungi (1 September 2003). \"Type-II Quantum Dots: CdTe\/CdSe(Core\/Shell) and CdSe\/ZnTe(Core\/Shell) Heterostructures\". Journal of the American Chemical Society. 125 (38): 11466\u201311467.<\/li>\r\n \t
  16. Norris, D. J.; Efros, A. L.; Erwin, S. C. (28 March 2008). \"Doped Nanocrystals\". Science. 319 (5871): 1776\u20131779.<\/li>\r\n \t
  17. Smith, A. M.; Lane, L. A.; Nie, S. (31 July 2014). \"Mapping the spatial distribution of charge carriers in quantum-confined heterostructures\". Nature Communications. 5: 4506.<\/span><\/li>\r\n \t
  18. Erwin, Steven C.; Zu, Lijun; Haftel, Michael I.; Efros, Alexander L.; Kennedy, Thomas A.; Norris, David J. (7 July 2005). \"Doping semiconductor nanocrystals\". Nature. 436(7047): 91\u201394.<\/li>\r\n \t
  19. Bussian, David A.; Crooker, Scott A.; Yin, Ming; Brynda, Marcin; Efros, Alexander L.; Klimov, Victor I. (14 December 2008). \"Tunable magnetic exchange interactions in manganese-doped inverted core\u2013shell ZnSe\u2013CdSe nanocrystals\". Nature Materials. 8 (1): 35\u201340.<\/span><\/li>\r\n \t
  20. Vlaskin, Vladimir A.; Beaulac, R\u00e9mi; Gamelin, Daniel R. (2009). \"Dopant-Carrier Magnetic Exchange Coupling in Inverted Core\/Shell Nanocrystals\". Nano Letters. 9 (12): 4376\u20134382.<\/li>\r\n \t
  21. Wang, Shizhong; Jarrett, Benjamin R.; Kauzlarich, Susan M.; Louie, Angelique Y. (1 April 2007). \"Core\/Shell Quantum Dots with High Relaxivity and Photoluminescence for Multimodality Imaging\". Journal of the American Chemical Society. 129 (13): 3848\u20133856.<\/span><\/li>\r\n \t
  22. Mandal, P; Srinivasa, R; Talwar, S; Major, S (15 June 2008). \"CdS\/ZnS core\u2013shell nanoparticles in arachidic acid LB films\". Applied Surface Science. 254 (16): 5028\u20135033.<\/li>\r\n \t
  23. Gu, Chaokang; Hui Xu; Minseo Park; Curtis Shannon (2009). \"Synthesis of Metal\u2212Semiconductor Core\u2212Shell Nanoparticles Using Electrochemical Surface-Limited Reactions\". Langmuir. 25 (1): 410\u2013414.<\/li>\r\n \t
  24. Trallero-Giner, C.; Comas, F.; Marques, G.; Tallman, R.; Weinstein, B. (1 November 2010). \"Optical phonons in spherical core\/shell semiconductor nanoparticles: Effect of hydrostatic pressure\". Physical Review B. 82 (20): 205426.<\/li>\r\n \t
  25. Zhou, Tiejun; Lu, Meihua; Zhang, Zhihua; Gong, Hao; Chin, Wee Shong; Liu, Bo (19 January 2010). \"Synthesis and Characterization of Multifunctional FePt\/ZnO Core\/Shell Nanoparticles\". Advanced Materials. 22 (3): 403\u2013406.<\/li>\r\n \t
  26. Ethayaraja, M.; Ravikumar, C.; Muthukumaran, D.; Dutta, K.; Bandyopadhyaya, R. (1 March 2007). \"CdS-ZnS Core-Shell Nanoparticle Formation: Experiment, Mechanism, and Simulation\". Journal of Physical Chemistry C. 111 (8): 3246\u20133252. doi:10.1021\/jp066066j.<\/li>\r\n \t
  27. Clark, Pip; Radtke, Hanna; Pengpad, Atip; Williamson, Andrew; Spencer, Ben; Hardman, Samantha; Neo, Darren; Fairclough, Simon; et al. (2017). \"The Passivating Effect of Cadmium in PbS \/ CdS Colloidal Quantum Dot Solar Cells Probed by nm-Scale Depth Profiling\". Nanoscale. 9: 6056\u20136067.<\/li>\r\n \t
  28. Xu, G.; et al. (2016). \"New Generation Cadmium-Free Quantum Dots for Biophotonics and Nanomedicine\". Chemical Reviews. 116: 12234\u201312327.<\/li>\r\n \t
  29. ^ Jump up to:a b Klostranec, J. M.; Chan, W. C. W. (4 August 2006). \"Quantum Dots in Biological and Biomedical Research: Recent Progress and Present Challenges\". Advanced Materials. 18 (15): 1953\u20131964.<\/li>\r\n \t
  30. Medintz, Igor L.; Uyeda, H. Tetsuo; Goldman, Ellen R.; Mattoussi, Hedi (1 June 2005). \"Quantum dot bioconjugates for imaging, labelling and sensing\". Nature Materials. 4 (6): 435\u2013446.<\/li>\r\n \t
  31. Jin, Shan; Hu, Yanxi; Gu, Zhanjun; Liu, Lei; Wu, Hai-Chen (1 January 2011). \"Application of Quantum Dots in Biological Imaging\". Journal of Nanomaterials. 2011: 1\u201313.<\/li>\r\n \t
  32. Pellegrino, Teresa; Kudera, Stefan; Liedl, Tim; Mu\u00f1oz Javier, Almudena; Manna, Liberato; Parak, Wolfgang J. (2004). \"On the Development of Colloidal Nanoparticles towards Multifunctional Structures and their Possible Use for Biological Applications\". Small. 1 (1): 48\u2013 63.<\/li>\r\n \t
  33. Coe, Seth; Woo, Wing-Keung; Bawendi, Moungi; Bulovi\u0107, Vladimir (19 December 2002). \"Electroluminescence from single monolayers of nanocrystals in molecular organic devices\". Nature. 420 (6917): 800\u2013803.<\/li>\r\n \t
  34. Steckel, Jonathan S.; Snee, Preston; Coe-Sullivan, Seth; Zimmer, John P.; Halpert, Jonathan E.; Anikeeva, Polina; Kim, Lee-Ann; Bulovic, Vladimir; Bawendi, Moungi G. (4 September 2006). \"Color-Saturated Green-Emitting QD-LEDs\". Angewandte Chemie International Edition. 45 (35): 5796\u20135799. PMID 16874835.<\/li>\r\n \t
  35. Sun, Qingjiang; Wang, Y. Andrew; Li, Lin Song; Wang, Daoyuan; Zhu, Ting; Xu, Jian; Yang, Chunhe; Li, Yongfang (18 November 2007). \"Bright, multicoloured light-emitting diodes based on quantum dots\". Nature Photonics. 1 (12): 717\u2013722.<\/li>\r\n \t
  36. Klimov, Victor I.; Ivanov, Sergei A.; Nanda, Jagjit; Achermann, Marc; Bezel, Ilya; McGuire, John A.; Piryatinski, Andrei (24 May 2007). \"Single-exciton optical gain in semiconductor nanocrystals\". Nature. 447 (7143): 441\u2013446.<\/li>\r\n \t
  37. Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. (28 July 2000). \"Full Color Emission from II-VI Semiconductor Quantum Dot-Polymer Composites\". Advanced Materials. 12 (15): 1102\u20131105.<\/li>\r\n \t
  38. Manthina V, Correa Baena JP, Liu G, Agrios AG (2012). \"ZnO\u2013TiO2 Nanocomposite Films for High Light Harvesting Efficiency and Fast Electron Transport in Dye-Sensitized Solar Cells\". The Journal of Physical Chemistry C. 116 (45): 23864.<\/li>\r\n \t
  39. Plank NO, Snaith HJ, Ducati C, Bendall JS, Schmidt-Mende L, Welland ME (2008). \"A simple low temperature synthesis route for ZnO\u2013MgO core\u2013shell nanowires\". Nanotechnology. 19 (46): 465603.<\/li>\r\n<\/ol>\r\n<\/div>","rendered":"
    \n
    \"epgp<\/a>
    \n<\/span><\/div>\n

     <\/p>\n

     <\/p>\n

     <\/p>\n

    Introduction<\/strong><\/p>\n

     <\/p>\n

    Core-shell semiconducting nanocrystals are the materials whose properties are intermediary to those of individual atoms\/molecules and bulk, semiconductor crystals. They have novel properties which dependent upon their size. A core-shell nanocrystal comprises a semiconducting QD core and a shell of different semiconductor. Core and shell usually comprise: type-II-VI, IV-IV, and III-V semiconductors. Typical core\/shell examples include CdS\/ZnS, CdSe\/ZnS, etc. Organically passivated QDs exhibit low quantum fluorescence yield because of surface related trap states. The shell protects the nanocrystal against environmental variations, degradation caused by photo-oxidation, and is another means to control the properties of the crystal. The emission wavelength can be varied over a broad range by precisely controlling the size, shape, and composition of both the core and shell. This is more effective way to control emission properties than individually controlling the core or shell. The material has extensive applications in biological systems as well as optics.<\/p>\n

     <\/p>\n

    \"\"<\/p>\n

     <\/p>\n

    Figure 1 TEM image of NaYF<\/strong>4<\/strong>:Yb,Tm nanoparticles coated with ZnO (top left), and the associated chemical maps confirming their chemical composition.<\/strong><\/p>\n

     <\/p>\n

    1. Background<\/strong><\/p>\n

     <\/p>\n

    Colloidal semiconductor nanocrystals or QDs, are ~1-10 nm in diameter and have organic ligands attached to their surfaces. Owing to the size dependent electrical and optical properties, these nanocrystals have extensive applications in nanoscaled photonics, photovoltaics, LEDs, etc. QDs are replacing organic dyes as fluorescent markers in biological imaging as well as biosensing because of their tiny size, adjustable emissions, and photostability.<\/p>\n

     <\/p>\n

    Luminescence in QDs results from exciton decay (electron hole recombination), which might be radiative or nonradiative. In radiative decay, electrons relax from conduction band to valence band via emitting photons. The wavelength of the emitted photons is equal to the bandgap of the semiconductor. In nonradiative recombination, the electrons relax by releasing their energy by emitting phonon or Auger recombination. At nanoscale, quantum effects result in size dependent increase in the bandgap, and\u00a0quantisation of energy levels. This <\/span>quantisation of energy levels in QDs renders their electronic structure intermediary to individual atoms\/molecules (having single HOMO-LUMO gap) and bulk semiconductors (having continuous energy bands).<\/span><\/p>\n<\/div>\n

    \n

    \"\"<\/p>\n

     <\/p>\n

    Figure 2 Electronic structure of Bulk, QD, and single molecule.<\/strong><\/p>\n

     <\/p>\n

    The crystal structure of semiconductor nanocrystals is usually identical to their bulk counterparts, however, the lattice periodicity abruptly stops at the surface of the nanocrystals. Thus, surface atoms have lower coordination numbers than the atoms present in the interior of the nanocrystal. Due to this incomplete bonding (in comparison to the interior atoms), atomic orbitals point away from the surface, resulting in \u2018dangling orbitals\u2019, also known as unpassivated orbitals. The dangling orbitals present at the surface of the nanocrystal are localised and are slightly charged (positive or negative). A hypothetical band structure is assumed to be formed owing to the weak interactions among inhomogeneously charged energy states present at the surface. Electrons and holes can be trapped at the surface, when the energy of dangling orbital is within the bandgap of the semiconductor, e.g., in CdSe QDs, Cd dangling orbitals trap electrons whereas Se dangling orbitals trap holes. Moreover, charge carriers can also be trapped by any surface defects in the nanocrystal.<\/p>\n

     <\/p>\n

    Due to the trapping of charge carriers of the surface of QDs, the probability of nonradiative decay increases, this results in a corresponding decrease in fluorescence quantum yield. To passivate these surface traps, organic ligands are used to coordinate with the surface atoms, e.g.,trioctylphosphine (TOP) and tri-n-octylphosphine oxide (TOPO) are generally used to passivate the surface traps and for growth condition control in high quality CdSe QDs. Even though, this technique produces high quality QDs with uniform size, and good crystallinity, their quantum yield is ~5-15%.<\/p>\n

     <\/p>\n

    2. Classification of Core-Shell nanocrystals<\/strong><\/p>\n

     <\/p>\n

    The properties of core-shell nanocrystals arise from relative alignment of conduction and valence band edges of core and shell materials. In the case of type-I semiconductor heterostructures, both the charge carriers (electron and hole) remain localised within the core, while for type-II, one carrier is localised in shell and the other in core.<\/p>\n<\/div>\n

    \"\"<\/p>\n

    \n

    Figure 3 Three types of core-shell nanocrystals. Upper and lower edges are the upper and lower energy edges of core (blue) and shell (red).<\/strong><\/p>\n

     <\/p>\n

    The core-shell structures can be classified in the following types:<\/p>\n

     <\/p>\n

    a. Type-I<\/strong><\/p>\n

     <\/p>\n

    In type-I, core has smaller bandgap than the shell, and the conduction and valence band edges of core lie within the bandgap of shell. This results in confinement of both electrons and holes within the core. This is shown in Figure 4<\/strong>, where an exciton at CdSe\/CdS interface occupies energy states in the CdSe core, corresponding to the lowest energy separation. Radiative recombination within the core emits the light of wavelength red-shifted relative to the uncoated CdSe. Typical examples include CdSe\/CdS, CdSe\/ZnS, InAs\/CdSe, etc.<\/p>\n

     <\/p>\n

    \"\"<\/p>\n

     <\/p>\n

    Figure 4 Energy level diagram of type-I core-shell heterostructure.<\/strong><\/p>\n

     <\/p>\n

    b. Reverse Type-I<\/strong><\/p>\n

     <\/p>\n

    In this case, the bandgap of core is larger than that of shell, and the edges of conduction and valence bands of shell lie within the bandgap of core. Lowest available exciton energy separation occurs when charge carries are localised in shell. The wavelength of the emitted light can be tuned by varying the thickness of the shell. Typical examples include CdS\/HgS, ZnSe\/CdSe, etc.<\/p>\n

     <\/p>\n

    c. Type-II<\/strong><\/p>\n

     <\/p>\n

    In this structure, both conduction and valence band edges of <\/span>core are both either higher or lower than those of the shell. <\/span>Figure 5<\/strong> shows a typical example of this case, where the band edges of the core (ZnTe) are higher than the shell (CdSe) band edges. Lowest energy separation of exciton occurs when <\/span>hole is confined in the valence band of <\/span>core (ZnTe), and <\/span>electron is confined in <\/span>conduction band of <\/span>shell (CdSe). The wavelength of the emitted light is determined by the energy difference between these occupied states (indicated by <\/span>red arrow in <\/span>Figure 5<\/strong>). Thus, the wavelength of emitted light is lower than either of the individual bandgaps, resulting in considerable red-shift in the emission wavelength in compatriot to the unpassivated core. Typical examples include ZnTe\/CdSe, CdS\/ZnSe, etc.<\/span><\/p>\n<\/div>\n

    \n

    \"\"<\/p>\n

     <\/p>\n

    Figure 5 Type-II core-shell ZnTe (2.26 eV)\/CdSe (1.74 eV) structure.<\/strong><\/p>\n

     <\/p>\n

     <\/p>\n

    d. Doped core-shell Semiconductor Nanocrystals<\/strong><\/p>\n

     <\/p>\n

    Doping can be used to effectively alter the optical properties of QDs. The impurity content in QDs produced via colloidal synthesis is lower than their bulk counterparts. The most popular applications of QDs which involve doping include magnetic memory and spin based electronics. Doping has been found to strongly influence the magnetic properties of core-shell nanocrystals. For example, Doping the shell with Mn in CdSe\/ZnS made the nanocrystal paramagnetic.<\/p>\n

     <\/p>\n

    3. Synthesis<\/strong><\/p>\n

     <\/p>\n

    Core-shell nanostructures have been prepared via various wet chemical methods including chemical precipitation, sol-gel, micro-emulsion, and inverse micelle formation. Chalcogenide core-shell nanoparticles have been prepared from these techniques with <\/span>emphasising better control over size, shape, and size distribution. To control the optical properties of these nanoparticles, various support matrices have been used, e.g., glass, zeolite, polymer, fatty acids, etc.<\/span><\/p>\n

     <\/p>\n

    Additionally, nanoparticles of sulfide, selenide, and telluride have been prepared via Langmuir-Blodgett film method. <\/span>Electrochemical technique is preferred over wet chemical methods, due to the possibility of using aqueous solvents instead of toxic organic solvents, <\/span>formation of conformal deposits, deposition at room temperature, inexpensive, precise control over composition and thickness of the semiconductor coating on metal nanoparticles. Nonetheless, electrochemical synthesis of core-shell structures is rather difficult because of the challenges involved in producing electrically addressable arrays of nanoparticles. There have been recent reports about the electrochemical synthesis of core-shell CdS and CuI\u00a0<\/span>nanoparticles on <\/span>a three-dimensional nanoelectrodes via layer-by-layer deposition of alternate layers of nanoparticles and polyoxometalate (POM).<\/span><\/p>\n

     <\/p>\n

    Colloidal synthesis of core-shell semiconductor nanocrystals can be performed with proper control of the reaction kinetics. This method provides great control over size and shape of the semiconductor nanoparticles and various types of nanostructures have been grown with this technique, such as dots, tubes, wires, etc. which demonstrate exciting electrical and optical size dependent properties. Owing to the synergistic effects of close contact between core and shell, core-shell nanoparticles offer novel properties and improved functions, not observed in individual particles.<\/span><\/p>\n

     <\/p>\n

    It is possible to control the size of core and thickness of shell during <\/span>synthesis process, e.g., <\/span>volume of H2S gas influences the size of CdSe core nanoparticles during synthesis, such that the size of the core decreases with an increase in the volume of H2S. Also, the size can be reduced by rapid cooling of the products. The thickness of shell depends upon the quantity of shell material added during <\/span>coating process.<\/span><\/p>\n

     <\/p>\n

    4. Characterization<\/strong><\/p>\n

     <\/p>\n

    The wavelength of the emitted light increase with an increase in either the size of <\/span>core, or thickness of <\/span>shell. To passivate the relaxation pathways and increase radiative states, the interface between the core and shell can be modified. Since the nanoparticles exhibit size dependent bandgap arising from quantum confinement, the photoluminescence color from the nanoparticles, can be tuned from blue to red by altering the size of the nanoparticles. Thus, the luminescence <\/span>colour as well as purity can be tuned by adjusting the size and shape of the nanoparticles. Nonetheless, the surface traps limit the quantum yield and luminescence brightness of the core-shell nanoparticles.<\/span><\/p>\n

     <\/p>\n

    Core-shell semiconductor nanocrystals can be <\/span>characterised by UV-Vis absorption spectra, X-ray diffraction, transmission electron microscope, and X-ray photoelectron spectroscopy.<\/span><\/p>\n

     <\/p>\n

    5. Applications<\/strong><\/p>\n

     <\/p>\n

    The key property of core-shell nanostructures is that the quantum dot cores exhibit strong fluorescence, which is essential property in their optical and biomedical applications. Further, the shells can also be manipulated to control the bulk properties, e.g. solubility, reactivity, etc.<\/span><\/p>\n

     <\/p>\n

    Biomedical Applications<\/strong><\/p>\n

     <\/p>\n

    The desirable properties of core-shell nanoscrystals for their biomedical applications <\/span>are: high quantum yield, narrow fluorescence emission, broad absorption spectrum, photostability, moderate fluorescent lifetime (~<\/span>20s), and high brightness.<\/span><\/p>\n

     <\/p>\n

    High quantum yield implies <\/span>minimum energy required to induce fluorescence in the QD. Narrow fluorescence emission is important to image multiple <\/span>colours simultaneously with no <\/span>colour overlap between different type of core-shell QDs. Broad absorption spectrum allows multiple QDs to be simultaneously excited at <\/span>same wavelength. This allows simultaneous imaging of multiple QDs. Time-resolved imaging requires around 20 s fluorescent lifetime. Additionally, the core-shell QDs are compatible with organic fluorophores. These QDs are stable against photo-bleaching, however, their performance vis-a-vis fluorophores needs further investigations. Besides, two-photon fluorescence efficiency of core-shell QDs is around 100 to 1000 times than that of the organic dyes. For biological applications, the quantum dot core can be covered by a biocompatible shell (such as DNA) for specific targeting. In addition to this, an organic molecule can also be used as the shell, with which another\u00a0<\/span>biomolecule may be conjugated at a later stage. Typical examples of core-shell materials used are: CdSe\/ZnS, CdSe\/CdS, etc. The addition of shell protects the core from photobleaching, as compared to using core material alone. <\/span>Fluorescence color can be tuned by controlling the particle size. Nonetheless, further study on the effects of shell molecules, pH, temperature, etc. in the properties of the core-shell nanoparticles is required.<\/span><\/p>\n

     <\/p>\n

    In-vitro Labeling<\/strong><\/p>\n

     <\/p>\n

    Owing to the possibility of imaging multiple <\/span>colours, core-shell QDs are important for cell <\/span>labelling. An important challenge to use core-shell nanocrystals in cell <\/span>labelling is the difficulties involved in passing them through the cell membrane. Various techniques have been adopted to achieve this, such as endocytosis, direct microinjection, and electroporation. After entering the cell, these nanoparticles aggregate in the nuclei and remain there for prolonged time spans. Since core-shell nanoparticles stay in the cells even after the cell division, both mother and daughter cells can be easily imaged using them. This has been demonstrated in the case of Xenopus embryos.<\/span><\/p>\n<\/div>\n

    \n

    \"\"<\/p>\n

     <\/p>\n

    Core-shell nanocrystals can also be used for tacking. This is achieved by growing cells on a 2D matrix, which is pre-embedded with core-shell nanocrystals. As cells move, they uptake these nanoparticles, thereby leaving a trail which can be seen in the form of the absence of the nanocrystals. In this way, the mobility of the cells can be observed, which is important since the metastatic potential of breast tissue cells has been shown to increase with mobility. Furthermore, five different core-shell nanoparticles have been shown to simultaneously detect five different toxins.<\/p>\n

     <\/p>\n

    Environmentally friendly and less toxic core-shell structures have been prepared by using silicon QDs coated with different shells. Silicon is ten times safer than cadmium and present work is focused to make silicon more hydrophilic and biocompatible. Particularly, silicon QDs with poly(acrylic acid) and allylamine shells have been used in cell labelling. Other in vitro uses are flow cyclometry, pathogen detection, and genomic and proteomic detection.<\/p>\n

     <\/p>\n

    In vivo and Deep Tissue Imaging<\/strong><\/p>\n

     <\/p>\n

    Core-shell QDs exhibit emission in NIR (700 to 900 nm) region of the electromagnetic spectrum, thus their imaging is not affected by the autofluorescence from the tissues (400 to 600 nm), and scattering effects. This has been used to map sentinel lymph-nodes in cancer surgery in animals. In a study, 1 cm deep lymph nodes were imaged and the excised nodes with nanoparticles accumulation were found to have the\u00a0highest probability for containing metastatic cells. Core-shell QDs have been demonstrated to remain fluorescent within the cells in vivo for up to 4 months. To track and diagnose cancer cells, labeled squamous carminoma cell-line U14 cells were used and fluorescent images were observed after 6h. Core-shell QDs conjugated to doxorubicin have been also used to target, image, as well as detect prostate cancer cells which express the prostate-specific membrane antigen proteins. Conjugating nanoparticles (having polymer shells) to a cancer-specific antibody is the most frequently used tumor targeted imaging.<\/span><\/p>\n

     <\/p>\n

    An important concern for use of core-shell nanoparticles to image in vivo is regarding their excretion and toxicity. There have been reports showing DNA damage and toxicity towards liver cells. Besides, the use of shells <\/span>have shown to reduce these ill-effects. Presently, <\/span>core made from rare-earth elements as well as silicon are being studied for reducing toxicity. Other concerns are limited commercial availability, variations in surface chemistries, non-specific binding, and instrument limitations.<\/span><\/p>\n

     <\/p>\n

    Optics<\/strong><\/p>\n

     <\/p>\n

    The optical properties of the core-shell QDs are decided by their bandgap, which can be tuned by modifying their size, shape <\/span>and composition. Thus, by tuning these attributes of the core-shell QDs, they can be <\/span>optimised for use in optical devices like LEDs, photodetectors, lasers, photodiodes, photovoltaics, etc.<\/span><\/p>\n

     <\/p>\n\n\n\n
    you can view video on Core-Shell Semiconductor Nanocrystals (CSSNCs)<\/strong><\/td>\n\"\"<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

     <\/p>\n

    References<\/strong><\/p>\n

      \n
    1. Dou, Qing Qing; Rengaramchandran, Adith; Selvan, Subramanian Tamil; Paulmurugan, Ramasamy; Zhang, Yong (2015). <\/span>“Core \u2013 shell upconversion nanoparticle \u2013 semiconductor heterostructures for photodynamic therapy”. Scientific Reports. 5: 8252.<\/span><\/li>\n
    2. Loukanov, Alexandre R.; Dushkin, Ceco D.; Papazova, Karolina I.; Kirov, Andrey V.; Abrashev, Miroslav V.; Adachi, Eiki (1 September 2004). “Photoluminescence depending on the ZnS shell thickness of CdS\/ZnS core\u2013shell semiconductor nanoparticles”. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 245 (1\u20133): 9\u201314.<\/li>\n
    3. Reiss, Peter; Proti\u00e8re, Myriam; Li, Liang (20 January 2009). “Core\/Shell Semiconductor Nanocrystals”. Small. 5 (2): 154\u2013168.<\/li>\n
    4. Peng, Xiaogang; Schlamp, Michael C.; Kadavanich, Andreas V.; Alivisatos, A. P. (1 July 1997). “Epitaxial Growth of Highly Luminescent CdSe\/CdS Core\/Shell Nanocrystals with Photostability and Electronic Accessibility”. Journal of the American Chemical Society. 119 (30): 7019\u20137029.<\/li>\n
    5. Bruchez Jr., M.; Moronne, M; Gin, P; Weiss, S; Alivisatos, AP (25 September 1998). “Semiconductor Nanocrystals as Fluorescent Biological Labels”. Science. 281 (5385): 2013\u2013 2016.<\/li>\n
    6. Makhal, Abhinandan; Yan, Hongdan; Lemmens, Peter; Pal, Samir Kumar (14 January 2010). “Light Harvesting Semiconductor Core\u2212Shell Nanocrystals: Ultrafast Charge Transport Dynamics of CdSe\u2212ZnS Quantum Dots”. The Journal of Physical Chemistry C. 114 (1): 627\u2013632.<\/li>\n
    7. Murphy, C.J. Coffer, J.L. Quantum Dots: A Primer. Appl. Spectrosc. 2002, 56, 16A-27A.<\/li>\n
    8. Smith, Andrew M.; Nie, Shuming (16 February 2010). “Semiconductor Nanocrystals: Structure, Properties, and Band Gap Engineering”. Accounts of Chemical Research. 43 (2): 190\u2013200.<\/span><\/li>\n
    9. Pokrant, S.; Whaley, K.B. (1 May 1999). “Tight-binding studies of surface effects on electronic structure of CdSe nanocrystals: the role of organic ligands, surface reconstruction, and inorganic capping shells”. European Physical Journal D. 6 (2): 255\u2013267.<\/span><\/li>\n
    10. Dabbousi, B. O.; Rodriguez-Viejo, J.; Mikulec, F. V.; Heine, J. R.; Mattoussi, H.; Ober, R.; Jensen, K. F.; Bawendi, M. G. (1 November 1997). “(CdSe)ZnS Core\u2212Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites”. The Journal of Physical Chemistry B. 101 (46): 9463\u20139475.<\/li>\n
    11. Talapin, Dmitri V.; Rogach, Andrey L.; Kornowski, Andreas; Haase, Markus; Weller, Horst (1 April 2001). “Highly Luminescent Monodisperse CdSe and CdSe\/ZnS Nanocrystals Synthesized in a Hexadecylamine\u2212Trioctylphosphine Oxide\u2212Trioctylphospine Mixture”. Nano Letters. 1 (4): 207\u2013211.<\/li>\n
    12. Mekis, Ivo; Talapin, Dmitri V.; Kornowski, Andreas; Haase, Markus; Weller, Horst (1 July 2003). “One-Pot Synthesis of Highly Luminescent CdSe\/CdS Core\u2212Shell Nanocrystals via Organometallic and “Greener” Chemical Approaches”. The Journal of Physical Chemistry B. 107 (30): 7454\u20137462.<\/li>\n
    13. Ivanov, Sergei A.; Nanda, Jagjit; Piryatinski, Andrei; Achermann, Marc; Balet, Laurent P.; Bezel, Ilia V.; Anikeeva, Polina O.; Tretiak, Sergei; Klimov, Victor I. (1 July 2004). “Light Amplification Using Inverted Core\/Shell Nanocrystals: Towards Lasing in the Single-Exciton Regime”. The Journal of Physical Chemistry B. 108 (30): 10625\u201310630.<\/li>\n
    14. Xie, R.; Zhong, X.; Basch\u00e9, T. (18 November 2005). “Synthesis, Characterization, and Spectroscopy of Type-II Core\/Shell Semiconductor Nanocrystals with ZnTe Cores”. Advanced Materials. 17 (22): 2741\u20132745.<\/li>\n
    15. Kim, Sungjee; Fisher, Brent; Eisler, Hans-J\u00fcrgen; Bawendi, Moungi (1 September 2003). “Type-II Quantum Dots: CdTe\/CdSe(Core\/Shell) and CdSe\/ZnTe(Core\/Shell) Heterostructures”. Journal of the American Chemical Society. 125 (38): 11466\u201311467.<\/li>\n
    16. Norris, D. J.; Efros, A. L.; Erwin, S. C. (28 March 2008). “Doped Nanocrystals”. Science. 319 (5871): 1776\u20131779.<\/li>\n
    17. Smith, A. M.; Lane, L. A.; Nie, S. (31 July 2014). “Mapping the spatial distribution of charge carriers in quantum-confined heterostructures”. Nature Communications. 5: 4506.<\/span><\/li>\n
    18. Erwin, Steven C.; Zu, Lijun; Haftel, Michael I.; Efros, Alexander L.; Kennedy, Thomas A.; Norris, David J. (7 July 2005). “Doping semiconductor nanocrystals”. Nature. 436(7047): 91\u201394.<\/li>\n
    19. Bussian, David A.; Crooker, Scott A.; Yin, Ming; Brynda, Marcin; Efros, Alexander L.; Klimov, Victor I. (14 December 2008). “Tunable magnetic exchange interactions in manganese-doped inverted core\u2013shell ZnSe\u2013CdSe nanocrystals”. Nature Materials. 8 (1): 35\u201340.<\/span><\/li>\n
    20. Vlaskin, Vladimir A.; Beaulac, R\u00e9mi; Gamelin, Daniel R. (2009). “Dopant-Carrier Magnetic Exchange Coupling in Inverted Core\/Shell Nanocrystals”. Nano Letters. 9 (12): 4376\u20134382.<\/li>\n
    21. Wang, Shizhong; Jarrett, Benjamin R.; Kauzlarich, Susan M.; Louie, Angelique Y. (1 April 2007). “Core\/Shell Quantum Dots with High Relaxivity and Photoluminescence for Multimodality Imaging”. Journal of the American Chemical Society. 129 (13): 3848\u20133856.<\/span><\/li>\n
    22. Mandal, P; Srinivasa, R; Talwar, S; Major, S (15 June 2008). “CdS\/ZnS core\u2013shell nanoparticles in arachidic acid LB films”. Applied Surface Science. 254 (16): 5028\u20135033.<\/li>\n
    23. Gu, Chaokang; Hui Xu; Minseo Park; Curtis Shannon (2009). “Synthesis of Metal\u2212Semiconductor Core\u2212Shell Nanoparticles Using Electrochemical Surface-Limited Reactions”. Langmuir. 25 (1): 410\u2013414.<\/li>\n
    24. Trallero-Giner, C.; Comas, F.; Marques, G.; Tallman, R.; Weinstein, B. (1 November 2010). “Optical phonons in spherical core\/shell semiconductor nanoparticles: Effect of hydrostatic pressure”. Physical Review B. 82 (20): 205426.<\/li>\n
    25. Zhou, Tiejun; Lu, Meihua; Zhang, Zhihua; Gong, Hao; Chin, Wee Shong; Liu, Bo (19 January 2010). “Synthesis and Characterization of Multifunctional FePt\/ZnO Core\/Shell Nanoparticles”. Advanced Materials. 22 (3): 403\u2013406.<\/li>\n
    26. Ethayaraja, M.; Ravikumar, C.; Muthukumaran, D.; Dutta, K.; Bandyopadhyaya, R. (1 March 2007). “CdS-ZnS Core-Shell Nanoparticle Formation: Experiment, Mechanism, and Simulation”. Journal of Physical Chemistry C. 111 (8): 3246\u20133252. doi:10.1021\/jp066066j.<\/li>\n
    27. Clark, Pip; Radtke, Hanna; Pengpad, Atip; Williamson, Andrew; Spencer, Ben; Hardman, Samantha; Neo, Darren; Fairclough, Simon; et al. (2017). “The Passivating Effect of Cadmium in PbS \/ CdS Colloidal Quantum Dot Solar Cells Probed by nm-Scale Depth Profiling”. Nanoscale. 9: 6056\u20136067.<\/li>\n
    28. Xu, G.; et al. (2016). “New Generation Cadmium-Free Quantum Dots for Biophotonics and Nanomedicine”. Chemical Reviews. 116: 12234\u201312327.<\/li>\n
    29. ^ Jump up to:a b Klostranec, J. M.; Chan, W. C. W. (4 August 2006). “Quantum Dots in Biological and Biomedical Research: Recent Progress and Present Challenges”. Advanced Materials. 18 (15): 1953\u20131964.<\/li>\n
    30. Medintz, Igor L.; Uyeda, H. Tetsuo; Goldman, Ellen R.; Mattoussi, Hedi (1 June 2005). “Quantum dot bioconjugates for imaging, labelling and sensing”. Nature Materials. 4 (6): 435\u2013446.<\/li>\n
    31. Jin, Shan; Hu, Yanxi; Gu, Zhanjun; Liu, Lei; Wu, Hai-Chen (1 January 2011). “Application of Quantum Dots in Biological Imaging”. Journal of Nanomaterials. 2011: 1\u201313.<\/li>\n
    32. Pellegrino, Teresa; Kudera, Stefan; Liedl, Tim; Mu\u00f1oz Javier, Almudena; Manna, Liberato; Parak, Wolfgang J. (2004). “On the Development of Colloidal Nanoparticles towards Multifunctional Structures and their Possible Use for Biological Applications”. Small. 1 (1): 48\u2013 63.<\/li>\n
    33. Coe, Seth; Woo, Wing-Keung; Bawendi, Moungi; Bulovi\u0107, Vladimir (19 December 2002). “Electroluminescence from single monolayers of nanocrystals in molecular organic devices”. Nature. 420 (6917): 800\u2013803.<\/li>\n
    34. Steckel, Jonathan S.; Snee, Preston; Coe-Sullivan, Seth; Zimmer, John P.; Halpert, Jonathan E.; Anikeeva, Polina; Kim, Lee-Ann; Bulovic, Vladimir; Bawendi, Moungi G. (4 September 2006). “Color-Saturated Green-Emitting QD-LEDs”. Angewandte Chemie International Edition. 45 (35): 5796\u20135799. PMID 16874835.<\/li>\n
    35. Sun, Qingjiang; Wang, Y. Andrew; Li, Lin Song; Wang, Daoyuan; Zhu, Ting; Xu, Jian; Yang, Chunhe; Li, Yongfang (18 November 2007). “Bright, multicoloured light-emitting diodes based on quantum dots”. Nature Photonics. 1 (12): 717\u2013722.<\/li>\n
    36. Klimov, Victor I.; Ivanov, Sergei A.; Nanda, Jagjit; Achermann, Marc; Bezel, Ilya; McGuire, John A.; Piryatinski, Andrei (24 May 2007). “Single-exciton optical gain in semiconductor nanocrystals”. Nature. 447 (7143): 441\u2013446.<\/li>\n
    37. Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. (28 July 2000). “Full Color Emission from II-VI Semiconductor Quantum Dot-Polymer Composites”. Advanced Materials. 12 (15): 1102\u20131105.<\/li>\n
    38. Manthina V, Correa Baena JP, Liu G, Agrios AG (2012). “ZnO\u2013TiO2 Nanocomposite Films for High Light Harvesting Efficiency and Fast Electron Transport in Dye-Sensitized Solar Cells”. The Journal of Physical Chemistry C. 116 (45): 23864.<\/li>\n
    39. Plank NO, Snaith HJ, Ducati C, Bendall JS, Schmidt-Mende L, Welland ME (2008). “A simple low temperature synthesis route for ZnO\u2013MgO core\u2013shell nanowires”. Nanotechnology. 19 (46): 465603.<\/li>\n<\/ol>\n<\/div>\n","protected":false},"author":3,"menu_order":30,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":["dr-s-s-islam"],"pb_section_license":""},"chapter-type":[],"contributor":[58],"license":[],"class_list":["post-378","chapter","type-chapter","status-publish","hentry","contributor-dr-s-s-islam"],"part":3,"_links":{"self":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/pressbooks\/v2\/chapters\/378","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/wp\/v2\/users\/3"}],"version-history":[{"count":4,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/pressbooks\/v2\/chapters\/378\/revisions"}],"predecessor-version":[{"id":388,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/pressbooks\/v2\/chapters\/378\/revisions\/388"}],"part":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/pressbooks\/v2\/parts\/3"}],"metadata":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/pressbooks\/v2\/chapters\/378\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/wp\/v2\/media?parent=378"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/pressbooks\/v2\/chapter-type?post=378"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/wp\/v2\/contributor?post=378"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/wp-json\/wp\/v2\/license?post=378"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}