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Open Access Nano Express

Novel hybrid organic/inorganic 2D quasiperiodic PC: from diffraction pattern to vertical light extraction

Lucia Petti1*, Massimo Rippa1, Jun Zhou2, Liberato Manna3, Marco Zanella3 and Pasquale Mormile1

Author Affiliations

1 Institute of Cybernetics "E. Caianiello" of CNR, Via Campi Flegrei 34, 80072 Pozzuoli, Italy

2 Institute of Photonics, Faculty of Science, Ningbo University, Ningbo, Zhejiang 315211, China

3 Fondazione Istituto Italiano di Tecnologia, via Morego 30, 16163, Genova, Italy

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Nanoscale Research Letters 2011, 6:371  doi:10.1186/1556-276X-6-371


The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/6/1/371


Received:21 February 2011
Accepted:4 May 2011
Published:4 May 2011

© 2011 Petti et al; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Recently, important efforts have been dedicated to the realization of a fascinating class of new photonic materials or metamaterials, known as photonic quasicrystals (PQCs), in which the lack of the translational symmetry is compensated by rotational symmetries not achievable by the conventional periodic crystals. As ever, more advanced functionality is demanded and one strategy is the introduction of non-linear and/or active functionality in photonic materials. In this view, core/shell nanorods (NRs) are a promising active material for light-emitting applications. In this article a two-dimensional (2D) hybrid a 2D octagonal PQC which consists of air rods in an organic/inorganic nanocomposite is proposed and experimentally demonstrated. The nanocomposite was prepared by incorporating CdSe/CdS core/shell NRs into a polymer matrix. The PQC was realized by electron beam lithography (EBL) technique. Scanning electron microscopy, far field diffraction and spectra measurements are used to characterize the experimental structure. The vertical extraction of the light, by the coupling of the modes guided by the PQC slab to the free radiation via Bragg scattering, consists of a narrow red emissions band at 690 nm with a full width at half-maximum (FWHM) of 21.5 nm. The original characteristics of hybrid materials based on polymers and colloidal NRs, able to combine the unique optical properties of the inorganic moiety with the processability of the host matrix, are extremely appealing in view of their technological impact on the development of new high performing optical devices such as organic light-emitting diodes, ultra-low threshold lasers, and non-linear devices.

PACS: 81.07.Pr Organic-inorganic hybrid nanostructures, 81.16.-c Methods of nanofabrication and processing, 42.70.Qs Photonic band-gap materials.

Background

Quasiperiodic crystals are a new class of materials that exhibit long-range aperiodic translational order and high rotational symmetries [1,2]. Unlike periodically arranged photonic crystals (or photonic band-gaps), photonic quasicrystals (PQCs) possess unique light localization and transport properties related to their complex, multi-fractal energy spectra [3-14].

PQCs possess photonic band-gaps (PBGs) with very interesting properties of light transmission [15,16], wave guiding, and localization [17-19] exploited in an enormous variety of electro-optical and photonic applications. The existence of high rotational symmetries and many not equivalent defective states [20] not achievable by conventional periodic crystals opens the possibility to realize versatile, robust PBG devices even for low dielectric contrast materials like polymeric ones. Different from random structures, PQCs are defined by the iteration of simple mathematical rules, rooted in symbolic dynamics and prime number theory, which possess very rich spectral features [5]. The structural complexity of PQCs is measured by their spatial Fourier spectra, which are discrete (singular) for quasiperiodic systems, singular-continuous, or absolutely continuous for pseudorandom structures of increasing complexity. Two-dimensional (2D) quasiperiodic lattices possessing high rotational symmetries have been largely studied in literature [21-26]. The experimental realization of 2D PQCs is a hard fabrication challenge. Two-beam and multiple-beam holographic lithography have been largely employed to realize periodic [27] and quasiperiodic [28] crystals at the mesoscale. Moreover, novel PBG aperiodic structures defined by recursive substitutional sequences like one-dimensional (1D) and 2D Thue-Morse patterns cannot be realized even in principle by multiple-beam interference. Experimental realization of photonic structures exhibiting large area 2D Thue-Morse arrangement has been recently reported by the authors for the first time to our knowledge [29].

Advances in 2D photonic structures are expected in the introduction of non-linear and/or active functionality into a 2D PQC. 1D semiconductor nanostructures are likewise promising materials both in fundamental research and in practical applications [30-33]. As an example, it has been shown that various types of semiconductor nanorods (NRs) have unique optical properties that make them appealing for applications in solar cells, light-emitting diodes, lasers, and cell labeling [33-36]. CdSe/CdS rods present the appealing characteristics of strong and tunable light emission from green to red, are highly fluorescent and show linearly polarized emission [37].

These characteristics open the way to a new class of hybrid devices based on polymers and colloidal NRs in which the unique optical properties of the inorganic moiety are combined with the processability of the host matrix to develop new high performing optical devices such as organic light-emitting diodes, ultra-low threshold lasers, and non-linear devices. One of the challenges of these applications is the incorporation of inorganic nanoparticles into organic polymer matrices, since this is usually accompanied by phase separation, aggregation of nanoparticles, loss of transparency, and luminescence quenching due to exciton energy transfer [38,39].

In this paper, for the first time to our knowledge, a 2D hybrid eightfold symmetric aperiodically ordered PQC which consists of air rods in a nanocomposite prepared by incorporating CdSe/CdS core/shell nanorods (NRs) in a polymer is proposed and experimentally demonstrated. The semiconductor nanocrystal based 2D PQC was fabricated by directly patterning and releasing a thin film of a functional material composed by NRs dispersed in a positive electronic resist.

Scanning electron microscopy (SEM) and optical measurements are used to characterize the structures. The vertical extraction of the light, by the coupling of the modes guided by the PQCs slab to the free radiation via Bragg scattering, consists of a narrow red emissions band at 690 nm with a FWHM of 21.5 nm.

Methods

Synthesis and fabrication

The dot/rod core/shell CdSe/CdS nanocrystals were synthesized by a "seeded growth" approach [40] which yields highly uniform nanocrystals in terms of distribution of rod lengths and diameters that additionally emit wavelength tunable, linearly polarized light with high quantum efficiency. In particular, the synthetic procedure relied on the rapid co-injection of preformed spherical CdSe nanocrystal seeds and elemental sulphur dissolved in trioctylphosphine, in a reaction flask that contains a solution of cadmium oxide in a mixture of trioctylphosphine oxide, hexylphosphonic acid, and octadecylphosphonic acid at high temperature (350°C) [40]. These NRs are dispersed in a solution of polymethylmethacrylate (PMMA) and toluene. In Figure 1a,b we report a sketch showing the seeded growth approach for the preparation of dot/rod core/shell CdSe/CdS nanocrystals and a photograph of the solution under visible light and UV light.

thumbnailFigure 1. Light emission properties of the colloidal solution. (a) Sketch of the seeded growth approach for the preparation of CdSe/CdS nanorods and (b) pictures of nanorods in toluene under visible light and UV showing photoluminescence.

A morphological analysis of the stock solution of CdSe/CdS NRs dissolved in toluene was carried out by means of transmission electron microscopy (TEM) and is reported in Figure 2.

thumbnailFigure 2. TEM analysis of the colloidal solution. A transmission electron micrograph of the stock solution of CdSe/CdS nanorods dissolved in toluene, deposited on copper grid coated with a thin amorphous carbon film, after solvent evaporation. The average rod diameter and length, as determined by TEM were 4 and 20 nm (aspect ratio 5), respectively.

We firstly fabricated CdSe/CdS NRs-PMMA polymer composite film by combining the CdSe/CdS NRs with PMMA of high optical transparency in the visible region and spin-coating the composite solution. Since PMMA is transparent in the visible spectral range and it is an electron-sensitive material, it was chosen as the embedding matrix for NRs. Clear nanocomposite films without bubbles and smooth surfaces with sizes up to 1.5 × 2.5 × 0.0000600 cm3 on an indium tin oxide (ITO) coated glass were obtained.

Our experimental structure was fabricated by using a high-resolution electron beam lithography (EBL) technique. The EBL facility employed consisted of a Raith 150 system. Such system enables the writing of patterns of arbitrary geometries with a spatial resolution up to 10 nm. The samples were obtained by exposing a layer of our mixture of CdSe/CdS NRs doped PMMA deposited on an ITO coated glass. The e-beam is locally focused on the sample to expose selected regions of material homogenously along the depth of the substrate according to the calculated desired pattern. Exposed areas were dissolved away leaving a 2D eightfold octagonal quasiperiodic structure made of air-filled cylinders, located at the vertices of the octagonal lattice, lying in the polymer matrix.

The resulting 2D QC is made of air rods embedded into a hybrid organic/inorganic matrix of CdSe/CdS NRs doped PMMA. The size of the patterned area is about 650 μm2. The air rods arrays were fabricated using EBL on quartz substrates with a 15 nm layer ITO for conduction. A 600-nm-thick layer of colloidal doped PMMA was spin-coated on top of the cleaned substrate. Subsequently, the nanopatterning was defined using the Raith 150 system with current and area dosage of 27.8 pA at 20 keV. The resist was developed in a 1:3 solution of methyl isobutyl ketone (MIBK) and isopropanol (IPA).

Recently, the importance of tiling geometry in the arrangement of the octagonal quasicrystal was discussed. The fabricated octagonal structure was determined by simulating the quasiperiodic transverse irradiance distribution given by 8-beam interference process and was theoretically analyzed through finite difference time domain (FDTD) simulations of the transmittance spectra [41]. The octagonal tiling gives rise to a wide photonic band-gap for low refractive index difference as low as 0.5 with an attenuation of the transmittance signal up to 50 dB [42]. Therefore, optoelectronics and photonics devices based on such octagonal photonic quasicrystal promise to be realized even in soft materials like polymer. The arrangement of the cylinders is shown schematically in Figure 3.

thumbnailFigure 3. Pattern design. Schematic figure showing the octagonal quasiperiodic arrangement of cylinders.

A SEM image of the eightfold rotational symmetry PQC realized in the hybrid nanocomposite film with a hole diameter of 535 nm is reported in Figure 4.

thumbnailFigure 4. SEM characterization of the realized hybrid nanostructure. Top view of SEM image of the 2D PC pattern SEM image characterization of the hybrid eightfold rotational symmetry photonic quasicrystal realized in the colloidal doped PMMA with EBL with a hole diameter d = 535 nm.

Results and discussion

Diffraction pattern

The experimental octagonal quasicrystal realized has been characterized in the direct space through SEM metrological measurements, whereas in the reciprocal space interesting properties of the aperiodic octagonal array can be provided through the experimental determination of the Fourier spectra of the samples. Diffraction patterns are produced as a result of the regional periodicity. Figure 5 shows the visible diffraction pattern produced by a normally incident green laser beam for the quasi crystal fabricated. The light source was a Ar+ laser operating on the TEM00 mode at the wavelength λ = 514.5 nm and having a beam waist w0 = 2 mm. After having spatially filtered the laser beam through a lens-pinhole-lens system, the source is incident on the sample. Through the spatial filter, the incident light is diffracted creating an airy pattern: the central zero-order has an excellent spatial coherence and represents a good approximation of a plane wave. Higher diffraction orders have highly divergent wave vectors with respect to the optical axis. The second pinhole filters only the central spot of the airy pattern. An optical system consisting of two confocal lenses is placed between the two pinholes providing a magnification ratio of 3.47 that ensures a uniform illumination of the experimental sample having a square surface of ~700 μm2. A charge-coupled device (CCD) array coupled with a focusing lens system is finally used to acquire the Fourier spectra of the structure.

thumbnailFigure 5. Far field diffraction characterization of the realized hybrid nanostructure. Far field diffraction pattern of the 2D octagonal quasicrystal. The red circles indicate the most pronounced peaks on the experimental diffraction pattern.

The diffraction pattern possesses eightfold rotational symmetry, and contains a series of spots of different intensity, surrounding the central undiffracted beam. These spots can be associated with vectors in reciprocal space. The observed peaks are sharp and symmetrically distributed. Each order has rings of spots at different cone angles around the zero-order central spot. The picture clearly reveals the presence of quasiperiodicity within the sample.

Optical transmission measurements

The light propagating in the glass substrate which is extracted by diffraction was measured at room temperature using a multimode fiber, a CCD imaging telescope (OL610), and a CCD-based spectroradiometer (OL770-LED).

A white light was introduced from an edge of the glass and was propagating inside the glass. The QPC area was seen as a red area because of the wavelength-selective diffraction by the hybrid QPC. Figure 6 shows spectrally integrated light intensity characteristics of the sample as detected normally from the sample surface. The sample shows three peaks (616, 655, and 690 nm) in the spectrum. The resonant peaks appear due to the QPC feedback effect, and the full width at half-maximum (FWHM) of spectrum is of 21.5 nm at the main peak. These resonant peaks are considered to originate from different transverse guiding modes with different modal indices, which are diffracted to the normal direction by the QPC effect.

thumbnailFigure 6. Spectrally integrated light intensity characteristics of the realized hybrid nanostructure. Measured emission profile of the fabricated sample as detected normally from the sample surface using a multimode fiber, a CCD imaging telescope, and a CCD-based spectroradiometer. A white light was introduced from an edge of the glass and was propagating inside the glass. The emission profile was measured along the ΓX direction of the octagonal QC. The main peak at 690 nm corresponds to the apparent resonant mode of the PC with a narrow FWHM of 21.5 nm.

It has been studied that when the period of a two-dimensional photonic crystal is equal to the cavity wavelength of the guided mode, the guided waves propagating to several in-plane directions are coupled to the radiation mode in the direction normal to the device surface since the Bragg diffraction condition is satisfied. The vertical extraction of the light, by the coupling of the modes guided by the PC slab to the free radiation via Bragg scattering, is ruled by a phasematching condition, namely by the conservation of the in-plane component of the momentum at the air-dielectric interface [43].

Conclusions

In this work we patterned, for the first time to our knowledge, a nanocomposite containing colloidal semiconductor quantum rods of nanometer size scale to fabricate a novel 2D hybrid eightfold symmetric aperiodically ordered PQC. Our hybrid organic/inorganic nanocomposite is formed by using inorganic NRs, core/shell CdSe/CdS quantum rods, as inclusions in an organic polymer matrix (PMMA). Our nanocomposite has been prepared as a film in which each domain/inclusion can perform a specific photonic or optoelectronic (combined electronic and photonic) function. This permits introducing multifunctionality, and each of these functions can independently be optimized.

A semiconductor nanocrystals based 2D QPC pattern with rods with a diameter of 500 nm and a depth of 700 nm has been uniformly formed by EBL in a large area of 650 × 650 μm2. The diffraction effect of our sample has been confirmed and consists of a narrow red emission with a FWHM of 21.5 nm. The measured resonances peaks are due to the QPC feedback effect.

The possibility to pattern hybrid materials open the route to the development of new high performing optical devices such as organic light-emitting diodes, ultra-low threshold lasers, and non-linear devices. In a recent work of some of the authors [36] CdSe/CdS colloidal quantum rods have already proved to be very suitable in lasing applications. In future studies we shall focus our attention to lasing effects obtained in hybrid QPC microcavities and based on band edge engineering.

Abbreviations

CCD: charge-coupled device; EBL: electron beam lithography; FDTD: finite difference time domain; FWHM: full width at half-maximum; IPA: isopropanol; ITO, indium tin oxide; MIBK, methyl isobutyl ketone; NRs: nanorods; 1D: one-dimensional; PQCs: photonic quasicrystals; PMMA: polymethylmethacrylate; PBGs: photonic band-gaps; SEM: scanning electron microscopy; TEM: transmission electron microscopy; TEM: transverse electromagnetic; 2D: two-dimensional.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

LP conceived of the study, carried out the nanocomposite preparation and the fabrication of the samples, the SEM characterizations and drafted the manuscript. MR carried out the far field diffraction characterization and the spectra measurements. LM supervised the synthesis of colloidal nanorods and provided the TEM characterization. MZ prepared NR solutions. JZ participated in the design of the study. PM participated in the design of the study and its coordination and helped to draft the manuscript. All authors read and approved the final manuscript.

Acknowledgements

We acknowledge Dr G. Zito for providing the pattern design. We gratefully thank Dr. Giuseppe Nenna of ENEA of Portici for very fruitful discussions and set-up support. This work is partially supported by the National Natural Science Foundation of China (60977048).

References

  1. Levine D, Steinhardt PJ: Quasicrystals: a new class of ordered structures.

    Phys Rev Lett 1894, 53:2477-2480. OpenURL

  2. Stadnik ZM: Physical Properties of Quasicrystals. New York: Springer; 1999. OpenURL

  3. Capaz RB, Koiller B, de Queiroz SLA: Gap states and localization properties of one-dimensional Fibonacci quasicrystals.

    Phys Rev B 1990, 42:6402. Publisher Full Text OpenURL

  4. Macia E: Physical nature of critical modes in Fibonacci quasicrystals.

    Phys Rev B 1999, 60:10032. Publisher Full Text OpenURL

  5. Macia E: The role of aperiodic order in science and technology.

    Rep Prog Phys 2006, 69:397-441. Publisher Full Text OpenURL

  6. Fujiwara T, Kohmoto M, Kokihiro T: Multifractal wave functions on a Fibonacci lattice.

    Phys Rev B 1989, 40:7413. Publisher Full Text OpenURL

  7. Dulea M, Johansson M, Riklund R: Localization of electrons and electromagnetic waves in a deterministic aperiodic system.

    Phys Rev B 1992, 45:105. Publisher Full Text OpenURL

  8. Kroon L, Lennholm E, Riklund R: Localization-delocalization in aperiodic systems.

    Phys Rev B 2002, 66:094204. OpenURL

  9. De Brito PE, Da Silva CAA, Nazareno HN: Field induced localization in Fibonacci and TM lattices.

    Phys Rev B 1995, 51:6096. Publisher Full Text OpenURL

  10. Luck JM: Cantor spectra and scaling of gap widths in deterministic aperiodic systems.

    Phys Rev B 1989, 39:5834. Publisher Full Text OpenURL

  11. Peng RW, Huang XQ, Qiu F, Wang M, Hu A, Jiang SS, Mazzer M: Symmetry-induced perfect transmission of light waves in quasiperiodic dielectric multilayers.

    Appl Phys Lett 2002, 80:3063. Publisher Full Text OpenURL

  12. Axel F, Terauchi H: High-resolution X-ray-diffraction spectra of Thue Morse GaAs-AlAs heterostrucutres: towards a novel description of disorder.

    Phys Rev B 1991, 66:2223. Publisher Full Text OpenURL

  13. Florescu M, Torquato S, Steinhardt PJ: Complete band gaps in two-dimensional photonic quasicrystals.

    Phys Rev B 2009, 80:155112. OpenURL

  14. Moretti L, Mocella V: Two-dimensional photonic aperiodic crystals based on Thue-Morse sequence.

    Opt Exp 2007, 15:15314. Publisher Full Text OpenURL

  15. Liu N: Propagation of light waves in Thue-Morse dielectric multilayers.

    Phys Rev B 1997, 55:3543. Publisher Full Text OpenURL

  16. Dal Negro L, Yi JH, Nguyen V, Yi Y, Michel J, Kimerling LC: Spectrally enhanced light emission from aperiodic photonic structures.

    Appl Phys Lett 2005, 86:261905. Publisher Full Text OpenURL

  17. Dal Negro L, Oton CJ, Gaburro Z, Pavesi L, Johnson P, Lagendijk A, Righini R, Colocci M, Wiersma DS: Light transport through the band-edge states of Fibonacci quasicrystals.

    Phys Rev Lett 2003, 90:055501. PubMed Abstract | Publisher Full Text OpenURL

  18. Jiang X, Zhang Y, Feng S, Huang KC, Yi Y, Joannopoulos JD: Photonic band gaps and localization in the Thue-Morse structures.

    Appl Phys Lett 2005, 86:201110. Publisher Full Text OpenURL

  19. Cheng Samuel SM, Li L-M, Chan CT, Zhang ZQ: Defect and transmission properties of two-dimensional quasiperiodic photonic band-gap systems.

    Phys Rev B 1999, 59:4091. OpenURL

  20. Jin C, Cheng B, Man B, Li Z, Zhang D, Ban S, Sun B: Band gap and wave guiding effect in a quasiperiodic photonic crystal.

    Appl Phys Lett 1999, 75:1848-1850. Publisher Full Text OpenURL

  21. Zhang X, Zhang Z-Q, Chan CT: Absolute photonic band gaps in 12-fold symmetric photonic quasicrystals.

    Phys Rev B 2001, 63:081105. OpenURL

  22. Hiett BP, Beckett DH, Cox SJ, Generowicz JM, Molinari M, Thomas KS: Photonic band gaps in 12-fold symmetric quasicrystals.

    J Mater Sci: Mater Electron 2003, 14:413. Publisher Full Text OpenURL

  23. Gauthier RC, Mnaymneh K: FDTD analysis of 12-fold photonic quasi-crystal central pattern localized states.

    Opt Commun 2006, 264:78. Publisher Full Text OpenURL

  24. Lei H, Chen J, Nouet G, Feng S, Gong Q, Jiang X: Photonic band gap structures in the Thue-Morse lattice.

    Phys Rev B 2007, 75:205109. OpenURL

  25. Romero-Vivas J, Chigrin DN, Lavrinenko AV, Sotomayor Torres CM: Resonant add-drop filter based on a photonic quasicrystal.

    Opt Exp 2005, 13:826. Publisher Full Text OpenURL

  26. Della Villa A, Enoch S, Tayeb G, Pierro V, Galdi V, Capolino F: Band gap formation and multiple scattering in photonic quasicrystals with a Penrose-type lattice.

    Phys Rev Lett 2005, 94:183903. PubMed Abstract | Publisher Full Text OpenURL

  27. Campbell M, Sharp DN, Harrison MT, Denning RG, Turberfield AJ: Fabrication of photonic crystals for the visible spectrum by holographic lithography.

    Nature 2000, 404:53. PubMed Abstract | Publisher Full Text OpenURL

  28. Wang X, Ng CY, Chan CT, Sheng P: Large-area two-dimensional mesoscale quasicrystals.

    Adv Mater 2003, 15:1526. Publisher Full Text OpenURL

  29. Matarazzo V, De Nicola S, Zito G, Mormile P, Rippa M, Abbate G, Zhou J, Petti L: Spectral characterization of two-dimensional Thue-Morse quasicrystals realized with high resolution lithography.

    J Opt 2010, 13:015602. OpenURL

  30. Hu JQ, Bando Y, Golberg D: Novel semiconductor nanowire heterostructures: synthesis, analysis, properties and applications.

    J Mater Chem 2009, 19:330. Publisher Full Text OpenURL

  31. Ye JF, Qi LM: Solution-phase synthesis of one-dimensional semiconductor nanostructures.

    J Mater Sci Technol 2008, 24:529. Publisher Full Text OpenURL

  32. Shen GZ, Chen D, Bando Y, Golberg D: One-dimensional (1-D) nanoscale heterostructures.

    J Mater Sci Technol 2008, 24:541. OpenURL

  33. Scholes GD: Controlling the optical properties of inorganic nanoparticles.

    Adv Funct Mater 2008, 18:1157. Publisher Full Text OpenURL

  34. Kislyuk VV, Dimitriev OP: Nanorods and nanotubes for solar cells.

    J Nanosci Nanotechnol 2008, 8:131. PubMed Abstract | Publisher Full Text OpenURL

  35. Cozzoli PD, Pellegrino T, Manna L: Synthesis, properties and perspectives of hybrid nanocrystal structures.

    Chem Soc Rev 2006, 35:1195. PubMed Abstract | Publisher Full Text OpenURL

  36. Zavelani-Rossi M, Lupo MG, Krahne R, Manna L, Lanzani G: Lasing in self-assembled microcavities of CdSe/CdS core/shell colloidal quantum rods.

    Nanoscale 2010, 35:931. OpenURL

  37. Wu KJ, Chu K-C, Chao C-Y, Chen Y-F, Lai C-W, Kang C-C, Chen C-Y, Chou P-T: CdS Nanorods Imbedded in Liquid Crystal Cells for Smart Optoelectronic Devices.

    Nano Lett 2007, 7:1908. Publisher Full Text OpenURL

  38. Rotello VM: Nanoparticles-building blocks for nanotechnology. In Semiconductor Nanoparticles: Synthesis, Properties, and Integration into Polymers for the Generation of Novel Composite Materials. Edited by Skaff H, Emrick T. New York: Springer; 2004:29-39. OpenURL

  39. Woelfle C, Claus RO: Transparent and flexible quantum dot-polymer composites using an ionic liquid as compatible polymerization medium.

    Nanotechnology 2007, 18:025402. Publisher Full Text OpenURL

  40. Carbone L, Nobile C, De Giorgi M, Della Sala F, Morello G, Pompa P, Hytch M, Snoeck E, Fiore A, Franchini IR, Nadasan M, Silvestre A F, Chiodo L, Kudera S, Cingolani R, Krahne R, and Manna L: Synthesis and micrometer-scale assembly of colloidal CdSe/CdS nanorods prepared by a seeded growth approach.

    Nano Lett 2007, 7:2942. PubMed Abstract | Publisher Full Text OpenURL

  41. Petti L, Matarazzo V, Rippa M, Zito G, De Nicola S, Abbate G, Mormile P: High resolution lithography as a tool to fabricate quasiperiodic crystals.

    Theoretical and Computational Nanophotonics (TACONA-PHOTONICS 2009) Proceedings of the 2nd International Workshop. AIP Conference Proceedings 2009, 1176:146-148. OpenURL

  42. Zito G, Rose TP, Di Gennaro E, Andreone A, Santamato E, Abbate G: Bandgap properties of low-index contrast aperiodically ordered photonic quasicrystals.

    Micro Opt Tech Lett 2009, 51:2732. Publisher Full Text OpenURL

  43. Fan S, Villeneuve PR, Joannopoulos JD, Schubert EF: High extraction efficiency of spontaneous emission from slabs of photonic crystals.

    Phys Rev Lett 1997, 78:3294. Publisher Full Text OpenURL