Architectural Photonics: A New Concept for Enhancing Opto-electronic Response in Material

1. Architectural Photonics:A New Concept for Enhancing Opto-electronic Response in Material Jung Y. Huang 黃中垚(http://www.jyhuang.idv.tw) Department of Photonics, Chiao Tung University August 6, 2009

2. An Overview • Nanotechnologycould serve as the technology platform of the second quantum revolution • Nanotechnology is dealing withfunctional systemsbased on the use of interactingsubunits with specific size-dependent quantum mechanical properties. • The subunits are combined in a direct manner toform a hierarchically organized structure on different levels of complexity.

3. An Overview • Current scientific research aims at the exploration of the collectivity of structures with dimensions between 1 nm and 100 nm(建構奈米組件). • Technologies offering access to these dimensions, forstructuring(製造),manipulating (操控),ormeasuring(量測)at high resolution, are strongly demanded.

4. An Overview Note that At the nanometer scales, the ratio of the numbers of atoms at the surface and in the bulk of a material increases rapidly. Interfacial properties of a nanostructured material could, therefore, enable new functional materials or devices. This is the new dimension we are exploring.

5. A profound question We are concerning how to tailor an existing material for an improved performance in a specific application. In this talk, an architectural photonics, a new concept for enhancing opto-electronic response in material is proposed, which is useful for optical detection and photovoltaic application.

6. Our model system of investigation Electronic Structure of nc-Si/MS • Silicon nanocrystals embedded in SiO2 behave like a "nutshell" of an internal core, an interface region, and the surroundings embedding matrix. • The electronic states close to the band edges are related to the NC core and partially to the O atoms at the interface. The states far from the band edges are related to the silica host. silica matrix

7. Optical Properties of nc-Si/MS The QDs in an ensemble have different sizes, resulting in a distribution of the quantum-confinement energy spacing. If there are no other nonradiative couplings, photo-excited carriers in anyQD eventually relax to the lowest levels and recombine togeneratea PL spectrum of a broad distribution of the lowest optical transition.

8. PL Characteristics of nc-Si/MS as grown nc-Si/MS annealed • Si nanocrystals sensitize the photo-generation of carriers, which are trapped in the interfacial defects and then recombine to yield the 460-nm PL. • After thermal annealing, the peak disappeared while a new luminescence band around 700 nm (depending on the size of nc-Si) was observed.

9. Optical Properties of nc-Si/MS • In a bulk semiconductor, an excited electrondelocalizes in a large volume. The number of vibrational modes (N) involved is the order of the number of atoms in the affected zone. Thus, the displacements of vibrational modes in excited states shall be very small that require the vibrational wave functions between the ground state and excited states are orthogonal.The interband electronic transition is allowed only at the same k.

10. Optical Properties of nc-Si/MS Two mechanisms can relax the crystal momentum conservation criterion for an optical transition: • The dynamic strain induced by localized excited carriers can relax the crystal momentum conservation and lead to multiphonon processes, or • An electron localized in a tiny volume around a deep defect can make an indirect transition in accompany of multiphonon processes. The greater the charge density of the captured electron, the greater the force on the nearby ions and hence the larger the lattice relaxation.

11. PL Characteristics of nc-Si/MS • The Si QDs in the ensemble have different sizes. • If the excited carriers can efficiently couple to interfacial phonons before it relaxes, the PL should appear at a spectral position where the QD has a correct particle size for fast interfacial phonon-assisted relaxation, leading to a narrow peak shifted from the excitation line by the energy of the interfacial phonon.

12. Pulsed PLE of nc-Si/MS • The energy interval between the energy of excitation and peak emission (Stokes shift)increases monotonically. • The Stokes shift is larger than the exciton splitting energy (57 meV, 456 cm-1) of c-Si, indicating the large Stokes shift to be caused by exciton-interface phonon interactions (Si-O=1050 cm-1).

13. 480 nm ↑ Stokes Shift of nc-Si/MS ↑ • Since the Si-O bond is polar, the coupling of excitons and stretch vibrations of surface species increase with localization of excitons in smaller dimensions.

14. Unique finger-printing capability of vibrational spectroscopy : • highly localized • well characterized by theory Vibrational Spectroscopy

15. Apparatus of sum-frequency vibrational spectroscopy (SFVS)---the Laser System

16. Probing interfacial bonding structure of nc Si/MS with SFVS • Broad resonance was observed between 2300 cm-1 and 1800 cm-1, attributed to the overtone (2100 cm-1) of Si-O stretch mode. • The broad resonance feature reveals the chemical bonds to be fairly complex at the interfaces of nc-Si and MS.

17. Polarization Switching of nc Si/MS • The polar structure at the interfaces in nc-Si/MS yields an electric polarization, whose direction is switchable with an electric field to displace the centers of gravity of the positive and negative charge distributions. • The measured remnant polarization (@ E=0) corresponds to a dipole moment of 4.2x10-18C-cm for each one-side bonded Si nanocrystal.

18. Polarization Switching of nc Si/MS in a MOSFET • A MOSFET device with a gate structure of Al/SiO2/nc-Si-in-MS/SiO2 reveals a hysteretic switching property. • A polarization-induced memory window of 5V, very low gate leakage and high ratio of the “on” state to the “off” state are demonstrated.

19. ImprovedPhoto-responsivity in Visible withnc Si/MS nc Si/MS PD MOS PD

20. ImprovedPhoto-responsivity of Photodetector in NIR MOSFET PD Hole trapping in the illuminated gate produces a built-in potential that adds to an external bias (Vg) to generate a negative shift (Vth) of threshold voltage, therefore, yields a higher drain current.

21. ImprovedPhoto-responsivity in NIR withnc Si/MS

22. ImprovedPhoto-responsivity in NIR withnc Si/MS

23. Self-assembled Crystal of Core-Shell Nanoparticles A strategy to realize 3D artificial crystal with both optical and magnetic activities: • Au nano shell provides a strong coupling with optical field, yielding an enhanced optical response via surface plasmon excitation. • Nanocrystals with Fe core is invoked to sense the magnetic response.

24. Self-assembled Crystal of Fe-Au Core-Shell Nanoparticles • Synthesis of the Fe–Au NPs using reverse micelles in which the dispersity in the sizes of these NPs can be well controlled. • Cetyltrimetylammonium bromide (CTAB) and tetraethylorthosilicate (TEOS) are added to form a self-assembled Fe–Au artificial crystal.

25. Self-assembled Crystal of Fe-Au Core-Shell Nanoparticles • One of the challenges in synthesizing Fe NPs is to prevent the Fe from oxidization. • A reduction of iron oxide-Au NPs in the presence of TEOS converts the S-Au/Fe oxide NPs into S-Au/Fe NPs.

26. Self-assembled Crystal of Fe-Au Core-Shell Nanoparticles • The Fe–Au artificial crystal exhibits a coercive field Hc = 70 Oe at 2 K with a hysteresis not the typical curve observed for ferromagnetism . • Fitting to the Langevin model yields µ = 400 B for the Fe–Au artificial crystal, revealing each Fe atom in single Fe–Au nanocrystal to have a magnetic moment of 0.44 B, about five times smaller than that observed in bulk Fe. Unfortunately, the distance between two closest nano particles is too large to allow the appearance of ferromagnetism.

27. TEM image of an array of 5-nm nc-Au capped with dodecanethiol. Sketch of a nc-CdSe capped with Sn2S64– ions TEM image of a three-dimensional superlattice of 5-nm nc-Au capped with (N2H5)4Sn2S6. Self-assembled Artificial Crystal of Nanoparticles • A solution to reduce the distance between nano particles in a self-assembled artificial crystal and increase the inter-particle coupling was proposed by using molecular metal chalcogenide surface ligands. M. V. Kovalenko, et al. Science 324, 1417 (2009)

28. Self-assembled Artificial Crystal of Nanoparticles Optical excitation in a semiconductor nanocrystal can delocalize in the artificial crystal. The artificial crystalline film was invoked to be the channel of a MOSFET. Combined functionality of transistor amplification and optical response was revealed.

29. Conclusions • A new concept of architectural photonicsis proposed to yield new functionality or improve opto-electronic response in a material. • Multifunctionality of nc SiQD/MS was demonstrated. • Both MOS PD and MOSFET PD made from nc SiQD/MSreveal high response in the visible and NIR regions. • New self assembly scheme becomes feasible to prepare an artificial crystal of nanoparticles with strong interparticle coupling. New opportunity is waited to be explored for a variety of applications from optical detection to PV.