Emitter Nanoparticles
M.Sc., Brown University, Providence, RI, USA, 2013
A Dissertation Submitted in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
in The Department of Physics at Brown University
PROVIDENCE, RHODE ISLAND
c© Copyright 2015 by Mingming Jiang
This dissertation by Mingming Jiang is accepted in its present form
by The Department of Physics as satisfying the
dissertation requirement for the degree of Doctor of Philosophy.
Date
Date
Date
Date
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Curriculum Vitae
Mingming Jiang was born on August 6, 1987 in Yangzhou, Jiangsu Province,
China. He entered Nanjing University in 2005 and received his Bachelor of Science
degree in Physics at 2009. During his undergraduate studies, he joined Prof. Jian-
min Zhu’s group working on an undergraduate student research project about the
fabrication and nanostructural analysis of multiferroic materials. In August 2009,
he was then admitted into the doctoral program in the Department of Physics at
Brown University. In 2010, he joined Prof. Rashid Zia’s laboratory and started to
work as a research assistant in the field of nanophotonics, focusing on self-assembly
of quantum emitters and their interactions with nanostructures. He received his M.
S. degree from the Department of Physics at Brown University in May 2013. He
became a full member of Sigma Xi (Scientific Research Society) in 2014.
iv
Acknowledgements
It has been almost six years from the first day I came to Brown University. I can still
remember that it was a lovely and sunny day. I was so lucky to meet a lot amazing
people at Brown. I want to thank all of them here.
First of all, I thank my adviser Prof. Rashid Zia, who is not only a great scholarly
mentor but also a helpful friend. With his guidance, encouragement and support, I
had a great learning and research experience in graduate school. I also learned a lot
as the personal treasure for my future life, from his intelligence, diligence and strong
sense of responsibility. It was my honor and luck to be his student.
Secondly, I thank my defense committee members, Prof. Arto V. Nurmikko and
Prof. Derek Stein, who were also on my preliminary exam committee. I want thank
them for their continual help and support during the past years for my thesis work,
which couldn’t be done without their inspiration. With their intelligent and valuable
comments, I developed a deeper understanding of this work and also improved my
thesis.
Thirdly, I thank my intelligent labmates Yana Cheng, Sebastien Cueff, Christo-
pher Dodson, Jonathan Kurvits, Sinan Karaveli and Dongfang Li. It was a great
and treasured experience to work with them. I learned a lot from the collaboration
and discussion with them. Especially, I thank Jonathan Kurvits for his great help
in improving my English and all the help on my thesis work. I thank Mr. Michael
v
Jibitsky, who taught me a lot useful techniques in the cleanroom which are critical
in my thesis work. I want to thank Mr. Anthony McCormick who showed my how
to use the TEM and SEM. I also thank Cuong Dang, Joonhee Lee, Son Le, Yao Lu
and Kwangdong Roh, who generously shared their nanofabrication experience and
knowledge of optics with me. I also want to thank Mrs. Barbara Dailey, who always
supported and helped me for both of my study and living at Brown.
I really thank all my friends I met at Brown, thanks a lot for sharing the pleasure
and pain with me during the past six years. I couldn’t be who I am without you.
I want to thank Lu Lu who I have known for ten years. I was lucky to have such
a friend and to be his roommate for these years. I want to thank Dongfang Li,
who is not only my labmate but also a great friend. We helped and supported each
other to survive from the tough time. I thank Wenyan Jiang, Kefei Lei, Libin Sun
and Shu Wang for all the meals they had with me. I thank Hao Tu, Zhen Ye and
Shaomin Zhang for the wonderful “Friday Hotpots”. I thank Jing Feng, Xinjun Guo
and Haoran Miao for all the joyful “Card Time”, and I thank Mingge Deng, Pei Liu,
Alex Loosley, Hong Zhang and Yifan Zhang for all the time they spent with me.
They are the most precious treasure in my life.
This work is dedicated to my beloved parents.
vi
Contents
Acknowledgments v
1 Introduction 1 1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Quantum Emitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 Nanoscale Positioning Methods . . . . . . . . . . . . . . . . . . . . . 5 1.4 Inorganic-Based Electrostatic Self-Assembly . . . . . . . . . . . . . . 9
2 Reusable Inorganic Template for Electrostatic Self-Assembly of Nanoparticles 15 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.2 Self-Assembly of Silica-Clad Quantum Dots . . . . . . . . . . . . . . 17
2.2.1 Silica-Cladding on Quantum Dots . . . . . . . . . . . . . . . . 17 2.2.2 Arrays of Silica-Clad Quantum Dots . . . . . . . . . . . . . . 19 2.2.3 Confocal Microscope for Fluorescence Measurement . . . . . . 22 2.2.4 Reusability of Inorganic Templates . . . . . . . . . . . . . . . 25
2.3 Extensions of Inorganic-Based Self-Assembly . . . . . . . . . . . . . . 28 2.3.1 QDs Self-Assembly on Other High IEP Templates . . . . . . . 28 2.3.2 Control Experiments with Low IEP Templates . . . . . . . . . 30 2.3.3 Self-Assembly of Upconversion Nanoparticles . . . . . . . . . . 31 2.3.4 Self-Assembly of NV center Nanodiamonds . . . . . . . . . . . 34
2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3 Photon Antibunching from Quantum Emitter Arrays 39 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.1.1 Intensity Autocorrelation Function . . . . . . . . . . . . . . . 40 3.1.2 g(2) of Quantum Dot . . . . . . . . . . . . . . . . . . . . . . . 42 3.1.3 g(2) of NV Center . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
vii
3.3 Photon Antibunching from QD . . . . . . . . . . . . . . . . . . . . . 49 3.4 Photon Antibunching from NV Center . . . . . . . . . . . . . . . . . 51 3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4 Integration of Quantum Emitters with Nanostructures 54 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.2 Silica-Clad QDs near Gold Nanorods . . . . . . . . . . . . . . . . . . 55 4.3 NV Center Nanodiamonds near Gold Nanorods . . . . . . . . . . . . 58 4.4 QDs Embedded in Dielectric Waveguides . . . . . . . . . . . . . . . . 61 4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5 Conclusion 64
A Dimension and Dipping Time Effect on the Coverage of QD-on-Pad 68
B Intensity Correlation Function for N Single Emitters 72
viii
A.1 QD coverage for different dimensions of pads . . . . . . . . . . . . . . 70
ix
List of Figures
1.1 Silica-clad QDs and the self-assembly process in the PDDA-based method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.2 The surface charge vs the pH values of its surrounding solvents . . . . 11 1.3 A novel self-assembly method of nanoparticles on substrate can be
achieved . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.1 Silica-clad QD growth . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.2 Scheme illustrating the fabrication and the self-assembly process for
the silica-clad QDs on Al2O3 pads arrays . . . . . . . . . . . . . . . . 20 2.3 Electrostatic self-assembly for the arrays of silica-clad QDs . . . . . . 21 2.4 Schematic for confocal scan microscope setup . . . . . . . . . . . . . 22 2.5 Spectrum of the commercial 620 nm QDs under Verdi pump . . . . . 23 2.6 The scan image is constructed by summing up all the “photon event
signals” between two adjacent “pixel signals” . . . . . . . . . . . . . . 24 2.7 Fluorescent imaging of a 40 µm × 40 µm of the QD arrays on a quartz
sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.8 Process of QDs re-deposition and cleaning . . . . . . . . . . . . . . . 25 2.9 The reusability experiment for Al2O3 pads arrays sample . . . . . . . 26 2.10 The QD deposition coverage for the one-month long test. . . . . . . . 27 2.11 Dark field microscope images of 160 nm silica-clad QD on the 150 nm
pads with a 2 µm pitch using different dielectric materials. . . . . . . 29 2.12 SEM images of SiO2 patterns . . . . . . . . . . . . . . . . . . . . . . 30 2.13 Left: Upconversion nanoparticle aggregations in some silica cladding
while nothing in others. Right: Upconversion nanoparticles with 40nm thick silica-cladding. . . . . . . . . . . . . . . . . . . . . . . . 31
2.14 Dark field image of a 40 µm×40 µm area of the arrays on a quartz sample. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.15 Fluorescent spectra of UCNPs under a pump power of 100 mW . . . 33 2.16 Confocal scan image of an area of 12 µm×12 µm on the same quartz
sample. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.17 Zeta potential of the diamond vs pH values of solution . . . . . . . . 34 2.18 Electrostatic self-assembly of NV center nanodiamonds on Al2O3 pads
arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.19 Spectra of NV centers under the 532 nm excitation. There is a small
peak around 637 nm which is its zero photon line (ZPL) . . . . . . . 36
x
2.20 Fluorescent imaging of a 40 µm × 40 µm of the nanodiamond arrays on a quartz sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.21 Dark field images showing the re-depositions of nanodiamonds on Al2O3 pad arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.1 A 2-level energy system. . . . . . . . . . . . . . . . . . . . . . . . . . 42 3.2 The intensity autocorrelation curves of single and cluster of CdSe/Zns
QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.3 A 3-level energy system, with the metastable state. . . . . . . . . . . 44 3.4 g(2) curves of NV centers . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.5 HBT setup combined with a confocal scan microscope . . . . . . . . . 47 3.6 The g(2) curve is built up by comparing the time difference of each
two photon event signals . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.7 Confocal scan image for a 12 µm × 12 µm area of QD arrays . . . . . 49 3.8 g(2) curves for all the bright sites in the scan image . . . . . . . . . . 50 3.9 Confocal scan image for a 12 µm × 12 µm area of the nanodiamond
arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.10 g(2) curves for all the bright sites in the scan image . . . . . . . . . . 52
4.1 Schematic of the two-step e-beam lithography method to fabricate the gold nanorod optical antennas with circular Al2O3 pads. . . . . . . . 56
4.2 Integration of the silica-clad QDs with various sizes gold nanorods in different positions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.3 Setup for the lifetime measurement of Silica-clad QDs with and with- out gold rods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.4 Lifetime measurements of QDs with and without near the gold nanorods. 58 4.5 The NV center nanodiamonds are positioned near the ends of the gold
nanorods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.6 Fluorescent spectra of gold nanorods with and without NV center
nanodiamond near. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.7 Lifetime measurements of nanodiamonds with and without adjacent
gold nanorods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.8 Schematic of the fabrication process of QD embedding in waveguides. 62 4.9 Fluorescent scanning image of the QD embedded in waveguides. . . . 62
A.1 There are almost no silica-clad QDs attracted on the 2.5 nm thick Al2O3 pads and film. . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
A.2 QD coverage for different dimensions of pads . . . . . . . . . . . . . . 70 A.3 QD coverage for different dipping time . . . . . . . . . . . . . . . . . 71
xi
Quantum emitters have received considerable attention as robust solid-state single
photon sources [1–4] in a wide variety of applications ranging from fundamental
studies of light-matter interactions [5–10] to emerging quantum information tech-
nologies [11, 12]. Their emission properties, such as spectral distribution, radiation
pattern and polarization, depend not only on the emitter’s intrinsic structure but
also its local optical environment. The latter can be tailored through integration of
emitters with optical nanostructures that modify the local density of optical states.
Therefore, a reliable and stable emitter-structure system is required to investigate
these characteristics. Furthermore, for quantum information processing, positioning
of individual or arrays of single photon sources needs to be achieved, while main-
taining compatibility for integration with nanostructures [13–15].
In this thesis, we present and investigate a new method to integrate quantum
emitters with optical nanostructures using reusable electrostatic self-assembly tem-
plates. This approach leverages the isoelectric point differences between dielectric
materials to generate Coulombic forces that help position individual emitters at pre-
defined locations on a patterned substrate. Using inorganic templates, we will show
how this method can enable repeated studies of the same nanostructures with dif-
ferent emitters and even different classes of emitters. The experimental setups and
analysis procedures developed to optimize and study these systems will be discussed.
This chapter provides the basic background for this research. We begin with a
discussion of the quantum emitters themselves and the specific emitter nanoparti-
cles that we studied. Then we discuss existing methods of positioning as well as
their relative strengths and weaknesses. Finally, we will focus on the electrostatic
2
self-assembly method and how we extended this technique to demonstrate reusable
inorganic templates.
1.2 Quantum Emitters
At the most fundamental level, the radiation of light by a quantum electronic sys-
tem usually occurs one photon at a time, and this is why the field of nanophotonics
generally uses the term “quantum emitter” to describe any discrete source. How-
ever, the physical nature of the quantum system, especially its level structure and
chemical host environment, help to distinguish most emitters in terms of important
properties such as their emission spectrum, photostability, and emitter density. Ide-
ally, we would like to be able to position nanoparticles that contain a single quantum
emitter, but there is a practical tradeoff between emitter density and photostability.
Therefore, in this thesis, we will investigate three different emitter systems: colloidal
semiconductor quantum dots (QDs), nitrogen-vacancy (NV) centers in nanodiamond,
and upconverting lanthanide-doped nanoparticles.
Colloidal semiconductor QDs are usually less than 10 nm in size, which results
in strong quantum confinement that determines its electronic structure. Radiative
transitions between these size-dependent energy levels produce emission lines that
can be tuned across the visible spectrum. As a result, QDs have been used in many
studies of light-matter interactions, and they are also becoming increasing popular in
commercial applications, such as television displays and solid-state lighting. In one
sense, QDs are ideal quantum emitter nanoparticles, because each individual QD
represents a single quantum emitter. However, QDs can get temporarily trapped
into dark states; this effect is referred to as blinking, because their emission appears
3
to blink on and off. Unfortunately, QDs also degrade with exposure to air and have
limited photostabilty. At room-temperature and modest pump rates, QDs often
photobleach. These effects can be mitigated to some extent by encapsulating QDs
within a protective polymer or inorganic matrix. Nevertheless, it is hard for QDs
to survive during long experimental studies, especially under continuous excitation
which accelerates their photobleaching. In this thesis, we will use core-multishell
QDs, which are known to exhibit relatively high quantum yields and good photosta-
bility. However, even these core-multishell QDs exhibit blinking and photobleaching,
and therefore, we also chose to examine more robust emitters.
Luminescent defects in solids, often called “color centers”, represent a more stable
class of emitters. For example, the NV center in diamond is a point defect, which
consists of a nitrogen atom substituting a carbon atom and a neighboring lattice
vacancy. The negatively charged NV center in diamond has received considerable
attention, because it exhibits spin-dependent photoluminescence and its spin state
can be optically addressed. Therefore, NV centers have become an important test
bed for spin-based quantum information processing, and there are also many studies
of single photon emission from NV centers. Compared to QDs, NV centers shows
very bright and stable single photon emission at room-temperature. (There are
some issues related to charge state changes in NV centers, but these are relatively
modest compared to QD blinking.) However, NV emission is generally broader
and more complicated than QD emission, because in addition to purely electronic
(so-called zero phonon line) emission, NV centers also exhibits very broad phonon
sidebands. These vibronic transitions involve electronic decay through both photons
and phonons creation, leading to longer wavelength emission. There is also an issue
with emitter density. While diamond nanoparticles with NV centers are commercially
available, these so-called “nanodiamonds” usually contain multiple defects. As a
4
result, additional screening is required to find a single NV center nanodiamond before
performing single emitter experiments.
Another type of “color center” are lanthanide dopants in solid-state hosts. Due
to the shielded 4f valence electron structure of lanthanide ions, these dopants often
behave as isolated, atomic-like systems. They have very well-defined multilevel elec-
tronic structures that exhibit very narrow emission lines with high photostability and
long lifetimes. The long lifetimes and multilevel electronic structure of lanthanide
ions make them ideally suited as upconversion materials. When pumped in the
near infrared, lanthanide doped upconverting nanoparticles (UCNPs) exhibit clear
visible emission with low background levels that could be especially important for
bioimaging applications [16] and solar cells [17]. As a result, there has been growing
interest in the development of very high-quality, regular-shaped UCNPs and their
integration with optical nanostructures [16,18–24]. However, it should be noted that
lanthanide-doped UCNPs are usually tens of nanometers in size, and therefore, a
single nanoparticle contains ∼105 lanthanide ions. Some tricks, such as resonance
fluorescence combined with monitoring fast 5d-4f emission lines, can be used to iden-
tify single lanthanide ions [25], but in general, the emission of even a single UCNPs
originates from a large ensemble of ions.
1.3 Nanoscale Positioning Methods
High specificity positioning of individual nanoparticles is a significant field of study in
itself. Given many recent advances, there are a wide range of existing methods that
can be (and have been) used to integrate emitter nanoparticles with optical nanopho-
tonic structures. These techniques can be loosely divided into three categories: repo-
5
sitioning, direct printing, and lithographic templating. As will be discussed below,
these approaches provide different degrees of scalability and resolution.
The most common repositioning technique is the dip-pen approach [26–28] which
uses an atomic force microscope (AFM) tip to manipulate and move the nanopar-
ticles to any desired positions on the substrate at a high-resolution. This approach
provides high resolution positioning, but it is sequential in nature and thus funda-
mentally slow. This method can only move one particle at a time, which makes the
positioning or integration with nanostructures of a large number of nanoparticles
very challenging.
A more scalable approach is direct printing of nanoparticles, including inkjet
printing [29], microcontact printing [30], and related nanoimprint methods [31].
While sequential inkjet printing is significantly faster than dip-pen methods, it has
lower resolution. However, microcontact and nanoimprint stamping are parallel ap-
proaches for printing at higher resolution. These methods use patterned stamps to
place nanoparticle “ink” at the desired locations across the sample. Although they
can achieve large scale positioning of particles, there are still some disadvantages.
For example, the nanoparticles must prefer to adhere to the substrate rather than
the patterned stamp, and therefore, often require chemical modification. The “ink”
droplets are also usually micron-size or larger and, therefore, cannot be readily used
to place single nanoparticles.
and scalability over a wide range of nanoparticle densities. Template-guided meth-
ods begin with the fabrication of pattern structures on the substrate and then use
either physical deposition and liftoff or selective self-assembly.
6
Pure physical templating does not require chemical functionalization. Using
electron-beam lithography, very small size openings (e.g., down to 10-20 nm) can
be readily defined in a PMMA resist layer. Then the nanoparticle emitters such as
colloidal QDs are spin-coated or drop-cast onto the sample, and standard lift-off pro-
cessing is used to remove the resist layer. This technique was recently used to place
bare colloidal QD clusters as small as tens of nanometers in scale [32,33]. There are
also similar approaches involving other patterning techniques such as photolithogra-
phy [34] and plasma lithography [35]. One can also combine these physical templates
with other particle deposition strategies, such as capillary force self-assembly [36,37],
where the templated substrate is dipped into a colloidal particle solution, and then
slowly pulled out and dried through the solvent evaporation so that particles are de-
posited into the patterned openings. Although all these methods are different in their
details, the main approach is the same: they use a physical template to achieve pat-
terned deposition on the sample. These techniques are simple and straightforward,
because they only require the creation of physical patterns on the substrate. How-
ever, the minimum particle densities are limited by the resolution of the lithography
technique itself, which can make single nanoparticle placement extremely difficult.
For example, in the e-beam lithography, the size of a written pattern normally can’t
be smaller than ∼10 nm, especially in most university research settings. Since the
size of colloidal QDs is usually less than 10 nm, these patterns will generally result
in the placement of multiple QDs.
More control over nanoparticle placement and density can be achieved by com-
bining lithographic templating with chemical functionalization. These methods can
be used to further guide and tether nanoparticles at desired locations on the sub-
strate. For example, using strepavidin-coated QDs and biotin-labeled gold patterns,
protein-DNA and protein-protein based hybridization have been used to assem-
7
bly QDs near gold nanostructures [38, 39]. Patterned functionalization can also
be achieved through scanning probe lithography [40] or in combination with tradi-
tional resist-based lithography approaches. For example, in recent works coupling
QDs to optical antennas, chemical functionalization was combined with lithography
and lift-off [7, 41]. In these papers, surface-modified QDs were covalently bounded
to functionalized regions that were defined in close proximity to the gold nanostruc-
tures through multistep electron-beam lithography. Then residual QDs elsewhere
were removed by the standard lift-off process. Such methods can realize very high
selectivity and precise placement of the nanoparticles, but single particle position-
ing is still challenging due to the size limits imposed by lithography resolution. To
achieve single emitter placement, one must control the nanoparticle density in solu-
tion and/or use surface modification to limit clustering.
A related technique that can offer direct control over particle density down to the
single emitter level is polymer-based electrostatic self-assembly [42]. This approach
exploits surface charge differences in solution between nanoparticles and lithograph-
ically patterned polymer regions. Coulombic attraction between particles and poly-
mer drives the self-assembly process. This approach is especially attractive as in-
dividuals QDs can be encapsulated by spherical silica cladding layers to produce
∼100 nm size nanoparticles that exhibit negative surface charge in solution, which
can then be assembled onto postively charged polydiallyldimethylammonium chlo-
ride (PDDA) polymer patterns. In 2008, Zhang et al. [42] developed this technique
and demonstrated how it could be used to realize large-scale arrays of single photon
emitters.
While all of the aforementioned techniques can be applied (with varying difficulty
and precision) to position quantum emitters near optical nanostructures, they all
suffer from a common problem; they are single use methods. Once fabricated, the
8
coupled emitter-structure patterns cannot be easily modified or reused. For example,
if the quantum emitters photobleach, there is no way to easily replace them. Or if you
want to investigate the coupling of different quantum emitters near the exact same
nanostructure, additional nanofabrication steps or new samples are required. This
process can be time-consuming and wasteful, and more importantly, there will always
be unknown variations between different samples. Thus, it can be difficult to perform
statistical characterizations where variations in the emission can be deconvolved from
variations of the nanostructures themselves. As a result, there is a need for iterative
placement of different quantum emitters on the same nanostructures.
1.4 Inorganic-Based Electrostatic Self-Assembly
The reusable self-assembly technique developed in this thesis builds upon the polymer-
based electrostatic self-assembly of Zhang and colleagues in Nurmikko Lab [42]. The
iterative placement of different emitter nanoparticles at the same location requires
a relatively general positioning strategy. In this context, electrostatic self-assembly
is very attractive, because the same positively charged template can be used to
position a variety of different negatively charged nanoparticles. Furthermore, in
contrast to chemical functionalization that uses strong covalent bonds, electrostatic
self-assembly forces can be relatively weak, and nanoparticles could be cleaned and
removed in solution from the template by strong agitation or sonication. However,
the key to develop a reusable electrostatic templates is to the create a robust tem-
plate that does not degrade with time and can also withstand aggressive cleaning
and sonication.
Figure 1.1 shows two main components of the polymer-based electrostatic self-
9
assembly of colloidal QDs by Zhang et al [42]. In this method, silica shells were
first grown on each colloidal QD in order to produce larger ∼100 nm nanoparticles
for easier physical positioning. Silica was chosen also as it naturally acquires a
negative surface charge in an ethanol solution, and therefore, silica-clad QDs were
attracted to the positively charged PDDA patterns (as opposed to the naturally
oxidized silicon surface which also acquires negative surface charge in solution). The
PDDA polymer templates are not robust; they readily degrade and cannot be easily
cleaned. However, the physical explanation for why the silica surface obtains negative
charge in solution does suggest an alternative approach, namely to find an inorganic
.
To understand why a silica surface acquires negative charge in the neutral solu-
tions, we must introduce the concept of isoelectric point (IEP). The IEP of a material
is defined to be the pH value at which the material’s surface has no net electrical
charge in solution. If a material’s IEP is greater than the pH of the solution, its
surface charge is positive, whereas the surface charge is negative if the IEP is below
the solution’s pH [44].
10
Isoelectric Point (IEP)
- - -- - -+ + + + + + + - -+
Figure 1.2: A plot of the surface charge vs the pH values of its surrounding solvents. Figure is reproduced from Ref. [45]
Generally, with the absence of surface modifications, the surface of oxide materials
in solutions naturally form with hydroxyl species, MOH, where M can be different
ions, such as Al3+, Si4+, etc. Thus, MO− species becomes predominant when the
solvent pH value is higher than the IEP (which means -OH rich), leading to negative
charge on the surface, while MOH+ 2 species provide positive charge when the pH
value is below the IEP (-H rich) [44]. Table 1.1 provides the IEPs of some common
materials at room temperature (25C). As listed in this table, the IEP value is
∼1.7-3.5 for SiO2, which means the surface charge on silica will be negative if it
is in a neutral solvent with pH value of ∼ 7 such as water. This will also be true
for solutions with a very small concentration of hydrogen ions, such as pure ethanol
where dissociation, CH3CH2OH CH3CH2O − + H+, leads to a large pKa=15.5 at
25C [46]. Normally, the ethanol solution used in labs contains a small percentage of
11
tantalum oxide, Ta2O5 2.7-3.0
manganese oxide MnO2 4-5
titanium oxide TiO2 3.9-8.2
iron oxide Fe3O4 6.5-6.8
aluminum oxide Al2O3 8-9
yttrium oxide Y2O3 7.15-8.95
zinc oxide ZnO 8.7-10.3
lead oxide PbO 10.7-11.6
magnesium oxide MgO 9.8-12.7
Table 1.1: Isoelectric points of some materials [44]
water which makes its effective pH value ∼ 7 (the measured pH value of the ethanol
solution we used is ∼ 6.9). This explains why the silica-clad QDs are attracted by the
positively charged PDDA patterns in the polymer-based electrostatic self-assembly
method.
Table 1.1 also shows that many dielectric materials have IEPs that are higher than
7, such as Al2O3(8-9). Note that the range of IEP values for Al2O3 as well as other
metal oxides will depend on the crystal structure and exposed facet. For amorphous
Al2O3, the IEP is also related to how they are prepared [47]. So it is reasonable
to predict that when a silicon substrate patterned with Al2O3 pads is dipped into
the neutral silica-clad QD ethanol solution, the positive surface charge of the Al2O3
(where AlOH + H+ AlOH+ 2 [47–50]) feature could attract the negatively charged
silica-clad QDs (where SiOH + OH− SiO− + H2O [48, 49]), while the silicon
substrate repels silica nanoparticles due to the negative surface charge of its native
12
oxide layer. Our hypothesis was that this combined effect of pad attraction and
substrate repulsion could allow for very selective and precise positioning of silica-
clad QDs at desired locations.
Al 2 O
2 O
3 Pads
- - - - - - - - + + + + + +
- - - - - -
Figure 1.3: By utilizing the electrostatic forces between different IEP materials, a novel self- assembly method of nanoparticles on substrate can be achieved.
Based on the hypothesis above, we proposed a general method for the inorganic
electrostatic self-assembly of nanoparticles, which is to utilize the attraction be-
tween different IEP materials. In principle, this method should be applicable to
any nanoparticles and templates with oppositely charged surfaces due to their IEP
differences. To be explicit, if the patterns are made of a high IEP material, such as
Al2O3 or MgO, and the nanoparticles have a low IEP, such as silica spheres, they
will attract each other in neutral solutions, as shown in the left schematic in Fig. 1.3.
Similarly, this method can be extended to particles with positive surface charge by
selecting a patterning material with a low IEP, as shown in the right side of Fig. 1.3.
In Chapter 2, we demonstrate this proposed method by modifying different quan-
tum emitter nanoparticles to engineer the surface charge, and fabricating inorganic
templates with the opposite charge to perform this inorganic-based electrostatic self-
assembly.
In Chapter 3, we verify that there are in fact single photon emitters in these
nanoparticles by performing the room-temperature antibunching measurements. The
number of quantum emitters in each nanoparticle then are obtained by fitting their
13
intensity autocorrelation curves.
In Chapter 4, we show three emitter-nanostructure examples to demonstrate the
ease and scalability of the inorganic-based electrostatic self-assembly technique for
the integration of quantum emitter with nanostructure.
Finally, in Chapter 5, we shortly summarize our inorganic electrostatic self-
assembly method and discuss its potential applications on fundamental light-matter
interactions.
14
2.1 Introduction
At the end of the last chapter, we proposed an inorganic electrostatic self-assembly
method based on isoelectric point (IEP) differences. To implement this method and
self-assemble quantum emitter nanoparticles, we only need to ensure a sufficiently
large difference between the IEP values of the nanoparticle surface and our patterned
templates.
In this chapter, we are going to: apply surface modifications on colloidal QDs
with silica cladding, which will result in a negative surface charge in neutral solu-
tions; fabricate Al2O3 templates which will be positively charged in the same neutral
solutions to attract those silica spheres to form QD arrays; check the self-assembly
results with different characterization techniques; and demonstrate the reusability of
these inorganic templates through daily re-deposition experiments over a one-month
period. Extended applications of this inorganic electrostatic self-assembly will be
presented by demonstrating the self-assembly of QDs on different high IEP mate-
rial templates as well as using different quantum emitters nanoparticles on the same
Al2O3 templates.
2.2.1 Silica-Cladding on Quantum Dots
To apply a low IEP material coating on colloidal QDs, we encapsulated them with
the same silica cladding used for polymer-based electrostatic self-assembly [42]. Both
custom-synthesized (CdSe-core CdS/Zn0.5Cd0.5S/ZnS-multishell, emission peak∼600
nm) and commercial (CdSe/ZnS, emission peak ∼620 nm, bought from NNLabs)
ODA-coated QDs were used in this work. A ∼160 nm-diameter silica sphere was
grown onto the QDs by utilizing a water-in-oil micro-emulsion growth technique,
which has been widely used to synthesize silica-clad colloidal particles [51–53]. For
this shell thickness, 30 mL of cyclohexane, 4 mL of polyoxyethylene (12) nonylphenyl
ether (NP-12), 100 µL of QDs in hexane (5 g/L), and 2000 µL of tetraethyl orthosil-
icate (TEOS) were added sequentially to a flask and stirred for 30 minutes. 600 µL
of aqueous ammonia hydroxide solution (28 wt. %) was then added to initiate the
polymerization process [54]. The final 160 nm-thick silica shell growth was completed
after 8 hours of continuous stirring. Silica spheres were isolated from the reaction so-
lution by centrifugal precipitation and washed with ethanol several times to remove
any reaction contaminants. Finally, the silica spheres were dispersed into ethanol
and sonicated for 20 minutes to form a uniform solution. Silica spheres ranging from
30 to 200 nm in diameter could be obtained by simply modifying the TEOS/QD
ratio in the reaction solution, as shown in Fig. 2.1(a-d). The diameter data shown
in Fig. 2.1(d) were fit to A× (VTEOS VQDs
)1/3 with a best fit value of A = 58.48nm.
17
50nm
100nm
40
80
120
160
200
Fit with 58.48*(VTEOS/VQD)^(1/3)
Figure 2.1: Silica-clad QD growth. (a), (b), (c) TEM images for 50 nm, 100 nm, 160 nm diameter silica spheres with single QDs inside. (d) Silica sphere diameter as a function of the TEOS/QD volume ratio.
18
2.2.2 Arrays of Silica-Clad Quantum Dots
After the QDs are encapsulated by a silica cladding, they acquire negative surface
charge in a neutral solution, e.g. of ethanol. To continue applying the proposed
inorganic electrostatic self-assembly method, a high IEP material template needs
to be fabricated, which acquires positive surface charge in the same solution. As
discussed in Chapter 1, there are several high IEP candidate materials for fabricating
the patterned template, such Al2O3, Y2O3, ZnO, PbO, MgO and so on. Al2O3 is a
very common dielectric material that can be easily grown by standard fabrication
methods such as e-beam evaporation, sputtering, and atomic layer deposition. As a
result, Al2O3 will be used as the high IEP template material.
Arrays of Al2O3 pads on a silicon wafer sample were made by the following
process. A bi-layer of poly(methyl methacrylate)(PMMA), 950A2, ∼60 nm, and
495A2, ∼40 nm, was spin-coated on a clean silicon wafer and baked at 180C for 30
minutes for each PMMA layer. The arrays of circular pads was patterned using e-
beam lithography followed by sputtering a 20 nm thick layer of Al2O3 and subsequent
lift-off. After being cleaned in acetone with sonication, the sample was then dipped
into an ethanol solution of silica-clad QDs (2.5 mg/L) for 8 hours to ensure good
coverage on the Al2O3 pads. Finally, the sample was dried with nitrogen and the
silica-clad QDs were held on the pads by van der Waals forces. A schematic of this
process is shown in Fig. 2.2.
The silica-clad QD self-assembly results were checked for different pad sizes by
scanning electron microscope (SEM) as shown in Fig. 2.3(a). When the Al2O3 pad
diameter is close to the silica-clad QD diameter (160 nm), each pad has only one
silica sphere, containing a single QD. As the pad size increases, the average number
of silica-clad QDs on each Al2O3 pad also increases, as shown in Fig. 2.3(c-e). Thus,
19
Ethanol, pH~6.9
IEP=1.7~3.5
Silica coated QDs, IEP=1.7~3.5
Quartz or silicon with a native oxide layer
++++++ ++ - - - - - - - -- - - - - - - -
- - - - - -- -
Figure 2.2: Scheme illustrating the fabrication and the self-assembly process for the silica-clad QDs on Al2O3 pads arrays
by matching the pad size with the diameter of a given silica sphere, it is possible
to achieve controllable placement of individual QDs with very high accuracy. Since
the QDs are centered in the silica spheres, the positioning resolution of this tech-
nique is limited by the diameter of the silica cladding and the size of pads. Here,
we have fabricated silica spheres as small as 30 nm on 50 nm Al2O3 pads, as shown
in Fig. 2.3(f). Note that the size of Al2O3 pads used here is limited by our fabrica-
tion equipment and methods (i.e. sputtering Al2O3 complicates lift-off for smaller
structure dimensions).
Dark field microscopy, as shown in Figs. 2.3(b), was also used to check the depo-
sition results. Side-by-side comparisons of SEM and dark field images were used to
validate this technique. For example, Fig. 2.3(b) shows a 5x5 array which is miss-
ing only one silica-clad QD on the central pad, consistent with the SEM image in
Fig. 2.3(a). Additional experiments to check the effects on QD-on-pad coverage from
the pad dimension and deposition time are discussed in Appendix A.
20
2μm
c ed f
Figure 2.3: Electrostatic self-assembly for the arrays of silica-clad QDs. (a) SEM images for the silica-clad QDs sitting on Al2O3 pads. (b) Dark field image shows the same deposition result as shown in the SEM image in (a). The dim site in the center is a pad without any silica-clad QDs on, as shown in (a). (c-e) shows 160 nm silica-clad QDs on different size pads, and (f) a 30 nm silica-clad QD on a 50 nm pad.
21
PicoHarp 300
CCD
Figure 2.4: Schematic for confocal scan microscope setup. SP: shortpass filter, DM: dichroic mirror, LP: longpass filter, BP: bandpass filter.
To check the fluorescent emission of these silica-clad QDs, a home-built fluores-
cence spectrometer and confocal scanning system based on a Nikon inverted micro-
scope was designed as shown in Fig. 2.4. A CW 532 nm (Coherent, Verdi, Nd:YVO4)
laser was focused by a 100x, NA1.3 oil immersion objective (spot size is ∼ 1 µm) to
excite the quantum emitters on a piezo-electric stage (Mad City Labs, NanoView-
M). The fluorescence is collected by the same objective, and then focus onto the
entrance slit of a spectrometer (Princeton Instruments, Acton) equipped with an
imaging camera (Princeton Instruments, PIXIS 1024B) to measure the QD’s spec-
tra, as shown in Fig. 2.5, when the mirror is switched to left.
When the mirror in Fig. 2.5 switches to right, the collected emission is coupled
into a 105 µm diameter multi-mode fiber where the fiber end is used as a pinhole for
confocal imaging (the diameter of imaging area is ∼ 1 µm since the magnification
of objective is 100), and then focused onto a single photon detector (PicoQuant, τ -
SPAD), where the arrival time of each detected photon event is recorded by a time-
22
0.5
1
1.5
2
2.5
ts )
Figure 2.5: Spectrum of the commercial 620 nm QDs under Verdi pump
correlated single photon counting (TCSPC) module (PicoQuant, PicoHarp 300). We
wrote a Matlab GUI to communicate with and control all the devices in this setup,
such that the stage could automatically scan the area of interest on the sample and
computer saved all the data acquired. For a confocal scan, the area of interest is
measured pixel by pixel, where the step size is typically set to∼0.1 µm. At each pixel,
as shown in Fig. 2.6, the scanning stage’s controller sends a time-correlated “pixel
signal” into the PicoHarp 300, merged with the time-correlated “photon incoming
event signals” in the same time sequence file. (There are actually a few “line signals”
which mark the times where the stage move to the next row of pixels, since the stage
moves line by line as well.) To construct the scan image, the Matlab GUI sums
up all the “photon event signals” between two adjacent “pixel signals”. The total
number of the signal counts will be the intensity on the pixel to which these two
“pixel signals” correspond.
For example, Fig. 2.7 shows the scan for a 40 µm × 40 µm area of a QD array
on a quartz sample. The scan image was constructed using an excitation of 20 µW
power, and it helped further confirm the high QD-on-pad coverage shown earlier in
the SEM and dark field images. Note that there are some intensity variations in this
fluorescent intensity scan image, which are most likely from variations in bare QD
sizes and crystal orientations [55], or empty/multiple QD occupied silica spheres.
23
Photon Events from SPAD
PicoHarp 300
{ { { { Constructed Image
Adding up all photon events between pixel signals gives the intensity at that pixel
Matlab
Figure 2.6: The scan image is constructed by summing up all the “photon event signals” between two adjacent “pixel signals” as the intensity.
0 20 40
m )
Figure 2.7: Fluorescent imaging of a 40 µm × 40 µm of the QD arrays on a quartz sample
24
After the nanoparticle-on-pad arrays are formed, electrostatic attraction is strong
enough to adhere the particles on the pads. This attraction is relatively weak, when
compared to others (e.g. chemical functionalization approaches). Indeed, the force
is so weak that the nanoparticles can be removed from the pads by applying a small
external force to them, such as sonication in a solvent. Once cleaned, the patterned
substrate can be dipped back into the nanoparticles solution for re-deposition on the
same pads and high pad occupancy can be recovered with a new set of emitters.
QD deposition Sonication
Figure 2.8: Process of QDs re-deposition and cleaning
To demonstrate this reusability, an Al2O3 pad array sample was fabricated with
the same pad size (160 nm diameter and 20 nm thickness) as the one showed in
Fig. 2.3(a). The pad arrays were first checked by dark field microscopy. These
initial images looked like Fig. 2.9(a), which shows a dark background due to the lack
of light scattering particles. After being dipped into the silica-clad QDs solution
for 12 hours to ensure a high QD-on-pad coverage, pad occupancy was determined
from further dark field microscope imaging, as shown in Fig. 2.9(b). After that,
the sample was immersed into acetone with sonication for 2 minutes, then cleaned
by ethanol and subsequently dried with nitrogen. This QD-removed sample is then
checked again by the dark field microscopy, an example image without QDs is shown
in Fig. 2.9(a). This shows that the silica-clad QD can be easily removed from Al2O3
pads by sonication when immersed in cleaning solvents.
25
a
h
Figure 2.9: The reusability experiment for Al2O3 pads arrays sample. (a) Optical dark field image of a pad arrays sample without any QDs on. (b)-(h) Dark field images for checking the QD coverage. All of these eight images are for the same arrays on the same sample and were captured under the same microscope setting. (d) Day 8 has a low QD coverage due to the build-up contaminants. (h) A re-deposition after 6 months.
Once cleaned, the Al2O3 pad arrays template can be immersed back into the
silica-clad QDs solution for re-deposition of with new nanoparticles. The electrostatic
attraction between the alumina surface and silica-clad QDs will occur again. This
process of cleaning and QD redeposition, as shown in Fig. 2.8, was repeated daily
for one month. Some dark field images of the self-assemblies are shown in Fig. 2.9(b-
g). Note that on day 8 and 9, QD coverage dropped to almost zero, as shown
in Fig. 2.9(d). This was due to both the contamination of the QD solution from
repeated use and build-up of contaminants on the sample surface. However, ∼100%
coverage could be recovered by a more thorough solvent cleaning process (5 minutes
in acetone, 5 minutes in methanol, 5 minutes in ethanol, all with sonication) and
replacement of the QD solution. After Day 9, the same thorough cleaning process
and solution replacement was repeated every 3 days.
To count the QD-on-pad coverage on each dark field image, a Matlab image
processing routine was written to automatically detect the groups of bright pixels in
26
Day
0
20%
40%
60%
80%
100%
Figure 2.10: The QD deposition coverage for the one-month long test.
the images based on certain signal to background ratios and then count the number
of detected groups which are determined to be a QD-on-pad site. The results of this
one month deposition process, shown in Fig. 2.10, demonstrate very high (> 95%)
coverage for most trials over this long time span. To show the long-term stability of
this template we re-deposited on the same sample after 6 months, and still obtained
high coverage, as shown in Fig. 2.9(h).
27
As discussed in Chapter 1, this inorganic-based electrostatic self-assembly should be
applicable for any nanoparticles and templates that accumulate opposing charges due
to their IEP differences. We have already demonstrated the successful self-assembly
of low IEP silica-clad QDs on high IEP Al2O3 pad arrays in ethanol solutions. Next
we are going to show the generality of this method by presenting the results for
silica-clad QDs on other high IEP material templates, such as Y2O3 and MgO, and
also different quantum emitter nanoparticles on the same Al2O3 templates, including
NV center nanodiamonds and UCNPs.
2.3.1 QDs Self-Assembly on Other High IEP Templates
The inorganic electrostatic self-assembly of silica-clad QDs should also work for other
dielectric materials as long as the solution’s pH value is between the IEPs of SiO2
and these dielectric materials, such as Y2O3(IEP ∼7-9) and MgO(IEP ∼12-13) [44].
To demonstrate this principle, we made three patterned silicon samples with the
same sized (160 nm diameter) pad arrays using three dielectric materials: Al2O3,
Y2O3, and MgO, and dipped them into the same silica-clad QD (160 nm diameter)
ethanol solution. After the QD self-assembly, optical dark field microscopy was used
to check the results.
All three samples were immersed in the same QD solution for a relatively short
dipping time, about 20 minutes, to allow only part of the pads in the arrays to
attract silica-clad QDs. Among the dark field images, the MgO arrays have the
highest coverage of QD-on-pad, which is consistent with the high IEP of MgO, while
28
MgOY2O3Al2O3
a b c
d e f
Figure 2.11: Dark field microscope images of 160 nm silica-clad QD on the 150 nm pads with a 2 µm pitch using different dielectric materials.
Al2O3 and Y2O3 showed similar coverage due to their close IEP values, as shown in
Fig. 2.11(a-c)
Then the three samples were dipped back into the the QD solutions for 8 hours
to obtain maximal coverage. The results were checked again by the dark field mi-
croscopy: Fig. 2.11(d-f) show that these different material pad arrays all have almost
100% QD-on-pad coverage for longer deposition times.
29
2.3.2 Control Experiments with Low IEP Templates
To confirm that the inorganic self-assembly process is based on electrostatic at-
traction between different IEP materials (rather than an artifact of the patterning
process), we ran a series of control experiments with low IEP SiO2 templates. We
patterned a silicon sample with pad arrays and large triangle patterns of 25-nm thick
SiO2 using e-beam lithography and liftoff followed by a solvent cleaning. (This pro-
cess was identically to that used for the high IEP templates, except for the deposited
oxide material.) Next, the low IEP templates were dipped into the silica-clad QD
solution for 8 hours, and SEM was used to check the result. From the SEM images
shown in Fig. 2.12, it is clear that these SiO2 patterns don’t have any silica-clad
QDs attracted on their surface. Since the patterns and silica-clad QDs have the
same surface charges, they repel each other. These and other control experiments
(such as deposition of silica-clad QDs on unpatterned substrates of Al2O3 and MgO)
were used to verify that this self-assembly is based on electrostatic attraction.
30um 5um
Figure 2.12: SEM images of SiO2 patterns (big triangle and circular pad array) after dipping in the silica-clad QD solution.
30
2.3.3 Self-Assembly of Upconversion Nanoparticles
The silica cladding process can be also applied to upconverting nanoparticles (UC-
NPs) by the same synthesis method, as long as they have a functionalized surface
similar to the colloidal QDs. To perform this silica cladding, commercially available
OA-coated NaYF4 nanoparticles doped with Er3+ and Yb3+ ions, with an average
size of 30 nm and emission line around 545 nm, were bought from Mesolight LLC.
Figure 2.13: Left: Upconversion nanoparticle aggregations in some silica cladding while nothing in others. Right: Upconversion nanoparticles with 40nm thick silica-cladding.
For the first several trials, we attempted the same recipe as used for QDs, but
the cladding results were not as good as expected. UCNPs aggregated in some silica
spheres while others contained no particles, as shown in left TEM image in Fig. 2.13.
These nanoparticles are much larger (∼30 nm) than the colloidal QDs (less than 10
nm), which caused them to precipitate to the bottom and aggregate. (In contrast,
the smaller bare QDs could always be dispersed uniformly in proper solutions.) As
a result, we found that longer and stronger sonication was needed for these UCNPs
to disperse them well in a hexane solution before injecting them into the reaction
solution for the silica encapsulation.
With the increased sonication, a silica cladding with shell thickness of 40 nm
was achieved by following the same synthesis steps in the QD recipe with: 10 mL of
31
cyclohexane, 1.3 mL of polyoxyethylene (12) nonylphenyl ether (NP-12), 400 µL of
UCNPs in hexane (10 g/L), and 50 µL of tetraethyl orthosilicate (TEOS), and 200
µL of aqueous ammonia hydroxide solution (28 wt. %). The final centrifuged silica
spheres were checked by TEM, as shown in the right TEM image of Fig. 2.13.
Figure 2.14: Dark field image of a 40 µm×40 µm area of the arrays on a quartz sample.
The Al2O3 pad arrays sample was then dipped into this UCNP ethanol solution
overnight to form the UCNP-on-pad structures, and then the coverage was checked
by dark field microscopy, as shown in Fig. 2.14. These demonstrated high coverage
similar to the results for silica-clad QDs .
The fluorescence emission spectra of these UCNPs, as shown in Fig. 2.15, was
obtained by a similar fluorescent measurement setup as used for the QDs, but with
an excitation by a 976 nm wavelength laser diode (BWT K98S09F). Fig. 2.16 shows
a 12 µm × 12 µm confocal scan on this sample, which also demonstrated high
particle-on-pad coverage consistent with the dark field image.
32
In te
ns ity
(c ou
nt s
×1 04 )
Wavelength (nm)
Figure 2.15: Fluorescent spectra of UCNPs under a pump power of 100 mW
12 um
12 u
m
100
500
1000
Figure 2.16: Confocal scan image of an area of 12 µm×12 µm on the same quartz sample.
33
2.3.4 Self-Assembly of NV center Nanodiamonds
Chemical modification methods can be used to apply functional groups to a diamond
surface, including hydrogenation (hydrogen-terminated,-H), oxidization (carboxylic-
COOH, carbonyl-CO, epoxide-COC, hydroxyl-OH and so on), and fluorination. In
Ref. [56], Chakrapani et al. studied the surface zeta potential for modified nanodia-
monds, their results are shown in Fig. 2.17
Figure 2.17: Zeta potential of the diamond vs pH values of solution. Figure is adapted from Ref. [56].
In order to perform electrostatic self-assembly method, properly functionalized
nanodiamonds need to be chosen according to the surface charges of the templates.
From Fig. 2.17, we can deduce that both oxidized and fluorinated diamonds have
a IEP below 3, which means they are negatively charged in a neutral solution. In
contrast, hydrogenated diamonds has an IEP near 8, and therefore, accumulate
slightly positive surface charge in a neutral solution. For the work of this thesis,
the carboxylited nanodiamonds were used due to their commercially availability
from Adamas Nanotechnologies. 40 nm carboxylated nanodiamonds containing 1-
4 NV/particle were chosen, which are negatively charged in DI-water due to the
34
carboxylic groups, where COOH+OH− COO−+H2O [57]. Thus, the same self-
assembly method used for silica-clad QDs and UCNPs can be applied to them.
a b
2µm
100nm
300nm
300nm
200nm
Figure 2.18: (a) Electrostatic self-assembly of NV center nanodiamonds on Al2O3 pads arrays. (b) SEM images for nanodiamonds on 70 nm Al2O3 pads arrays. Inset shows nanodiamonds on pads of different sizes (70 nm, 100 nm, 200 nm, 300nm).
Arrays with different sizes of Al2O3 pads were patterned to silicon wafers sample
by the same fabrication method used above, and then dipped into the nanodia-
monds DI-water solution with a concentration of 5 mg/L. A schematic of the self-
assembly process is shown in 2.18(a), and the results are shown in the SEM images
in Fig. 2.18(b).
About 400 nanodiamond-on-pads were counted to obtain the coverage of this
nanodiamond deposition. ∼77.3% have nanodiamond particles on each pad and
about 48.3% have a single nanodiamond. Compared to the nearly 100% of single
QD-on-pad coverage in the silica-clad QD self-assembly, this coverage is relatively
low. The reasons for this difference could be the rough surface of small Al2O3 pads
due to the limitation of our fabrication techniques as well as the small size and
irregular shapes of nanodiamond particles, all of which could affect the attraction
force between the pads and nanodiamond particles.
The fluorescence spectra shown in Fig. 2.19, was obtained by pumping the nan-
35
500 550 600 650 700 750 800 850 900 950
532
~637
Wavelength (nm)
In te
ns ity
Figure 2.19: Spectra of NV centers under the 532 nm excitation. There is a small peak around 637 nm which is its zero photon line (ZPL)
odiamonds with ∼0.5 mW at 532 nm. To acquire the scanning fluorescence intensity
image shown in Fig. 2.20, NV center nanodiamond array on a quartz substrate was
measured. There are also some intensity variances in this image, which is due to
both the number of particles per pad and number of NV centers per particle (aver-
age number is 1-4).
)
Figure 2.20: Fluorescent imaging of a 40 µm × 40 µm of the nanodiamond arrays on a quartz sample
A short-span reusability measurement on the Al2O3 pads template was also per-
formed for NV center nanodiamonds, as show in Fig. 2.21(a-d). Since single 40 nm-
size nanodiamonds are too small to be imaged under dark field microscopy, Al2O3
pad arrays with a diameter of 200 nm were used, which attracted more nanodia-
36
b dca
Figure 2.21: (a-d) Dark field images showing the re-depositions of nanodiamonds on Al2O3 pad arrays, top ones are after self-assembly and bottom ones are cleaned by acetone with sonication. All dark filed images were captured under same microscope setting.
monds on the surface to obtain clearer imaging from the stronger scattering of more
nanodiamonds. Four nanodiamond deposition cycles are showed, and it can be seen
that each of them has a high nanodiamond-on-pad coverage. However, it is also
observed that there are an increasing number of diamond nanoparticles that are de-
posited at locations between pads with each reuse. As shown in the upper row of
dark field images, the number of ”off-pad” defects increases from 1 (in 400) to 7,
10, and 25. For the 20 by 20 pad region, these values represent between 0.25% to
6.25% defects, and probably originates from the build-up of contamination in the
nanodiamond solution as well as on the sample surface. It was also noticed that the
Al2O3 pads gradually got thinner, which can be seen from the dimer and dimer dark
field images of the pads. This is because there was a slow erosion of the Al2O3 in
water. Eventually, after 5 cycles, the pads were too thin to attract nanodiamond
particles.
Y2O3 and MgO pad arrays were also used to repeat the same nanodiamonds
assembly experiment. Y2O3 pads showed similar assembly result to the Al2O3 pads.
37
However, it was observed that the MgO pad arrays disappeared after the sample was
dipped into water for a short while, which was very likely due to the fact that MgO
reacts with water (MgO + H2O Mg(OH)2, which is soluble in water).
2.4 Conclusion
In this chapter, we applied surface modifications with low IEP materials on different
quantum emitter nanoparticles to obtain negative charge on their surfaces in neu-
tral solutions, which allowed for inorganic electrostatic self-assembly onto positively
charged high IEP material templates. Several characterization methods were used to
check the assembly results, including SEM, dark field imaging and fluorescent imag-
ing. The generality of this technique was shown by presenting the self-assembly of
different nanoparticles and template materials. All the self-assembly results demon-
strated the ease, high precision, and efficiency of this inorganic-based method for
nanopositioning nanoparticles. More importantly, the reusability of these inorganic
template was showed by month-long and even year-long re-deposition and cleaning
experiment. The ability to iteratively position individual quantum emitter nanopar-
ticles at the same location on the solid substrates could potentially help enable a
more meaningful statistical studies of the emitter-structure interactions, as well as
more robust samples for quantum information researches by replenishing new photon
sources particles in the same optical circuits. While it has been clearly demonstrated
the successful placement of individual nanoparticles, such as silica spheres and nan-
odiamonds, experimental verification of single photon statistics must be used to con-
firm that these nanoparticles contain single quantum emitters. These experiments
will be discussed in the next chapter.
38
3.1 Introduction
In the last chapter, we demonstrated that single nanoparticles can be precisely and
efficiently positioned at desired locations on solid substrates by fabricating arrays of
individual silica-clad QDs and nanodiamonds with NV centers onto alumina pads
on both silicon and quartz substrates. To verify that there are in fact single photon
emitters in these silica spheres and nanodiamonds, room-temperature antibunching
measurements need to be performed. The number of quantum emitters can be then
obtained by fitting the antibunching result.
3.1.1 Intensity Autocorrelation Function
The antibunching characteristic of single photon source can be demonstrated by
measuring the normalized second-order autocorrelation function of the emission light,
which is also called the intensity autocorrelation function [58]. It is defined as
g(2)(t) = < I(t0)I(t0 + t) >
< I(t0) >2 . (3.1)
Here, <> indicates the time average, and g(2)(t) is the probability of measuring
an intensity I(t0 + t) at time t0 + t when the value of the intensity at time t0 is I(t0).
In the quantum picture, the intensity of light can be described as the number of
photons, obtained by applying number operator “n” on the photon state |n >. Thus,
the intensity autocorrelation function can be written in the raising and lowering
operators:
40
< a†(t0)a(t0) >2 . (3.2)
A g(2)(0) value of less than 1 shows that antibunching is occuring. For an ideal
a single photon source (single emitter) g(2)(0) = 0, which means the probability of
measuring two photons at the same time is 0, but in the presence of noise, any value
belows 0.5 is indication of a single emitter.
For N independent quantum emitters,
< I >= N < i > . (3.3)
Given that fluorescence intensity of each emitter < i > is dependent on its prob-
ability of emission, which in turn depends on the time-dependent excited state pop-
ulation or density on the excited level, σex(t), the g(2) can be described by following
equation [59] (see Appendix B for more details):
g(2)(t) = (N − 1)
g(2)(t) = σex(t)
σex(∞) . (3.5)
3.1.2 g(2) of Quantum Dot
Although the electron transition of a real quantum dot is much more complicated,
a simple 2-level electronic structure model is usually used to describe its emission.
Here k12 and k21 are the transition rates between two levels. σ1 and σ2, are the
normalized electron densities in each energy level. At time t=0, after one photon is
just emitted, σ1=1, and σ2=0.
k12 k21
d
dt
σ1
σ2
σ1
σ2
. (3.6)
By solving above equation with the initial conditions, σ1(0)=1, σ2(0)=0,
σ1(t) = e(−k12−k21)tk12 + k21
k12 + k21 , (3.7)
k12 + k21 , (3.8)
and one can derive the second-order autocorrelation function g(2)(t) for a single
QD emitter (N=1) by inserting Equation 3.8 into Equation 3.5:
42
or into Equation 3.4 for multiple emitters (N>1):
g(2)(t) = 1− 1
If we write τ = 1/(k12 + k21), then
g(2)(t) = 1− 1
where, τ is the decay lifetime in this system.
.
Figure 3.2: The intensity autocorrelation curves of single and cluster of CdSe/Zns QDs: single one shows a clear g(2)(0) dip while the cluster has a flat characteristic, adapted from Michler et al. [60].
43
3.1.3 g(2) of NV Center
In the electronic structure of the negatively charged NV center, there is a meta-stable
level between the excited and ground states, which will cause photon bunching when
the excitation power saturates the absorption of NV centers. The experimental result
of high-power pumped single NV centers show that g(2) goes above 1 for t 6= 0 and
then decays to 1 at longer time. To describe this behavior, a 3-level system which
considers the metastable shelving state needs to be used [4, 61].
As the scheme shown in Fig. 3.3, the 3-level system is described by k12, k21, k23,
and k32 which are the transfer rates between energy levels, and σ1, σ2, and σ3 which
are the normalized electron densities on each energy level. At time t=0, after one
photon is just emitted, σ1(0)=1, σ2(0)=0, and σ3(0)=0. The system can be written
as follows:
k12 k21
Figure 3.3: A 3-level energy system, with the metastable state.
d
dt
σ1
σ2
σ3
σ1
σ2
σ3
. (3.12)
44
By solving these equations with the above initial conditions, the second-order
autocorrelation function for a single NV center can be derived as shown in Equa-
tion 3.13:
g(2)(t) = σ2(t)
B = (k12 + k21 − k23 − k32)2 + 4k21k23, (3.16)
C = 2k12k23 + k32(k12 + k21 − k23 − k32)√
Bk32 . (3.17)
This equation shows that the g(2) of NV center is a bi-exponential function when
there are electrons decaying to the meta-stable level, which matches the experimental
result when the pump power is high. Fig. 3.4 shows the g(2) curves obtained for a
NV center when it is pumped by 532 nm laser with different powers, and the fitting
to the 3-level g(2) function by Beveratos et al. [3]. The g(2) curve is similar to the one
of the 2-level model when the excitation power is relatively low. However, bunching
peaks appear due to the meta-stable level in NV centers is higher when the excitation
is stronger.
45
Figure 3.4: g(2) curves of NV centers when pump power is from 0.3 to 31 mW, shown in circles, where the solid lines are the fittings. Figure is adapted from Ref. [61]
3.2 Experimental Setup
A Hanbury-Brown-Twiss (HBT) setup [62] is usually used to measure the intensity
autocorrelation function of the light by splitting it equally into two identical detec-
tors. A schematic of home-built HBT setup is shown in the right side of Fig. 3.5:
The emission is coupled into a multimode fiber after being collected by the objective
and then divided by a 50/50 beam-splitter into two equal beams. The beams are
focused onto two identical single photon avalanche detectors (PicoQuant, τ -SPAD),
by which photon arrival times in both SPADs are recorded in a PicoHarp 300. Two
650 nm short pass filters are placed in front of each photon detector to reduce the
cross-talking between them, which is due to the emission from the SPADs themselves
during avalanche events. The left side in the figure is the same confocal microscope
we built for fluorescence measurement used in Chapter 2.
46
MadCityLab
Figure 3.5: HBT setup combined with the confocal scan microscope. SP: shortpass filter, DM: dichroic mirror, LP: longpass filter.
As described in Chapter 2, automated confocal scan measurements are performed
on the emitter arrays sample. After the scan image is constructed, the Matlab
GUI can recognize all the bright pixels according to the preset signal-to-noise ratio
(SNR), and record the center of each bright pixel group as the coordinates of the sites
of interest for future measurements. This process uses the same image processing
routine developed for QD-on-pad coverage counting in Chapter 2.
The stage will automatically move to the positions of detected sites of interest
to perform the antibunching measurement, where SPADs send their “photon event
signals” sequences into PicoHarp 300. By comparing differences between the arrival
time of each photon signal from both the SPADs, the g(2) curve for the quantum
emitters on each site of interest can be built up. The number of the quantum
emitters on each site can then be estimated by fitting the g(2) curve to a proper
model function, as discussed in the previous section.
This home-built system was designed to allow automatic measurements for large
47
Time
Signal
{
SPAD1
SPAD2
Δt=0
t1 and t2 contributes one to the Δt with a value of t1-t2
Comparing the time difference of each photon events between the same two pixel signals from both SPADs gives the g(2) curve
Adding up all photon events between two pixel signals gives
the intensity at that pixel
Pixel Signal from Scanning Stage
Figure 3.6: SPADs send the time-related “photon event signals” sequences into PicoHarp 300 with the “pixel signals”. The scan image is constructed by summering up all the counts between adjacent two pixel signals. The g(2) curve is built up by comparing the time difference of each two photon event signals. The vertical blue box on the two sequences is an example of comparing two photon events from two detectors.
scales quantum emitters arrays, and in the following sections we discuss it use to
obtain the intensity autocorrelation functions for the QD and NV center arrays
prepared by inorganic electrostatic self-assembly.
48
3.3 Photon Antibunching from QD
To to characterize the photon statistics of the QD arrays, the quartz sample with
QD-on-pad arrays (2 µm pitch) was placed on the stage under an excitation of 532
nm (Verdi) laser with a 20 µW incident power.
0 6 12
3)
Figure 3.7: Confocal scan image for a 12 µm × 12 µm area. The 25 bright sites are marked by green (number of QD is 1), blue (number of QD is more than 1) and red (insufficient signal for fit) circles.
Fig. 3.7 shows a 12 µm ×12 µm, scan that includes 36 QD sites. 25 bright sites
(marked in circles) were recognized above the set SNR value of ∼2, while there 11
sites were too dim to be detected due to the intensity variances as explained before
in Chapter 2. After locating all the bright sites, the stage moved to each site and
the silica-clad QD was pumped for ∼10 minutes to acquire sufficient counts to build
up an intensity autocorrelation curve.
After normalizing the curves to the average number of counts at long times, each
49
Time
Figure 3.8: g(2) curves for all the bright sites in the scan image, g(2) < 0.5 is shown in green frames and ≥ 0.5 is shown in blue ones. The dashed lines indicate a g(2) = 0.5, and the time range is -45∼45 ns
curve was fit to the 2-level model as described in section 3.1.2,
g(2)(t) = 1− 1
N e−t/τ , (3.18)
which allowed us to estimate the number of QDs per site. Here, g(2)(t) is the intensity
autocorrelation function, N is the number of quantum emitters, τ is the lifetime of
emitter, and t is the acquisition time difference between two single photon detectors.
It was noticed that some of the silica-clad QDs blinked off or became photo-bleached
during the continuous pumping of the 10-minute-long measurement. As a result,
these QDs did not give a sufficient signal to obtain the g(2), as marked by red circles.
50
Fig. 3.8 shows a 12 µm ×12 µm scan with a 6 × 6 array (2 µm pitch). The result of
all these fits is displayed in Fig. 3.8, in which 16 of them show a clear dip of single-
QD (g(2)(0) < 0.5 is required to demonstrate single photon emission [63]) and 5
sites have the multiple-QD characteristics. Note that the noise in these experiments
primarily comes from the intrinsic dark counts of the SPADs as well as background
fluorescence from the substrates.
3.4 Photon Antibunching from NV Center
Similar antibunching measurements were also performed for the NV center arrays
on a quartz substrate under a ∼0.5 mW incident power of a 532 nm laser. An area
of 12 µm ×12 µm with 2 µm pitch, including 36 sites , was scanned, as shown in
Fig. 3.9.
)
Figure 3.9: Confocal scan image for a 12 µm × 12 µm area of the nanodiamond arrays. The 29 bright sites are marked by green (number of NV centers is 1) and blue (number of NV centers is more than 1) circles.
51
Time
Figure 3.10: g(2) curves for all the bright sites in the scan image, g(2)(0) ≤ 0.5 is shown in green frames while > 0.5 is shown in blue ones.
29 bright sites were recognized above the set value of SNR, while 7 sites were too
dim to be detected due to the intensity variances. Because the NV center emission is
much more stable than QDs, they can survive when being pumped for long time while
still emitting photons. Each nanodiamond-on-pad site was pumped for 20 minutes
to obtain intensity autocorrelation curves. Since the NV centers were excited by a
relatively low laser power [61], and there was no obvious bunching near t = 0 dip
in the obtained g(2) curves, as shown in Fig. 3.10, the 2-level model is sufficient
to fit these results. All the fits are displayed in Fig. 3.10, in which most show a
clear g(2)(0) dip, indicating a low number of NV centers. The average number of NV
centers in each nanodiamond is about 3.3, which is close to the specification provided
52
by Adamas Nanotechnology, that the average number of NV centers is ∼1-4.
3.5 Conclusion
In this chapter, we built an automated system for the confocal scanning and HBT
measurement of the quantum emitter arrays. The antibunching characteristics of
the QDs and NV centers were presented by showing the dips at t = 0 in their g(2)(t)
curves. The successful placement of single quantum emitter or photon source was
verified from the fitting results of quantum emitter numbers on each Al2O3 pad site.
These experimental results demonstrate that our inorganic-based electrostatic self-
assembly method is a simple and very efficient technique to position single photon
sources. In the following chapter, we will show how these fabrication and measure-
ment techniques can be leveraged to study the coupling of single emitters with optical
nanostructures.
53
with Nanostructures
4.1 Introduction
We have already demonstrated in the previous chapter, that this new developed
inorganic-based electrostatic self-assembly method allows for easy, precise and effi-
cient positioning of individual quantum emitters on the solid substrates. For the
study of light-matter interaction, emission properties, such as spectral distribution,
radiation pattern and polarization, depend on both the emitter’s intrinsic structure
and its local density of optical states which can be modified by nearby nanostructures.
As discussed in Chapter 1, several nanopositioning methods have been developed for
the integration of emitters with nanostructures. In this chapter, the ease and scal-
ability of our inorganic-based electrostatic self-assembly technique will be shown by
demonstrating three examples of quantum emitter nanoparticles integration with
different nanostructures.
4.2 Silica-Clad QDs near Gold Nanorods
A simple example is the integration of QDs with rod-shaped optical antennas. To
make this emitter-structure system, Al2O3 pads were fabricated near gold nanorods
by a two-step e-beam lithography process. Two layers of PMMA (950A2, ∼60 nm,
and 495A2, ∼40 nm) were spin coated on a silicon wafer substrate and baked at
180 C for 30 minutes for each PMMA layer. Gold nanorods with different sizes
were fabricated on the substrates by e-beam lithography followed by deposition of
a 5 nm Ti adhesion layer and a 50 nm gold film. After lift-off, a bilayer of PMMA
was coated and baked again, and then circular Al2O3 pads were e-beam written at
specific locations relative to the nanorods followed by sputtering of a 20-nm-thick
layer of Al2O3 and subsequent lift-off. The sample was dipped in the silica-clad
55
QD ethanol solution for 8 hours and dried with nitrogen. The fabrication process
schematic is shown in Fig. 4.1.
First Lithography and Depostion
Silica-Clad QD
Figure 4.1: Schematic of the two-step e-beam lithography method to fabricate the gold nanorod optical antennas with circular Al2O3 pads.
The electrostatic self-assembly result was checked by the SEM in Fig. 4.2. The
images clearly demonstrate that this technique allows for controlled positioning of
individual silica-clad QDs near nanostructures.
200nm 200nm 1µm
a b c
Figure 4.2: Integration of the silica-clad QDs with various sizes gold nanorods in different posi- tions.
Gold nanorods can help enhance light-matter interactions. For example, they
can enhance spontaneous emission through the Purcell effect [64]. To observe this
56
effect, lifetime measurements were carried out by excitation with a 2.5 MHz 405 nm
pulsed laser (PicoQuant LDH series 405 nm) at a 5 µW average power. Emitted
photons were detected by the SPAD after a 620±10 nm bandpass filter. Histograms
of photon arrival times were recorded with the PicoHarp 300.
PicoHarp 300
MadCityLab
Figure 4.3: Setup for the lifetime measurement of Silica-clad QDs with and without gold rods. SP: shortpass filter, DM: dichroic mirror, LP: longpass filter, BP: bandpass filter.
Silica-clad QDs without and with integrating gold nanorods on the same quartz
substrate were fabricated. On this sample, there were arrays both QDs on the Al2O3
pad similar to the ones used in the antibunching experiment, and QDs near the 400
nm-long and 50 nm-wide gold nanorods. About 50 QD sites of each were measured,
and lifetime information was obtained by fitting the decay traces to a stretched
exponential function I(t) = I0 · e−(t/τ) β
+ Ibackground, where τ is the lifetime [65, 66].
The result of these fits are summarized in Fig. 4.4(a), and two decay traces from the
QD only and QD with gold nanorods are shown in Fig. 4.4(b). The results show
a shift to shorter lifetimes, as expected, when QDs are close to the end of the gold
nanorods.
57
0 50 100 150 200 250 300 350 40010 2
10 3
10 4
10 5
10 6
Time (ns)
Co un
ts
20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 0
2
4
6
8
10
12
14
16
18
20
Lifetime, τ (ns)
ts
a b QD without rods: τ=59.18 ns QD near gold rods: τ=34.24 ns
Average 50.6 ns
Average 39.6 ns
Figure 4.4: Lifetime measurements of QDs with and without near the gold nanorods. 160-nm diameter silica-clad QDs are self-assembled near the ends of gold rods (50 nm × 400 nm × 50 nm). (a) Distributions of QD lifetimes. There is a shift to shorter lifetimes when QDs are near the gold rod. (b) Two typical decay traces and their fits for QDs with and without gold nanorods.
4.3 NV Center Nanodiamonds near Gold Nanorods
We also studied the modified emission on NV center near the gold naonorod an-
tennas. Modify spontaneous emission from NV centers has been demonstrated in
many studies recently, such as coupling NV centers with different nanostructures,
(e.g. optical antenna, cavities and photonic waveguides), by growing NV centers in
bulk diamond with predefined masks [67–69], spin-coating nanodiamond particles
containing NV centers [70, 71], or nanoposition with dip-pen method [8,72].
Here we apply our inorganic-based electrostatic self-assembly method to integrate
the NV center nanodiamonds with simple gold nanorod optical antenna structures,
and demonstrate Purcell enhancement of the spontaneous emission by showing the
changes of their fluorescent lifetimes.
The same two-step e-beam lithography and evaporation process were used to
fabricate the gold nanorods arrays and Al2O3 pads on a quartz sample. Then the
sample was dipped in the nanodiamond DI-water solution for the electrostatic self-
58
Ti
Figure 4.5: The NV center nanodiamonds are positioned near the ends of the gold nanorods.
assembly. Fluorescent spectra of the gold nanorods with and without NV center
nanodiamonds were acquired with a spectrometer (Princeton Instrument, Acton)
and imaging CCD (Princeton Instrument, ProEM 512BK), as shown in the Fig. 4.6.
The background fluorescence from the gold nanorod extends from 550 nm to 900
nm, which overlaps with the emission range of the NV centers. Note that the 630
nm line in both spectra was from the fluorescence of the immersion oil between the
objective and quartz. To minimize the effect of the background emission from gold
nanorods, a bandpass filter of 680±30 nm was used in the lifetime measurement
below. This range was chosen to avoid overlap with the immersion oil background
but, unfortunately, also eliminate the emission of NV center zero photon line.
500 600 700 800 900 1000
1000
1500
2000
2500
3000
630nm
532nm
wavelength (nm)
Figure 4.6: Fluorescent spectra of gold nanorods with and without NV center nanodiamond near.
The experimental setup for NV center lifetime measurement was basically identi-
cal to the one for QDs, however a 532 nm picoseconds pulsed laser (Spectra-Physics,
High-Q, 78 MHz) was used instead of the 405 nm pulsed laser, due to the very low
absorption around 400 nm wavelength excitation of NV centers.
59
About 40 sites of interest were detected in each of the two scan images of nan-
odiamonds on Al2O3 arrays with and without gold nanorods (400 nm length and 50
nm width). The stage moved to focus the objective on each site to record the fluo-
rescent decay traces for later analysis. For the nanodiamonds on Al2O3 arrays, their
lifetimes were obtained by fitting the experimental decay curves to a bi-exponential
function and calculating the amplitude-weighted lifetime [73,74], as shown in Fig. 4.7
(b), which gave an average lifetime around 10 ns. The summary of all the fittings
is shown in the histograms in Fig. 4.7 (a). For the nanodiamonds near the gold
nanorods, a triple-exponential function [73, 74] was used to fit the decay, in which
the shortest lifetime component was attributed to background emission from the
gold antennas. This gold emission lifetime was measured to be around 0.9 ns by
only pumping the gold nanorod arrays, shown in Fig. 4.7 (b) with black dots. By
fixing the shortest lifetime to be 0.9 ns, the other decay curves were fit and an aver-
age lifetime was obtained around 4.2 ns, as shown in Fig. 4.7 (a), for the NV centers
near gold nanorods. By comparing the lifetime results of the two sets of NV centers
without and with gold nanorods, we observed an average Purcell enhancement of 2×
(i.e. 2× shorter lifetime)
NV near Gold Rods : 4.36 ns
NV on Al2O3 Pads : 10.91 ns
0 8642
C o
u n
0
a b
Figure 4.7: Lifetime measurements of nanodiamonds with and without adjacent gold nanorods. Nanodiamonds are self-assembled near the ends of gold nanorods (50 nm × 400 nm × 50 nm). (a) Distributions of NV center lifetimes. There is a shift to shorter lifetimes when NV centers are near the gold nanorods. (b) Decay curves and their fits for NV centers with and without gold nanorods, and gold nanorods only.
4.4 QDs Embedded in Dielectric Waveguides
For the quantum information applications, single photon emitters are required to be
positioned at specific locations in the optical circuits. For example, single emitters
might be positioned at the input end of the waveguides as the single photon sources,
or inside an optical cavity [75]. To show the capability of our inorganic-based self-
assembly method in these applications, the silica-clad QDs are embedded at the
intersection of the perpendicularly overlapping Al2O3 waveguides.
A two-step photo-lithography technique was used to fabricate such a structure
on a quartz substrate, the process schematic of which is shown in Fig. 4.8. The first
Al2O3 waveguide patterns were written by the photo-lithography. Photoresist AZ
5214 was spin-coated and baked on a quartz