Conference Programme

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Overview
Session
B-02: Symp B
Time:
Monday, 19/Jun/2017:
4:00pm - 6:15pm

Session Chair: Stephen John Sweeney, University of Surrey
Location: Rm 300

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Presentations
4:00pm - 4:30pm
Invited

Novel Semiconductor Nanowires for Energy and Photodetector Applications

Chennupati JAGADISH

Australian National University, Australia

In this talk, I will introduce III-V/II-V compound semiconductor nanowires and discuss about the suitability of these nanowires and nanostructures for energy applications. I will discuss the role of light trapping in nanowires for solar cells applications. I will present the challenges associated with fabricating nanowire solar cells. I will discuss the requirements for artificial photosynthesis and how nanowires could be used in photoelectrochemical cell for water splitting to create clean hydrogen as an energy source. I will also discuss about use of nanowires for THz detectors and how to improve their performance. I will introduce the role of ordered nanowires in engineering neuronal networks and neuronal function in the engineered nanowires.


4:30pm - 5:00pm
Invited

Full-Spectrum Nanowire Light-Emitters on Nonconventional Substrates

Boon OOI, Chao ZHAO, Bilal JANJUA, Tienkhee NG

King Abdullah University of Science and Technology, Photonics Laboratory, Saudi Arabia

Group-III materials are the basis of solid-state lighting (SSL) because their emission can cover the whole visible spectrum. However, the planar nitride thin film used for commercial light-emitting diodes (LEDs) is typically grown on sapphire substrates, which results in high threading dislocation densities because of their thermal- and lattice-mismatch. Moreover, the degraded crystal quality, increased piezoelectric field lead to decreasing quantum efficiency as the indium composition higher than 25%, forming the “green-yellow gap” for LEDs. The roll-over of quantum efficiency under high injection current, called “efficiency droop”, also becomes pronounced with increasing indium composition. In the past, GaN and SiC bulk substrates have been used to address these issues at the expense of high material costs. Recently, Group III-nitride vertically aligned nanowires and devices grown on silicon have been investigated, due to their numerous advantages over planar epitaxial LEDs, such as a reduced polarization field, wavelength tunability by increasing indium composition, and light extraction. However, the high density of surface states due to the large specific surface limited the quantum efficiency and output power of nanowire devices. The carrier and phonon confinement in these nanowires also led to reduced heat dissipation and high junction temperature. In this paper, we will report our results on nanowires LEDs grown on nonconventional substrates emitting at ultraviolet-to-near infrared wavelength range using molecular beam epitaxy (MBE). The approach in surface states passivation in InGaN/GaN quantum-disk in nanowires LEDs, as well as low self-heating by growing nanowires on metal substrates, will also be presented, which demonstrated the potential of nanowires emitters for versatile applications.


5:00pm - 5:30pm
Invited

Carbon Nitride Films from Vacuum Free Depositions for Photoelectrochemical Applications

Ruiqin ZHANG, Juncai BIAN, Chao HUANG

City University of Hong Kong, Hong Kong S.A.R. (China)

Graphitic carbon nitride (g–CN) are promising materials for applications as photocatalysts for fuel conversion, degradation of organic pollutants, bioimaging and sensing, owing to their moderate band gaps, low cost and nontoxicity. In this presentation, We shall first describe their electronic and optical properties derived from both computations [1] and experimental studies [2]. We shall then introduce our work on synthesising g-CN films using a thermal vapor condensation method which is vacuum free and g-CN applications in photoelectrochemical (PEC) H2 generation. The g-CN film shows photocurrent density as high as 0.12 mA cm-2, the highest to date for g–CN based photoanode, at the bias of 1.35 V vs reversible hydrogen electrode with Na2S as the sacrificial reagent [3]. Further, by copolymerizing 2, 6-Diaminopyridine molecules with melamine, C domains are incorporated into g-CN to adjust the electronic properties of g-CN, leading to significantly improved optical, electronic and PEC properties of the modified CN films. The optimal modified CN film exhibits a photocurrent density four times higher than the photocurrent density exhibited by the pristine g-CN film [4].

References:

[1] X. Ma, Y. Lv, J. Xu, Y. Liu, R.Q. Zhang, and Y. Zhu, “A Strategy of Enhancing the Photoactivity of g-C3N4 via Doping of Nonmetal Elements: A First-Principles Study”, Journal of Physical Chemistry C, 116, pp 23485–23493 (2012)

[2] J. Bian, J. Li, S. Kalytchuk, Y. Wang, Q. Li, T. C. Lau, T. A. Niehaus, A. L. Rogach and R.Q. Zhang, “Efficient Emission Facilitated by Multiple Energy Level Transitions in Uniform Graphitic Carbon Nitride Films Deposited by Thermal Vapor Condensation”, ChemPhysChem, 16, 954 –959 (2015)

[3] J. Bian, Q. Li, C. Huang, J. Li, Y. Guo, M. Zaw, R.Q. Zhang, “Thermal Vapor Condensation of Uniform Graphitic Carbon Nitride Films with Remarkable Photocurrent Density for Photoelectrochemical Applications”, Nano Energy, 15, Pages 353–361 (2015)

[4] J. Bian, L. Xi, J. Xu, C. Huang, A.L. Kathrin, M. Antonietti, R.Q. Zhang, M. Shalom, “Efficiency Enhancement of Carbon Nitride Photoelectrochemical Cells via Tailored Monomers Design”, Advanced Energy Materials, 1600263 (2016)


5:30pm - 5:45pm
Oral

Multistage Growth of Monocrystalline ZnO Nanowires and Twin-nanorods: Oriented Attachment and Role of the Spontaneous Polarization Force

Baolu FAN, Yumeng ZHANG, Ruolin YAN, Jiyang FAN

Southeast University, China

Our understanding of crystal growth mechanisms has changed deeply in the past few decades. Particularly, the oriented attachment of intermediate nanoparticles has been accepted to be a crucial crystal growth mechanism that is distinct from the traditional one involving nucleation and Ostwald ripening.However the details of the oriented attachment process are not readily observed experimentally, and little is known about the driving force and the dynamics involved in oriented attachment. In this respect, ZnO is an ideal material because it possesses strong spontaneous polarization which may easily drive oriented attachment during its crystal growth. We study experimentally and theoretically the complete crystal growth process (from primitive amorphous nanoclusters to ultimate single nanocrystals) of one-dimensional ZnO nanocrystals growing in water/ethanol at high temperatures. The results reveal that both axial (along the direction of the polarization axis) and lateral oriented attachment of the intermediates occurs during the growth process of the one-dimensional ZnO nanocrystals. Calculation based on the force and interaction model reveals that the axial oriented attachment driven by the spontaneous polarization force dominates the crystal growth of ZnO nanocrystals, and the van der Waals force also plays a role in driving oriented attachment. The study shows that oriented attachment of intermediate nanoparticle ensembles induces formation of the symmetric twin-nanorods. These results improve our understanding of the growth mechanism of nanocrystals in a liquid medium.


5:45pm - 6:00pm
Oral

Optical Investigation of Wurtzite Structured Silicon Nanowires

Saurabh DIXIT, A. K. SHUKLA

Indian Institute of Technology Delhi, India

Wurtzite structured silicon nanowires (w-SiNWs) has been attracted immense research interest since last decade. In this work, w-SiNWs are synthesized using metal assisted chemical etching (MACE) of p-type silicon substrate. Presence of wurtzite structured SiNWs is revealed by first-order and second-order Raman spectra. Variation of etching durations of silicon substrates is explicitly studied using scanning electron microscope (SEM). Furthermore, absorption studies are performed for different etching durations in the visible and NIR region. Enhancement in absorption coefficient as a function of etching duration is reported here. The quantum confinement effect along with surface stress is mainly responsible for formation of stable w-SiNWs.


6:00pm - 6:15pm
Oral

Size Dependent Tunnel Diode Effects in Gold Tipped CdSe Nanodumbbells

Deepashri SARAF1, Ashok KUMAR2, Dilip Govind KANHERE1, Anjali KSHIRSAGAR1

1Savitribai Phule Pune University, India; 2Central University of Punjab, India

In a recent work [1] on Nano dumbells of CdSe, it is shown that as a consequence of gold clusters at the ends as electrodes, there are induced gap states on CdSe nanorods that are highly localized. Furthermore, the electronic structure characteristics are shown to have different behaviour as a function of length. This warranted an investigation, particularly, for response to electric field. As we demonstrate in this work, the tunneling characteristics display length dependent Negative differential resistance (NDR) effects.

In this work we [2] report simulation results for scanning tunneling spectroscopy of gold-tipped CdSe nanodumbbells of lengths ∼27 Å and ∼78 Å. First, fully relaxed ab initio calculation is crarried out with gold clusters as electrodes at both the ends. Then Bardeen, Tersoff, and Hamann formalism is employed for computing tunneling current, which takes inputs from ab initio calculations. For the shorter nanodumbbell, the current-voltage curves reveal negative differential conductance, the characteristic of a tunnel diode. This behaviour is attributed to highly localized metal induced gap states that rapidly decay towards the center of the nanodumbbell leading to suppression in tunneling. In the longer nanodumbbell, these gap states are absent in the central region, as a consequence of which zero tunneling current is observed in that region. The overall current-voltage characteristics for this nanodumbbell are observed to be largely linear near the metal-semiconductor interface and become rectifying at the central region, the nature being similar to its parent nanorod. The cross-sectional heights of these nanodumbbells also show bias-dependence where we begin to observe giant Stark effect features in the semiconducting central region of the longer nanodumbbell.

References:

[1] D. Saraf and A. Kshirsagar, Phys. Chem. Chem. Phys. 16, 10823 (2014).

[2] Work based on accepted manuscript :Journal of Chemical Physics, Feb 2017.



 
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