Transcript Slides - Jung Y. Huang

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
An Overview
Nanotechnology could serve as the technology
platform of the second quantum revolution
Nanotechnology is dealing with functional systems based
on the use of interacting subunits with specific size-dependent
quantum mechanical properties.
The subunits are combined in a direct manner to form a
hierarchically organized structure on different levels of
complexity.
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, for
structuring (製造), manipulating (操控), or measuring (量
測) at high resolution, are strongly demanded.
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.
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.
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
Optical Properties of nc-Si/MS
The QDs in an ensemble
have different sizes, resulting in
a distribution of the quantumconfinement energy spacing.
If there are no other
nonradiative couplings, photoexcited carriers in any QD
eventually relax to the lowest
levels
and
recombine
to
generate a PL spectrum of a
broad distribution of the
lowest optical transition.
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.
Optical Properties of nc-Si/MS
 In a bulk semiconductor, an excited electron delocalizes 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.
Ee (Q )  Ee0  S Q
S  1 N  1 2 Q 2  eQ 2
 0 no phonon involved
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.
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.
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).
Stokes Shift of nc-Si/MS
↑
480 nm
↑
 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.
Vibrational
Spectroscopy
Unique finger-printing capability of
vibrational spectroscopy :
 highly localized
well characterized by theory
Apparatus of sum-frequency vibrational spectroscopy
(SFVS)---the Laser System
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.
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.
8 (a)
2
Polarization (C/cm )
 The measured remnant polarization (@ E=0) corresponds to a dipole
moment of 4.2x10-18 C-cm for each one-side bonded Si nanocrystal.
E
P
4
1
1
E
0
P
2
-4
-8
E
2
3
P
3
1 kHz
-1 0
1
Electric field (MV/cm)
Polarization Switching of nc Si/MS in a MOSFET
Off-state
0
-5
-9
10
Id (A)
-7
10
-1
-11
10
=0 sec
-4
0
0
Vg(V)
1
Vth(V)
10
-2
-13
10
4
(b) -3
On-state
2
3
10 10 10 10
Retention time (sec)
 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.
Improved Photo-responsivity in Visible with nc Si/MS
nc Si/MS PD
MOS PD
Improved Photo-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.
Drain current Id ( A)
6
dark
1310 nm
1550 nm
5
4
(a)
1310 nm
1550 nm
(b)
3
3 4 nW/m2
Vg=1.6V
2
2
1
1
4 nW/m2
Vg=1.6V
0
-1
Vg=1.2V
0
1
4
2
Vd (V)
3 0
1
2
Vd (V)
0
3
-1
Photoresponse ( A/W)
Improved Photo-responsivity in NIR with nc Si/MS
Improved Photo-responsivity in NIR with nc Si/MS
Photoresponse (A/W)
10
1
Si-QD MOSFET PDs
10
0
Si-QD MOS PDs
10
-1
10
-2
Si PDs
-3
10
300
600
900
1200 1500 1800
Wavelength (nm)
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.
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.
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 SAu/Fe oxide NPs into S-Au/Fe NPs.
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.
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.
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 threedimensional superlattice of
5-nm nc-Au
capped with (N2H5)4Sn2S6.
M. V. Kovalenko, et al.
Science 324, 1417 (2009)
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.
Conclusions
 A new
concept of architectural photonics is 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/MS
reveal 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.