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7/23 Wednesday - Training Session

08:00 AM - 10:00 AM All Presenters

7/25 Friday - Electronic and Optical Properties of Nanoparticle Complexes

07:45 AM Getting Ready

08:00 AM Qu-Quan Wang

Shao-Ding Liu, Xiong-Rui Su, Mu-Tian Cheng, and Qu-Quan Wang

We also discussed the coupling, propagations, and far-field emissions of surface plasmons in a pair of Au nanowires (NWs) with a SQD dipole emitter using the finite-difference time domain method (FDTD). The surface plasmon wavelength is tunable from 650 to 380 nm by adjusting the distance between the two NWs, which leads to an enhancement of coupling constant and density of states of the surface plasmon. The converted energy from the SQD nanosized dipole emitter to the propagating surface plasmon as well as the far-field emission intensity of a pair of Au NWs increase to approximately four times as large as those of a single NW.

[1] M. T. Cheng, S. D. Liu, H. J. Zhou, Z. H. Hao, and Q. Q. Wang, Opt. Lett. 32, 2125 (2007).

[2] M. T. Cheng, S. D. Liu, and Q. Q. Wang, Appl. Phys. Lett. 92, 162107 (2008).

[3] S. D. Liu, M. T. Cheng, Z. J. Yang, and Q. Q. Wang, Opt. Lett. 33, 851 (2008).

[4] J. Y. Yan, W. Zhang, S. Q. Duan, X. G. Zhao, and A. O. Govorov, Phys. Rev. B 77, 165301 (2008).

[5] J. Y. Yan, W. Zhang, S. Q. Duan, and X. G. Zhao, J. Appl. Phys. 103, 104314 (2008).

08:30 AM Xingyu Jiang

Yang Zhou, Shixing Wang, Ke Zhang and Xingyu Jiang

National Center for NanoScience&Technology, Beijing, China

The gold NPs are functionalized with azide-/alkyne- terminated thiols. When we mixed the two kinds of functionalized gold nanoparticles, with the presence of Cu(I) as a catalyst (from the reduction of Cu ²⁺), the color of gold nanoparticles changed from red to purple and finally formed precipitates. UV-visible spectrostrocopy and transmission electron microscopy confirmed the aggregation of NPs. The color changes and formation of precipitates in a homogeneous solution of gold NPs can thus be used to judge the presence of Cu²⁺. The selectivity for Cu²⁺ was evaluated by testing the response of the assay to other metal ions such as Al³⁺, Fe²⁺, Mg²⁺, Mn²⁺, Zn²⁺, Pd²⁺, Ca²⁺, Co²⁺, Na⁺ and K⁺. The results showed these metal ions do not interfere with the assay. Furthermore, even mixtures of these cations do not interfere with the selectivity of the assay. The detection limit of [Cu²⁺] is about 50 microM by the naked eye. The visual detection provided the convenience for detection of Cu²⁺ in aqueous solution without any advanced instruments.

Because of the compatibility of the underlying chemistry (that mainly involves azides, alkynes and their reactions catalyzed by copper) with biomolecules, we are attempting to extend current work to applications in biological assays. We anticipate this methodology to find applications wherever sensors for Cu²⁺ are required.

09:00 AM Jianbiao Zhang

J. B. Zhang, X. S. Chen, W. D. Hu, W. Lu

National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Science, Shanghai, China

We study the second harmonic (SH) wave generation in one-dimensional multilayer GaAs/air nanostructure. Due to the strong localization of the light near the defective region, the conversion efficiency from the fundamental frequency (FF) wave to the SH wave has been significantly enhanced. As the FF wave intensity reaches a certain value, the intensity of the generated SH wave saturates to a constant value.

The one-dimensional multilayer structure is constructed by GaAs and air layers alternately, with l_GaAs = 361.2 nm and l_air = 240.8 nm. A defect is introduced by changing the thickness of the central GaAs layer to l_defect = 782.6 nm. The numbers of periods on both sides of the defect layer are set to be the same, so that the multilayer nanostructure is bilaterally symmetrical relative to the central defective layer. Two sharp transmission peaks appear in the first and second band gaps in the transmission spectrum, which is due to the defective central layer induces dual-localized modes, λ_FF = 1335.8nm and λ_SH = 667.9nm. The nonlinear coefficient of GaAs material is 0.166 nm/V.

By using a nonlinear transfer matrix method and an iteration way [1, 2], it is found that the transmission coefficient of the FF wave is reduced while the SH generation and FF reflectivity are evidently enhanced with the increasing incident FF intensity (in Fig. 1). When the incident FF intensity reaches 3.8 KW/cm², the SH conversion efficiency is gradually tend to saturate in both the forward and backward directions with the total conversion efficiency of 40% [3]. We have also investigated the effect of the period number and nonlinear efficiency of the composite material on SH wave generation. It is shown that the multilayer nanostructure can be used as a high-efficiency compact SH generator.

Fig. 1. Forward SH conversion efficiency η_+SH; backward SH conversion efficiency η_-SH, FF reflectivity R_FF; and FF transmission T_FF vs FF wave input intensity. The number of the periods is 6 and the nonlinear coefficient of GaAs material (d) is 0.166 nm/V.

1.    A. Yariv, P. Yeh, Optical waves in crystals: propagation and control of laser radiation (New York: John Wiley & Sons), 526 (1984).

2.    Y. Jeong and B. Lee, IEEE J. Quantum Electron 35, 162 (1999).

3.    Yong Zeng, Xiaoshuang Chen and Wei Lu, J. Appl. Phys. 99, 123107 (2006).

09:15 AM Wei ZHANG

We investigate theoretically the effects of interaction between an optical dipole (semiconductor quantum dot or molecule) and metal nanoparticles. The calculated absorption spectra of hybrid structures demonstrate strong effects of interference coming from the exciton-plasmon coupling. In particular, the absorption spectra acquire characteristic asymmetric lineshapes and strong anti-resonances. We present an exact solution of the problem beyond the dipole approximation and find that the multipole treatment of the interaction is crucial for the understanding of strongly-interacting exciton-plasmon nano-systems. Interestingly, the visibility of the exciton resonance becomes greatly enhanced for small inter-particle distances due to the interference phenomenon, multipole effects, and electromagnetic enhancement. We find that the destructive interference is particularly strong.

Using our exact theory, we show that the interference effects can be observed experimentally in the exciting systems even at room temperature.

This study was performed in collaboration with Alexander Govorov et al.

7/25 Friday - Characterization of Nanostructured Materials for Solar and Optoelectronic Devices

09:45 AM P. C. Kelires

P. C. Kelires^1,2, G. Hadjisavvas²

¹ Department of Mechanical Engineering & Materials Science and Engineering, Cyprus University of Technology, P. O. Box 50329, 3603 Lemesos, Cyprus

²Department of Physics, University of Crete, P. O. Box 2208, 710 03 Heraclion, Crete, Greece

Silicon nanocrystals (NCs) embedded in amorphous dielectric matrices (a-SiO₂) have attracted considerable attention both for their fundamental properties and potential applications in photonics and nanoelectronics. These nanocomposite systems exhibit strong optical activity (efficient light emission, optical gain) which makes them suitable for optical devices. The embedded NCs can also be efficiently charged which makes them suitable for charge storage elements in non-volatile memory devices. It is universally accepted that these properties strongly depend on the nature of the interface between the NCs and the embedding medium, but exactly this parameter is the least well understood issue of the whole problem.

 We review in this talk our recent studies [1,2] which shed light on several questions regarding the stability of the interface as a function of the NC size, its structure, both topological and chemical, and the effects produced by variations in these parameters on the electronic and optical response of the system. We discuss the interplay between quantum confinement (QC) effects, oxygen interfacial states, and topological deformations in controlling the photoluminescence (PL) energies. We identify two distinctly different regimes. For NCs larger than 3-4 nm, the QC effects (free excitonic recombination) are shown to be the dominant factor in agreement with experimental studies [3,4]. For smaller sizes, we propose that the observed pinning of the gap and the associated redshift of the PL energies is heavily influenced by the topological distortions, and not so much by the localized states due to interface oxygen bonds, as previously thought. We also discuss the issue of faceting of the embedded NCs [5]. Comparison with other theoretical and experimental studies of the above subjects will be done in order to assess the state-of-the-art in this important field.

 [1] G. Hadjisavvas and P. C. Kelires, Phys. Rev. Lett. 93, 226104 (2004).

[2] G. Hadjisavvas and P. C. Kelires, Physica E 38, 99 (2007).

[3] M. Volkin et al., Phys. Rev. Lett. 82, 197 (1999).

[4] E. Lioudakis et al., Physica E 38, 128 (2007).

[5] G. Hadjisavvas, I. Remediakis, and P. C. Kelires, Phys. Rev. B 74, 165419 (2006).

(Invited) InN on GaN(0001): A Model Case Study for the Spontaneous Growth of III-Nitride Nanostructures

10:15 AM E. Iliopoulos

E. Iliopoulos¹, E. Dimakis¹, A. Georgakilas¹

¹Physics Department, University of Crete,Heraklion-Crete, Greece

Epitaxial growth of InN on GaN(0001) surfaces, by plasma assisted molecular beam epitaxy, is challenging, due to the fact that InN decomposition rate is higher than the corresponding In metal desorption rate, at any growth temperature. [1] The surface kinetics of growth has been investigated in details. [2] Four distinct growth regimes are observed, due to the temperature dependence of the indium adatom’s surface mobility and of  the InN decomposition rate.

Apart from the kinetics factors that govern the epitaxial growth, thermodynamic effects were observed to play an important role, at the substrate temperature regime where indium adatoms’ mobility is large. In this case a self-regulating mechanism of InN islands’ shape takes place. This mechanism is correlated with the decrease of surface energy for near-stoichiometric coverages of nitride surfaces with metal and nitrogen adatoms. [3] The exploitation of this self-regulating mechanism permits accurate control of the dimensions of engineered InN nanostructures (surface coverage, nanostructures’ diameter and height).

This self-regulating mechanism is correlated to (i) high adatom mobility of group-III atoms, (ii) low-residence time on the surface of group-V adatoms and (iii) high surface energy of nitrogen covered surfaces. Therefore it is expected to be a universal feature of epitaxial growth of the whole III-Nitrides family and may prove an important key for the development of III-Nitrides nanotechnology.

Fig1. The ratio of growth rate along the c-axis GR_[0001] to incident active nitrogen flux F_N, of InN epitaxial structures, plotted versus the incident, during growth, ratio of In to N fluxes. AFM micrographs (1x1 μm²) depict characteristic morphologies at three different flux ratios F_N/F_In : (a) 1, (b) 2.8 and (c) 3.6.

1.    E. Dimakis, E. Iliopoulos, K. Tsagaraki, Th. Kehagias, Ph. Komninou, A. Georgakilas, J. Appl. Phys. 97, 113520 (2005).

2.    E. Dimakis, E. Iliopoulos, K. Tsagaraki, A. Georgakilas, Appl. Phys. Lett. 86, 133104 (2005).

3.    E. Dimakis, E. Iliopoulos, K. Tsagaraki, A. Georgakilas, Phys. Stat. Sol. (a) 203, 1686 (2006)

10:45 AM A. G. Nassiopoulou

11:15 AM T.Yu. Bilyk

T.Yu. Bilyk ^1, M.M. Melnichenko ², O.M. Shmyryeva ¹, K.V. Svezhentsova³ 

¹Kiev National Technical University of Ukraine “KPI”,Ukraine

²Physics Department, Taras Shevchenko Kiev National University,Ukraine
Email: realcrystallab@univ.kiev.ua

³Institute of Semiconductor Physics of NASU,Ukraine

 
         Alternative and renewed sources of energy, such as a wind and solar, hydro and geothermal energy, all over the world attract more and more attention. The growing interest to them can be explained by ecological reasons, on the one hand, and limitation of traditional terrestrial resources — on another hand. The special place among alternative and renewed sources of energy is occupied by photo-electric converters of a solar energy. Today it is possible to speak about obvious advantages of solar power before traditional sources, namely it is practically inexhaustible, and there is no pollution of the environment and no wastes. Prospects of development of photo-electric converters recently have opened on the basis of single-crystal silicon with application of nanostructured (nanoporous) silicon. The nanostructured silicon has unique gettering, passivation, clarifying and optical properties.

         It is known, that a thin layer of nanostructured silicon on a textured substrate of single-crystal silicon considerably reduces reflection of light in comparison with usual textured substrates. However methods and mechanisms of formation of nanostructured silicon are ambiguous and are not yet completely investigated.

And that is why the study of mechanisms of formation and properties of nanostructured silicon on textured substrates requires detailed research, necessary for increase of efficiency of transformation of solar cells.

         The authors of the paper studied the process of formation of nanostructured silicon, the influence of the formation technique type on its structural, photo-luminescent and anti-reflecting properties. The layers of nanostructured silicon have been formed on the textured surface of solar cells. The layer of the nanostructured silicon was created by chemical etching in a mix of acids HF and HNO₃. The thickenss of a layer of the nanostructured silicon (3 – 30 nm) during chemical modification of textured surface of single-crystal of silicon was supervised by preset parameters of technological process and was determined by a method of Auger electron spectroscopy. It has been demonstrated that the use of nanostructured silicon reduces the anti-reflecting coefficient and guarantees high value of conversion efficiency in the short-wave spectral region due to the phenomenon of light re-emission.

11:30 AM Ioannis Alexandrou

Ioannis Alexandrou¹, Emmanouil Lioudakis², Christos Markos¹ and Andreas Othonos²

¹Electrical Engineering & Electronics, University of Liverpool, Liverpool L69 3GJ, UK

²Research Center of Ultrafast Science, Department of Physics, University of Cyprus P.O. Box 20537, 1678, Nicosia, Cyprus

In this presentation, we show how a combination of electrical and optical characterizations can be used to probe the response of charge at the polymer-nanotube bulk junctions. The samples examined were prepared by mixing P3HT and single wall nanotubes (SWNTs) from solution. Processing and measurements were performed in ambient conditions. Current-voltage measurements on composites reveal a percolation threshold of about 0.75 %w.t. SWNT concentration, showing good dispersion of SWNTs. Capacitance-voltage (C-V) measurements were used to probe charge transport and storage at the polymer-SWNT junctions for SWNT concentration up to 1 %w.t.  At the vicinity of the percolation threshold charge trapping is clearly monitored via the increase in depletion capacitance with increasing the SWNT concentration (Fig 1). Charge is trapped or released from the SWNTs as the concentration of holes in the polymer matrix varies during accumulation and depletion. Understanding the behavior of P3HT-SWNT composites shows that this methodology can be used to compare different polymer fillers against their efficiency to exchange charge with the polymer matrix. By varying the measurement frequency one can also assess the time response of the junctions.

Fig. 1. C-V characteristics of P3HT-SWNT composites. Holes trapped in the SWNTs are released during depletion, leading to increased depletion capacitance.

The optical absorption spectra of these composites were measured by multiwavelength spectroscopic ellipsometry.  A critical characteristic determining the optoelectronic response (solar cells and detectors) of these composites is the ability of the polymer-SWNT junctions to dissociate excitons created at their vicinity. Degenerate and non-degenerate transient absorption measurements were used to compare charge relaxation dynamics for pure SWNTs as well as for P3HT-SWNT composites while varying the SWNT concentration. With the addition of SWNTs photoexcited carriers at resonance with the P3HT excitonic energy levels appear to relax progressively faster and always in the sub 5 ps timescale. However, these measurements can probe the response of the P3HT-SWNT junctions only at high SWNT concentrations, providing complimentary study to the C-V characterization.

1. A. Star, Y. Lu, K. Bradley, and G. Gruner, Nano Letters 4, 1587 (2004).
2. A. Nish, J.-Y. Hwang, J.Doig and R. J. Nicholas, Nanotechnology 19, 095603 (2008)

12:00 PM Demetra Tsokkou

Research Center of Ultrafast Science, Department of Physics, University of Cyprus P.O. Box 20537, 1678, Nicosia, Cyprus

Ultraviolet femtosecond spectroscopy has been utilized to investigate the carrier dynamics in a set on nanocrystalline silicon films with thickness ranging from 30 nm down to 5 nm [1]. We have performed transient non-degenerate photoinduced absorption measurements using ultrashort amplified pulses generated with an Optical Parametric Amplifier in the UV spectral region [2]. Following excitation at 300 nm (4.12 eV), carriers are excited into energy states near the direct energy gap (critical point-Γ₂) of the first Brillouin zone and a white light supercontinuum was utilized to determine the carrier relaxation. The observed complex photoinduced absorption behaviour for the different samples may be attributed to the various available energy states where the photogenerated carriers can relax and then can be re-excited to higher energy states. A simple model based on contributions from various energy states has been utilized to explain the results. Contributions from quantum confinement and surface-related states to the photogenerated carrier relaxation appear to become important for the thinner sample. We have time-resolved carrier transitions from the L to the X valley in the first Brillouin zone, which appear to be dependent on the presence of surface-related states and the deformed energy band structure of the nanomaterials.

Figure 1. Non-degenerate time resolved transient absorption measurements of the ultrathin silicon film with thickness 20 nm using ultrafast UV excitation pumping pulses at 300 nm and probing pulses in the range of 450-900 nm

[1] Andreas Othonos, J. Appl. Phys., 83, 1789 (1998)

[2] Andreas Othonos, Demetra Tsokkou and Emmanouil Lioudakis, Res. Lett. Phys., 2008, 837503 (2008)

Electronic States and Light Absorption in Cylindrical Quantum Dot with Thin Falciform Cross-Section

12:15 AM Ani. A. Tshantshapanyana

Karen G. Dvoyan^a, David B. Hayrapetyan^a,b, Eduard M. Kazaryan^a, and
Ani. A. Tshantshapanyan^a

^aDept. of Applied Physics and Engineering, Russian-Armenian State University, Yerevan, Armenia

^b Dept. of Physics, State Engineering University of Armenia, Yerevan, Armenia

 Within the framework of adiabatic approximation the energy levels and direct interband light absorption in cylindrical quantum dot with thin falciform cross-section are studied. Analytical expressions for the particle energy spectrum are obtained. The one dimensional “fast” subsystem wave function amplitude and phase dependence on cylindrical quantum dot thin falciform cross-section geometry is revealed. For “slow” subsystem both parabolic and modified Pöschl-Teller effective potentials cases are considered. It is shown that for lower energy levels the particle energy spectrum has an equidistant character for both cases. Absorption edge frequencies and absorption coefficient in strong size quantization regime are investigated. Corresponding selection rules for quantum transitions are revealed.

12:30 PM Andreas Mandelis

 Center for Advanced Diffusion-Wave Technologies (CADIFT), Department of Mechanical and Industrial Engineering, University of Toronto, Toronto M5S 3G8, Canada

A two-beam photo-carrier radiometry (PCR) technique of semiconductors has been developed. The technique operates on the superposition of superband-gap and subband-gap laser beams which results in the cross-modulation of the backscattered subband-gap laser intensity by the harmonically varying free-carrier-wave density-dependent infrared absorption coefficient. A theory of this two-beam cross-modulation approach and various experimental configurations applied to the imaging of electronic contamination and defects in silicon wafers are presented. Owing to the nonlinear interaction of the two beams, the configuration revealed a new optoelectronic effect, the decrease of the residual subband-gap absorption coefficient due to the decreased carrier capture cross-section brought about by the depletion of occupied band-gap states in the presence of photons produced by radiative recombination. Quantitative values of the optoelectronic constant B associated with the rate of depletion of free-carrier capture cross-section with superband-gap intensity, as well as of IeR, the

intensity of radiative recombination emissions, were obtained. These values cannot be measured by conventional PCR or other single-ended optoelectronic techniques. The theory explains the experimental dependence of electronic transport properties on the intensity of the subband-gap beam and accounts for optoelectronic imaging contrast amplification in contaminated or defect semiconductors. The two-beam cross-modulation PCR was further shown to enhance the imaging contrast of a certain electronic contamination type (Fe in p-Si). A dramatic phase contrast enhancement of subsurface defects made by low-dose proton implantation was demonstrated at superband-gap laser intensity levels one order of magnitude lower than possible with single-ended optoelectronic imaging methodologies. This is tentatively attributed to relatively low-injection trap-filling well below optoelectronic trap saturation.

01:00 PM Athos Petrou

M. Yasar, A. Petrou, T. Ali

Department of Physics, University at Buffalo, Buffalo, NY, USA

Naval Research Laboratory, Washington DC, USA

M. Korkusinski, and P. Hawrylak

Institute for Microstructural Sciences NRC, Ottawa, Canada

 
We have studied the electroluminescence (EL) spectra from Fe/AlGaAs(n)/GaAs(i)/AlGaAs(p) spin LEDs which incorporate a single layer of InAs quantum dots (QDs) at the center of the GaAs quantum well.  The QDs were grown at the rate of 0.001 ML/s, resulting in low density (7×10⁸ dots/cm²) with narrow size distribution.  By varying the device current we were able to observe gradual population of the atomic-like energy s-, p-, d-, and f – shells. This is illustrated in the figure.  Similar effects have been previously observed in photoluminescence studies of  the same system.[1]  The EL maxima are interpreted as due to recombination of excitons associated with conduction and valence band shells.  The circular polarization P of EL features associated with filled shells is near zero and does not exhibit any evidence of spin injection from the Fe contact.  In contrast,  P   for partially filled shells is high and shows the characteristic behavior of spin-injection.[2]  The circular polarization as function of photon energy exhibits well defined maxima in-between the EL intensity peaks.  The magnetic field dependence of P of these maxima provides convincing evidence of spin injection from the ferromagnetic contact.  These experimental data have been compared to a calculation of the emission spectra using exact diagonalization techniques for multi-exciton complexes up to 6 electron-hole pairs.   The calculation predicts the presence of polarization maxima which are shifted in energy with respect to the EL intensity maxima, in agreement with experiment.  The shifts are attributed to the enhanced exchange energy between spin-down electrons occupying the shells.  This work has yielded experimental values for the average exchange energies  V_X(sp) and V_X(pd) between the s- and p- shells,  and the p- and d-shells, respectively.

Work at SUNY Buffalo was supported by ONR (N000140610174) and NSF (ECS0524403).   Work at NRL was supported by the Office of Naval Research through core programs at NRL.  Work at IMS NRC was supported by the Canadian Institute for Advanced Research, by QuantumWorks, and by a NRC-CNRS collaborative research grant.

1.             Raymond, S., S. Studenikin, A. Sachrajda, Z. Wasilewski, S.J. Cheng, W. Sheng, P. Hawrylak, A. Babinski, M. Potemski, G. Ortner, and M. Bayer, Physical Review Letters. 92(18): p. 187402, (2004).

2.             Hanbicki, A.T., O.M.J. van 't Erve, R. Magno, G. Kioseoglou, C.H. Li, B.T. Jonker, G. Itskos, R. Mallory, M. Yasar, and A. Petrou. Applied Physics Letters. 82(23): p. 4092, (2003).

 Fig1. EL Spectra of InAs QDs at different bias voltages.

7/25 Friday - Physics and Application of Carbon Nanotubes

01:30 PM Yonggang Huang

02:15 PM Mark C. Hersam

Large-scale production of chirality-resolved single-walled carbon nanotubes (SWNTs) has the potential to enable and/or improve many applications for SWNTs such as thin film transistors, transparent conductors, optical amplifiers, and biosensors [1].  Recently, we have developed a scalable and flexible technique for sorting SWNTs by their physical and electronic structure using density gradient ultracentrifugation (DGU) in aqueous solution [2-4].  For sorting by physical structure, DGU exploits inherent differences in buoyant density as a function of SWNT diameter.  Alternatively, for sorting by electronic structure, DGU takes advantage of subtle differences in buoyant density that result from differential adsorption of co-surfactant mixtures to metal versus semiconducting SWNTs.  This talk will delineate recent developments in DGU including efforts to improve purity, yield, and throughput of chirality-resolved SWNTs.  In addition, this talk will explore the improvements that chirality-resolved SWNTs enable in both fundamental experiments and applied technologies.  For example, DGU-sorted SWNTs allow exciton energy transfer [5] and exciton decay dynamics [6] to be quantified.  From a technology perspective, chirality-resolved SWNTs permit the optical and electrical properties of semi-transparent, conductive films to be independently tuned [7].  Specifically, the ability to sort metallic SWNTs by diameter enables the formation of conductive films with tunable optical adsorption throughout the visible and infrared portions of the electromagnetic spectrum.  The properties and potential applications of this semi-transparent, conductive, and mechanically flexible SWNT “stained glass” will be discussed.

1.    M. C. Hersam, “Progress towards monodisperse single-walled carbon nanotubes,” Nature Nanotechnology, advance online publication, 30 May 2008, doi:10.1038/nnano.2008.135.

2.    M. S. Arnold, et al., “Enrichment of single-walled carbon nanotubes by diameter in density gradients,” Nano Letters, 5, 713 (2005).

3.    M. S. Arnold, et al., “Sorting carbon nanotubes by electronic structure via density differentiation,” Nature Nanotechnology, 1, 60 (2006).

4.    A. A. Green and M. C. Hersam, “Ultracentrifugation of single-walled carbon nanotubes,” Materials Today, 10, 59 (2007).

5.    H. Qian, et al., “Exciton energy transfer in pairs of single-walled carbon nanotubes,” Nano Letters., 8, 1363 (2008).

6.    T. Gokus, et al., “Exciton decay dynamics in individual carbon nanotubes at room temperature,” Applied Physics Letters, 92, 153116 (2008).

7.    A. A. Green and M. C. Hersam, “Colored semitransparent conductive coatings consisting of monodisperse metallic single-walled carbon nanotubes,” Nano Letters, 8, 1417 (2008).

(Invited) Carbon nanotubes: optimized growth for applications and practical use of large CNT structures

03:00 PM Robert Vajtai

Robert Vajtai¹, Géza Tóth², Krisztián Kordás², Xiaohong An³, Pulickel M. Ajayan¹

¹Department of Mechanical Engineering & Materials Science, Rice University, Houston, TX 77005 USA

²Microelectronics and Materials Physics Laboratories, Department of Electrical and Information Engineering, and EMPART research group of Infotech Oulu, P.O. Box 4500, FIN-90014 University of Oulu, Finland  

³Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, NY

Polymer crystallization induced wrapping of carbon nanofibers

03:30 PM Gaurav Mago

Gaurav Mago¹, Dilhan M. Kalyon² and Frank T. Fisher¹

¹Department of Mechanical engineering, Stevens Institute of Technology, Hoboken, NJ, USA 07030
Email: fgaurav@stevens.edu

²Department of Chemical Engineering and Materials Science, Stevens Institute of Technology, Hoboken, NJ, USA 07030

 Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have attracted significant interest due to their excellent mechanical, electrical, and physical properties. Recent advances in chemical functionalization strategies are anticipated to extend their utility in various applications. For example, the dispersion of nanoparticles within a polymer matrix can be significantly improved by the attachment of compatible organic, inorganic and biological species to the CNT or CNF surface. However, while covalent functionalization schemes allow one to chemically tailor the surface properties of the nanostructure, this is often at the expense of creating defects at the nanostructure surface. In this study, CNFs were non-covalently functionalized using a solution crystallization technique. The nanopaticles were coated with crystals of various technologically-relevant semicrystalline polymers such as PBT and PVDF. While working with dilute solutions of PBT and PVDF in the presence of CNFs, it was found that CNFs can be coated (functionalized) with small polymer crystals, with CNFs acting as nucleating agents and the crystal size dependent on the polymer concentration. Such structures with a periodic patterning of polymer crystals on the surface of CNFs are commonly referred to as nano-hybrid shish-kebabs (NHSKs). This solution crystallization technique thus provides an alternative strategy to alter and control the nanostructure/polymer interface, which the resulting nano-hybrid structures being useful in a broad range of applications including electronic devices and sensors.

03:45 PM Ming Zheng

04:15 PM Junhong Chen

7/28 Monday - Epitaxial Semiconductor Nanostructures

07:45 AM Getting Ready

08:00 AM Alexander Govorov

08:45 AM Emanuele Pelucchi

MOVPE growth on (111)B GaAs substrates patterned with inverted pyramids allows producing site-controlled quantum dots (QDs) via capillarity-driven segregation and self ordering. Very small size distributions of strongly confined QDs (inhomogeneous photoluminescence broadening <4-8 meV, >50 meV s-p state separation) can be routinely achieved. Reproducible signatures of confined excitonic states are observed in the optical spectra, from which single and correlated photon emission can be obtained. The inverted pyramid technique is demonstrated as a particularly versatile system for controlling dot shape, stacking and confinement potential, allowing, for example, for the control of the polarization state of the QD optical emission. [1]

We will here review recent results in the field and discuss perspectives and future milestones.

1.    E. Pelucchi, M. Baier, Y. Ducommun, S. Watanabe, and E. Kapon, Phys.  Stat. Solidi B 238, 233 (2003); M. Baier, E. Pelucchi and E. Kapon, S. Varoutsis, M. Gallart, I. Robert-Philip, and I. Abram,  Appl. Phys. Lett. 84, 648 (2004); K. F. Karlsson, V. Troncale, D. Oberli, A. Malko, E. Pelucchi, A. Rudra and E. Kapon,  Appl. Phys. Lett. 89, 251113 (2006); E. Pelucchi, S. Watanabe, K. Leifer, B. Dwir, Q. Zhu, P. De Los Rios and E. Kapon, NanoLetters 7, 1282 (2007); Q. Zhu, K. F. Karlsson, E. Pelucchi, E. Kapon,  NanoLetters 7, 2227 (2007); K. Leifer, E. Pelucchi, S. Watanabe, F. Michelini, B. Dwir and E. Kapon, Appl. Phys. Lett. 91, 081106  (2007).

Crystallographically oriented Zn nanocrystals inside ZnO induced by Zr⁺-implantation

09:15 AM Ya-Jun Li

Y. J. Li, B. Zhang, and W. Lu

National Lab for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yu Tian Road, Shanghai 200083, China

The semiconductors containing dispersed metallic nanocrystals (NCs) have recently begun to receive considerable attention because of their promising applications in producing nonvolatile flash memory devices with ultralow power consumption and ultrahigh density[1,2]. The materials also exhibit a large nonlinearity in the third-order optical susceptibility at the surface plasmon resonance frequency[3]. Ion implantation is a very powerful method for producing this type of the materials.

single-crystal ZnO wafers with (0001) orientation used in this study were grown by hydrothermally method. Zr⁺ ions were implanted into these wafers at room temperature with a dose of 5×10¹⁶/cm². The energy of Zr ⁺-implantation was 400 keV and the ion beam flux was ~2.3×10¹³cm^-2s^-1. After implantation, the samples were subject to a 5 min rapid thermal annealing (RTA) at 725 °C in flowing N₂ ambient. To investigate the microstructures of the samples, XRD was carried out in a 2D diffraction system (Bruker-AXS, D8 Discover with GADDS) operating with Cu Kα radiation (λ=1.5418Å) at 40kV and 40mA.

Fig1(a) shows the θ-2θ XRD patterns for the sample with the implantation dose of 5×10¹⁶/cm². As expected, intense ZnO (002) and (004) peaks are observed (ZnO has a hexagonal structure with a=0.3429 and c=0.5206 nm). Careful examination of the ZnO (002) peak shows a shoulder-like structure at 2θ~36.2° which corresponds to the nanocrystal metallic precipitated Zn (002) peak (Zn also has a hexagonal structure with a=0.2665 and c=0.4947 nm). Fig1(b) is the f-scan of ZnO and precipitated Zn NCs with grazing incidence XRD patterns (GIXRD). The f-scan shows a sixfold symmetry of  both ZnO and Zn NCs at the same azimuthal position and confirms the epitaxial relationships [100]_Zn//[100]_{ZnO matrix}, [010]_Zn//[010]_{ZnO matrix}, [001]_Zn//[001]_{ZnO matrix}. The results of  Raman spectra and high-resolution TEM (not shown here) are in good agreement with the XRD results.

during implantation, most of Zr⁺ ions implanted into ZnO matrix are interstitial atoms. cit_bfcit_bf   In RTA, the interstitial Zr⁺ atoms activated thermally and occupied the Zn site. And some Zn atoms came out of ZnO lattice and aggregated into Zn clusters energetically favourable. As a result, Zn NCs were precipitated and grew bigger and bigger in implanted ZnO matrix.

In summary, Crystallographically oriented Zn NCs are produced inside ZnO matrix by Zr+-implantation.

Fig1. XRD patterns of  Zr⁺-implanted and annealed ZnO sample. (a) q-2q scan, (b) f-scan of ZnO and nanocrystal Zn with grazing incidence XRD patterns (GIXRD)

1.    Z. Liu, C. Lee, V. Narayanan, G. Pei, and E. Kan, IEEE transactions on electron devices 49, 1606 (2002).

2.    Z. Liu, C. Lee, V. Narayanan, G. Pei, and E. Kan, IEEE transactions on electron devices 49, 1614 (2002).

(Invited) Growth and luminescence of site-controlled single InAs quantum dots

09:30 AM P. Atkinson

P. Atkinson¹, M. Benyoucef², S. Kiravittaya¹, A. Rastelli², O.G. Schmidt²

¹ Max-Planck-Institut für Festkörperforschung, Heisenbergstr.1, D-70569 Stuttgart, Germany

²Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstr.20, D-01069 Dresden, Germany

Many proposed quantum dot based devices for quantum cryptography and optical quantum computing require the placement of a single dot, or in some instances, two dots, in the centre of a photonic crystal [1]. This site control can be achieved by MBE overgrowth on pre-patterned GaAs substrates containing small pits in the surface [2] and the remaining challenge is to achieve high quality photoluminescence from such site-controlled dots.

 In this work we use ex-situ electron-beam lithography followed by conventional wet etching to pattern holes ~ 100nm wide and ~ 20nm deep in a (001) GaAs substrate. During the overgrowth of a ~ 10nm GaAs buffer layer these holes change shape, and then completely infill during subsequent InAs deposition. The difference in strain above these infilled holes and the surrounding planar substrate leads to preferential dot nucleation occurring over the patterned sites. The change in hole shape during the buffer growth, due to the difference in GaAs migration over the arsenic and gallium terminated step edges lining the hole, can lead to each patterned site effectively providing two preferential nucleation sites, as shown by the ordered array of pairs of site-controlled quantum dots shown in figure 1.

 We discuss here the effect of the pattern size and the growth conditions on the site-controlled dot growth, and the effect of the regrowth interface on the luminescence for different oxide removal techniques [3,4]. From these investigations it appears that the dot-interface separation should be greater than 30nm to avoid significant degradation of the luminescence, which may be achieved by vertical stacking of site-controlled quantum dots.

1.    A. Imamoglu et al.; PRL  83   4204 - 4207 (1999)

2.    O.G. Schmidt (Ed.) Lateral alignment of epitaxial quantum dots. Springer, Berlin. (2007)

3.    T.M. Burke et al.; J. Cryst. Growth 175/176 416-421 (1997)

4.     Z.R. Wasilewski et al.; J. Vac. Sci. Technol. B 22 1534-1538 (2004)

10:00 AM Xifeng Yang

X. F. Yang, X. S. Chen and W. Lu

National Lab for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yutian Road, 200083, Shanghai, People's Republic of China

The calculated bandoffset (doted + line) and energy level (dotted line) of the three different shaped QD when strain included.

[1] M. A. Naser, M. J. Deen, and D. A. Thompson, J. Appl. Phys. 102, 083108 (2007).

[2] V. Ryzhii, V. Mitin, and M. Stroscio, Appl. Phys. Lett. 78, 3523 (2001).

[3] Z. H. Chen, O. Baklenov, E. T. Kim, I. Mukhametzhanov, J. Tie, A. Madhukar, Z. M. Ye, and J. C. Campbell, J. Appl. Phys. 89, 4558 (2001).

[4] S. Y. Wang, S. D. Lin, H. W. Wu, and C. P. Lee, Appl. Phys. Lett. 78, 1023 (2000).

[5] J. Jiang, S. Tsao, T. O'Sullivan, W. Zhang, H. Lim, T. Sills, K. Mi, M. Razeghi, G. J. Brown, and M. Z. Tidrow, Appl. Phys. Lett. 84, 2166 (2004).

[6] J. L. Birman, Phy. Rev. 111, 1510 (1958).

[7] M. J. P. Musgrave and J. A. Pople, Proc. Roy. Soc. London A, 268, 474-84 (1962).

The conception of e-transformations in wave-function and phonon engineering

10:15 AM Rustam R Bashirov

Topologically, a great deal of quantum semiconductor heterostructures with nontrivial 2D and 3D geometry, fall into three large classes of X-, T-, and L- shaped quantum wells(wires) and superlattices made up of them [1,2]. Within each class, the objects inherit some general features conditioned by their topology. The problem of producing an object with desirable properties is a great challenge for band structure engineering. In this work we propose a new method for analytical treatment of carrier spectrum problem for such objects. The approach is based on non-smooth transformations, first applied for pulse exited systems [3].

            It has been shown that Schrödinger equation with potential (1) has analytical solutions. These solutions appear to be interesting themselves, giving eigen-values and eigen-functions for two crossing quantum wires or three-terminal T-shaped quantum knot generated by e_s, for instance. Another example is a quantum well grown on the cleaved edge of a superlattice – a pattern, generated by e_s and e_p and had been investigated numerically earlier [4]. Besides, analytical solutions available now grant us an unprecedented opportunity for wave-function and phonon engineering in common case of complex heterostructures of 2D and 3D geometry.

Fig1. The three patterns for potential V(x,y): T-shaped single quantum knot, 2D antidot array, and 2D dot array.

1.    Madhud Menon, Antonis N. Andriotis, and Deepak Srivastava. Phys. Rev. Lett. 91 (2003), p.145501

2.    L. Pfeiffer, K.W. West, H.L. Stormer, J.P. Eisenstein, K.W. Baldwin, D. Gershoni and J. Spector Appl. Phys. Lett. 56 (1990), p. 1697

3.    V.N. Philipchuk. Jornal of Sound and Vibration 249 (2) (2002), pp. 399-404

4.    Z. S. Gribnikov, R. R. Bashirov, H. Eisele, V. V. Mitin, and G. I. Haddad. J. Appl. Phys. 93 (2003), p. 330

10:30 PM Andreas Othonos

²IMEL/NCSR Demokritos, Terma Patriarchou Grigoriou, Aghia Paraskevi, 153 10 Athens-Greece

We have studied ultrafast carrier dynamics in oxidized silicon nanocrystals (NCs) and the role that surface-related states play to the various relaxation mechanisms over a broad range of photon excitation energy corresponding to energy levels below and above the direct bandgap of the formed NCs. Transient photoinduced absorption techniques have been employed to investigate the effects of surface-related states on the relaxation dynamics of photogenerated carriers in 2.8 nm oxidized silicon NCs. Independent of the excitation photon energy, non-degenerate measurements reveal several distinct relaxation regions corresponding to relaxation of photoexcited carriers from the initial excited states, the lowest indirect states and the surface-related states. Furthermore, degenerate and non-degenerate measurements at difference excitation fluences reveal a linear dependence of the maximum of the photoinduced absorption signal and an identical decay suggesting that Auger recombination does not play a significant role in these nanostructures even for fluence generating up to 20 carriers/NCs.

10:45 AM David B. Hayrapetyan

Karen G. Dvoyan¹, David B. Hayrapetyan^1,2, and Eduard M. Kazaryan<sup>1

1</sup>Dept. of Applied Physics and Engineering, Russian-Armenian State University, Yerevan, Armenia

 ²Dept. of Physics, State Engineering University of Armenia, Yerevan, Armenia 

            Within the framework of adiabatic approximation the energy levels and direct interband light absorption in a strongly prolated ellipsoidal quantum dot are studied. Analytical expressions for the particle energy spectrum and absorption threshold frequencies in strong, intermadiate and weak regimes of size quantization are obtained. Selection rules for corresponding quantum transitions are revealed. Absorption edge and absorption coefficient for three regimes of size quantization are also considered. To facilitate the comparison of obtained results with the probable experimental data, size dispersion distribution of growing quantum dots by the small semiaxe in the regimes of strong and weak size quantization by two experimentally realizing distribution functions have been taken into account. Distribution functions of Lifshits-Slezov and Gaussian have been considered.

(Invited) Antibonding hole ground state in artificial molecules

11:00 AM Juan Ignacio Climente

J.I.Climente^1,2, M. Korkusinski³ , M.F. Doty⁴, M. Scheibner⁴, A.S. Bracker⁴, D. Gammon⁴ and P. Hawrylak³

¹ National Research Center S3, CNR-INFM, Modena, Italy;

² Departament de Química Física i Analítica, Universitat Jaume I, Castellon, Spain

³ Institute of Microstructural Sciences, National Research Council, Ottawa, Canada

⁴ Naval Research Laboratory, Washington, USA

 Resonant tunneling of carriers between vertically coupled quantum dots enables the formation of hybridized, molecular-like orbitals which are important in many quantum dot-based devices, including those aiming at optically-controlled quantum information storage.[1] The differences in size and composition of quantum dots is overcome by the application of the vertical electric field, which brings the two quantum dot levels into resonance and induces either electron or hole tunneling.[2] The tunneling of electrons is now well understood[3], it leads to the formation of  bonding molecular ground states in analogy to natural diatomic molecules. However, tunneling of holes does not have a counterpart in diatomic molecules and is less understood. In fact, previous atomistic calculations suggested a reversal of bonding and antibonding hole molecular ground states as the interdot barrier distance increases.[4-6]

In this work, we present theory and experimental observation of the formation of the antibonding hole molecular ground state. Using a 4-band k·p approximation, the hole states are described as Luttinger spinors[7], which contain all the relevant symmetries. It is shown that the strong spin-orbit interaction in the valence band breaks the parity in the growth direction, mixing bonding and antibonding heavy- and light-hole components of the spinor. This mixing destabilizes (stabilizes) the otherwise pure bonding (antibonding) states, leading to the state reversal. Molecular ground states are then found to have  up to ~95% antibonding character. These conclusions are reproduced by numerical, atomistic multi-million-atom calculations using a sp³d⁵s^* tight-binding model applied to the realistic self-assembled InGaAs/GaAs double quantum dot structures, including strain, structural asymmetries and vertical electric fields. The results are in qualitative agreement with the k·p theory and predict a bonding-to-antibonding ground state reversal at interdot distances of d»2 nm. Clear experimental evidence of this peculiar hole behavior is found in magneto-photoluminescence experiments of double dots. The character of the hole molecular orbitals is identified from the electric field dependence of the Zeeman splitting of the neutral exciton when resonant hole tunneling is induced[8]. Comparison of samples with different inter-dot separation shows the bonding-to-antibonding ground state reversal in agreement with theory.[9]

1. M. Bayer et al., Science 291, 451 (2001); E.A. Stinaff et al., ibid 311, 636 (2006).

2. A.S. Bracker et al., Appl. Phys. Lett. 89, 233110 (2006).

3. H.J. Krenner et al., Phys. Rev. Lett. 94, 057402 (2005); G. Ortner et al. ibid 94, 157401 (2005).

4. W. Jaskolski, Acta Phys.Pol. A 106, 193 (2004); Phys. Rev. B 74, 195339 (2006).

5. G. Bester et al., Phys. Rev. Lett.  93, 047401 (2004); Phys. Rev. B 71, 075325 (2005).

6. M. Korkusinski et al., Proceedings of the 27^th Int. Conf. Phys. Semicond., 685 (2005).

7. J.M. Luttinger, and W. Kohn, Phys. Rev. 97, 869 (1955). L.Rego et al . Phys. Rev. B 55, 15694 (1997).

8. M.F. Doty et al., Phys. Rev. Lett. 97, 197202 (2006).

9. M.F. Doty et al. arXiv:0804.3097 (2008).

11:30 AM Elisabetta Romano

G. F. Cerofolini¹, D. Narducci¹, Elisabetta Romano¹, P. Amato²

¹CNISM and Department of Materials Science, University of Milano-Bicocca, Milano, Italy

²Numonyx, Agrate Brianza (MI), Italy

Self-similar patterns are frequently observed in Nature, from cauliflower to the system of blood vessels. [1,2] Their reproduction is possible on a 10² -10⁵ nm length scale with lithographic methods, but seems impossible on the nanometer length scale. Here, it is shown that this goal may be achieved via a multiplicative variant (referred to as SⁿPT_x) [3] of the multi-spacer patterning technology (SⁿPT), in this way permitting the controlled preparation of fractal surfaces. Actually, for any assigned simple pattern with sufficiently high aspect ratio, SⁿPT_x allows the multiplication by a factor of 2 of the same pattern with a reduced width. Reiterating n times the process, this technique allows the production of 2ⁿ patterns with much smaller width than their original size—the repetition of 3 SPT_x cycles could reduce the spacer width from the lithographic width of 0.1 μm to the very nanometric size of 12.5 nm, with minimum separation of 25 nm (Figure 1).

In future, SⁿPT_x crossbars embedded in a complementary metal-oxide-semiconductor (CMOS) circuitry may be used for molecular electronics, [4] allowing the tera scale integration of the integrated circuit technology. Moreover, other interesting applications could be the highly parallel and real-time sensing of single cells and the preparation of super-hydrophobic surfaces, exploiting the ability of self-similar structures to impart the desired roughness to surfaces.

Fig1. Plan-view of the crossbar structure which can be obtained via S³PT_x: the light-blue lines are the two crossing seeds, originally defined by lithography; the pink squares are the cross-points of the final crossbar.

1.    Y. Gazit, D. A. Berk, M. Leunig, L. T. Baxter, and R. K. Jain, Phys. Rev. Lett. 75, 2428 (1995).

2.    B. B. Mandelbrot, The Fractal Geometry of Nature (Freeman: New York), (1982).

3.    G. F. Cerofolini, G. Arena, M. Camalleri, C. Galati, S. Reina, L. Renna, D. Mascolo, and V. Nosik, Microelectr. Eng. 81, 405 (2005).

4.    G. F. Cerofolini and E. Romano, Appl. Phys. A  91, 181 (2008).

Fabrication of large area periodic nanostructures using Nanosphere Photolithography

11:45 AM Wei WU

Wei WU, Dibyendu Dey, Alex Katsnelson, Omer G. Memis, Hooman Mohseni

EECS Department, Northwestern University, Evanston, IL, USA

[2] S. M. Weekes, F. Y. Ogrin, and W. A. Murray, Langmuir 20, 11208 (2004).

[3] A. G. Brolo, E. Arctander, R. Gordon, B. Leathem and K. L. Kavanagh, Nano Lett. 4, 2015 (2004).

[4] W. Wu, D. Dey, O. G. Memis, A. Katnelson and H. Mohsen, Nanoscale Res. Lett. 3, 123 (2008).

12:00 PM Hui (Claire) Xiong

7/28 Monday - Lasers and Sound Waves for Nano-Synthesis and Characterization

12:15 PM Makaiko L. Chithambo

Department of Physics and Electronics, Rhodes University, PO BOX 94, Grahamstown 6140, South Africa

Optically stimulated luminescence is the luminescence that is emitted from a previously irradiated material, usually an insulator, when it is exposed to light of certain wavelengths.  The phenomena is exemplified in quartz (SiO₂) in its natural and synthetic forms where stimulation using light of wavelengths within 400 - 600 nm produces luminescence over the wavelength range 300 - 600 nm.  The basis of optically stimulated luminescence is that certain extrinsic or intrinsic defects in materials can act as charge traps or luminescence sites.  Ionizing radiation can transfer electrons or holes to such sites.  The trapped electrons can then be released by optical energy provided by the stimulating light. The subsequent radiative recombination of the released electrons with trapped holes can then be recorded as a time-dependent signal otherwise termed optically stimulated luminescence.  The aim of time-resolved optical stimulation is to separate in time the stimulation and emission  of luminescence.  The method produces a time-resolved spectrum consisting of two parts, the signal during stimulation and the luminescence after the light pulse.  Each spectrum can be resolved into components with distinct principal and secondary lifetimes.  This presentation develops conceptual models for analysis of spectra by comparing experimental results and computer simulations of time-resolved spectra.  The evaluation of principal and subsidiary luminescence lifetimes from time-resolved spectra measured during or after pulsed optical stimulation is illustrated as are various methods for calculating the activation energies for thermal quenching, thermal assistance and the frequency factor for the thermal quenching process using the temperature dependence of both luminescence lifetimes and luminescence intensity.  The notion of the luminescence throughput, that is, the amount of luminescence measured after the light pulse of duration as a proportion of the total signal will then be used to examine cases where time-resolved optical stimulation may achieve an improvement over steady-state optical stimulation. 

12:45 PM Gift KATUMBA

G. Katumba¹, B Mwakikunga² and T.R. Mothibinyane¹

¹ CSIR – National Laser Centre, Building 46A, P O Box 395 Pretoria 0001, South Africa.

² CSIR – National Centre for Nano-structured materials, Building 19B, P O Box 395, Pretoria 0001, South Africa.

 Selective solar absorber coatings of carbon nanoparticles dispersed in SiO₂, ZnO and NiO matrices on aluminium substrates have been fabricated by a sol-gel technique. Spectrophotometry was used to determine the solar absorptance and the thermal emittance of the composite coatings with a view to apply these as selective solar absorbers.  Raman spectroscopy was used investigate the properties of the composite coatings.  Cross-sectional high resolution transmission electron microscopy (X-HRTEM) was used to study the fine structure of the samples.  Chemical composition analysis was done by energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS).  The crystal structure of the samples was also investigated with X-ray diffraction (XRD) technique.  X-HRTEM, EDS and EELS studies revealed a nanometric grain size for all types of samples.  The C-SiO₂, C-ZnO and C-NiO coatings contained amorphous carbon nanoparticles embedded in nano-crystalline SiO₂, ZnO and NiO matrices, respectively.  Selected area electron diffraction (SAED) showed that a small amount of Ni grains of 30 nm diameter also existed in the NiO matrix.  The thermal emittances of the samples were 10% for SiO₂,  6% for the ZnO and 4% for the NiO matrix materials.  The solar absorptances were 95%, 71% and 84% for the SiO₂, ZnO and NiO matrix samples, respectively.  Based on these results, C-NiO samples proved to have the best solar selectivity behaviour followed by the C-ZnO, and last were the C-SiO₂ samples.

Fig1:  X-HRTEM image of a C-SiO₂ sample.

1. G Katumba, L Olumekor, A Forbes, G Makiwa, B Mwakikunga, J Lu and E Wackelgard, Optical, thermal and structural characteristics of carbon nanoparticles embedded in ZnO and NiO as selective solar absorbers, Solar Energy Materials & Solar  Cells, in press.

2. G Katumba, J Lu, L Olumekor, G Westin and E Wackelgard, Low cost selective solar absorber coatings:  characteristics of carbon-in-silica synthesized by sol-gel technique, J. Sol-Gel Science and Technology, 36, 33-43 (2005).

01:15 PM Mahmoud Mahmoud

Mahmoud Mahmoud^*, Mostafa El-Sayed

Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 USA

Nanocatalysts that possess large amts. of atoms on sharp corners and edges and high indexed sites are known to be more catalytically active. [1] We report here on a novel yet simple method to synthesize in large yields a very active platinum nanocatalyst; the multiarmed nanostar single crystal.  We utilize a seed mediated method using tetrahedral nanoparticles that are also synthesized by a new and simple technique.  High-resoln. TEM shows that the nanostar has many arms, varying from a few to over 30, whereby even the largest ones are found to have single-crystal structures.  This strongly suggests that they are formed by a growth mechanism of the seed crystals and not by the aggregation of seed crystals, which should produce twinning planes.  Due to the redn. reaction of ferricyanide by thiosulfate, the nanostars are found to have an activation energy, [2] which is nearly 60% of that of the tetrahedral seeds themselves, both having the same PVP capping agent.  This is undoubtedly due to the multiarms with edges, corners, and the presence of high indexed facets in the nanostar catalyst.

Silver nanocubes (72 nm) and Gold nanocages (75 nm) were assembled by varying the average interstitial particle distances by the Langmuir Blodgett technique. [3] The shape of the surface pressure-area isotherms for the two types of nanoparticles are discussed in terms of the degree of surface capping of each nanoparticle which depends on the surface quality. Monolayer films formed at different surface pressures were transferred by the vertical dipping method to silicon and glass substrates where their optical spectra and SEM images were examined. A red-shift of the strong plasmon peak maximum was observed as the average particle distance was decreased. From the SEM images, the morphology as well as the percent area of the substrate surface covered at different pressures was determined. A linear relationship was observed between the percent of covered area and the plasmon peak maximum the slope of which reflects the average plasmon field felt by the nanoparticles. The increase in the average field with increasing percent covered area is a result of the decrease in the average nanoparticles separation and/or the increase in the number of nanoparticles in each cluster in the distribution formed. The strength of the plasmon field resulting from gold cages is found to be much higher than that from the silver cubes, and increases as the gold wall thickness decreases. These results are discussed in terms of the coupling between the two separate fields of the cage wall surfaces.

1. M. A. Mahmoud et al. J. Am. Chem. Soc. 130, 4590. (2008),

2. M. A. Mahmoud and M. A. El-Sayed J. Phys. Chem. C 111, 17180, (2007),

3. M. A. Mahmoud and M. A. El-Sayed J. Phys. Chem. C (2008) (in press).

01:45 PM Bonex W. Mwakikunga

B. W. Mwakikunga^1,2,3, A. Forbes^4,5, E. Sideras-Haddad¹, C. J. Arendse¹

¹Council for Scientific and Industrial Research, Nanoscience Research Group, Materials Science and Manufacturing, P.O. Box 395, Pretoria 0001, South Africa

Email: bmwakikunga@csir.co.za

²School of Physics, University of the Witwatersrand, Private Bag 3, P.O. Wits 2050, Johannesburg, South Africa

³Department of Physics and Biochemical Sciences, University of Malawi, The Polytechnic, Private Bag 303, Chichiri, Blantyre 0003, Malawi

⁴Council for Scientific and Industrial Research, National Laser Centre, P.O. Box 395, Pretoria 0001, South Africa

⁵School of Physics, University of Kwazulu-Natal, Private Bag X54001, Durban 4000, South Africa

We show that oxygen carrier gas gives a higher yield of WO₃ nanowires by laser pyrolysis than acetylene. The latter also shows trace amounts of multi-walled carbon nanotubes. The solid-vapour-solid mechanism [1] is found to be the possible mechanism that explains the manner of growth of the nano–wires. The data on length and diameter of the nano-wires leads to the following conclusion: the bigger the diameter of the nanowires, the longer it is and the smaller the diameter the longer. This is in sharp contrast with the VLS  growth mechanism [2] which requires faster growth for large diameter 1D structures. The theory of the SVS growth mechanism is developed from basic statistical mechanics and validated with experimental data.

Fig. 1 Scatter plots of (a) length of the WO₃ nano–wire versus the corresponding diameter (b) length v.s 1/D² (c) aspect ration (L/D) vs diameter and (d) aspect ratio (L/D) vs 1/D³. The linearized plots (b) and (d) have the similar slopes within experimental error as predicted by our theory [(6.22 ± 2.77)´ 10^-20 m³ and (6.25±0.831)´10^-20 m³ respectively]

1. B. W. Mwakikunga, E. Sideras-Haddad, A. Forbes, C. Arendse, M. J. Witcomb   J. Nanosci. & Nanotechnol (2008) in press

2. R. S. Wagner, W. S. Ellis, Appl. Phys. Lett. 4, 89 (1964)

7/28 Monday - Conference Close with Keynote

2:00 PM Gehan Amaratunga

[1] “Polymer Photovoltaic Cells: Enhanced Efficiency via a network of internal donor-acceptor junctions” G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heege, Science, 270, 1759 (1995).

[2] “Single-wall carbon nanotube conjugated polymer photovoltaic devices” E. Kymakis and G. A. J. Amaratunga , App. Phys. Letts.. 80, 112 (2002).

[3] “High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends”

G Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery and Y. Yang, , Nature Mat. 4 864(2005) 864.