Spatially selective assembly of quantum dot light emitters in an LED using engineered peptides.

Abstract

Semiconductor nanocrystal quantum dots are utilized in numerous applications in nano- and biotechnology. In device applications, where several different material components are involved, quantum dots typically need to be assembled at explicit locations for enhanced functionality. Conventional approaches cannot meet these requirements where assembly of nanocrystals is usually material-nonspecific, thereby limiting the control of their spatial distribution. Here we demonstrate directed self-assembly of quantum dot emitters at material-specific locations in a color-conversion LED containing several material components including a metal, a dielectric, and a semiconductor. We achieve a spatially selective immobilization of quantum dot emitters by using the unique material selectivity characteristics provided by the engineered solid-binding peptides as smart linkers. Peptide-decorated quantum dots exhibited several orders of magnitude higher photoluminescence compared to the control groups, thus, potentially opening up novel ways to advance these photonic platforms in applications ranging from chemical to biodetection.

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February 23, 2011
C2011 American Chemical Society
Spatially Selective Assembly of
QuantumDotLightEmittersinanLED
Using Engineered Peptides
Hilmi Volkan Demir,†,^,*Urartu Ozgur Safak Seker,†,^Gulis Zengin,†Evren Mutlugun,†Emre Sari,†
Candan Tamerler,‡,§and Mehmet Sarikaya§,*
†Department of Electrical and Electronics Engineering, Department of Physics and UNAM, Institute of Materials Science and Nanotechnology, Bilkent University,
06800Ankara,Turkey,‡MolecularBiologyandGenetics,MOBGAM,IstanbulTechnicalUniversity,Maslak34469,Istanbul,Turkey,§GeneticallyEngineeredMaterials
Science and Engineering Center, Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98105, United States, and
^Luminous! Center of Excellence for Semiconductor Lighting and Displays, School of Electrical and Electronic Engineering, Microelectronics Division, School of
Physical and Mathematical Sciences, Physics and Applied Physics Division, Nanyang Technological University, 639798 Singapore
S
have been extensively utilized as functional
materials in a variety of photonic and elec-
tronicdeviceplatforms.1-5QDotsareuseful
becauseoftheiropticalandelectronicprop-
erties including size-tunable optical emis-
sion, high quantum yield, and photo-
stability. Recently, for example, capacity of
QDots has also been demonstrated for
white light generation and tunability in
color-conversion white-light-emitting diodes
(LEDs).6-10To fully exploit useful character-
istics of QDots, however, two significant
limitations still have to be overcome simul-
taneously during the processing of real-life
device settings. First, QDots have to be
assembled with specific spatial distribution
in space with respect to one another for
enhanced functionality; and second, the
dots have to be positioned at desired loca-
tions, at the micrometer scale, on devices
that typically have several different material
components.
In previous studies, the distribution pro-
blem is partially solved by performing the
assembly of QDots, in host polymers (e.g.,
PMMA),11,12layer-by-layer assembly using
thin films of polyelectrolytes (e.g., PAA and
PSS),13-15or the particles were assembled
as closely packed self-aggregates without
using a host scaffold or an additional film.
Noneoftheseapproaches,however,hasthe
potential of assembling the QDots in pre-
specified locations on a practical device,
which usually has multilateral micropat-
terned architectures. For instance, a photo-
nic LED platform consists of at least three
materials, including metal (e.g., gold),
and electroluminescence of LED dictate the
emiconductor quantum dots (QDots),
such as colloidal nanocrystals made of
CdSe/ZnS in core-shell architecture,
semiconductor (e.g., InAlGaN), and a dielec-
tric (e.g., silica) in a complex patterned
architecture.6,16In a standard LED, by driv-
ing a forward current through its metal
contacts, electrons and holes are injected
into the semiconductor. The recombination
of electrons and holes in the active region
gives rise to emission of light by sponta-
neous process ineverydirection. Theresult-
ing photons are coupled out through an
optical emission cone determined by the
ratio of the refractive indices of the semi-
conductor with respect to air and the loca-
tion of charge injection into the active
region. The working principle of color-con-
version LEDs involves optically pumping of
their integrated luminophors, such as phos-
phorsandQDots,throughemissionfroman
optical window. The subsequent collection
of the photoluminescence of luminophors
*Address correspondence to
volkan@bilkent.edu.tr,
hvdemir@ntu.edu.sg,
sarikaya@u.washington.edu.
Received for review November 18, 2010
and accepted February 23, 2011.
Published online
10.1021/nn103127v
ABSTRACT Semiconductor nanocrystal quantum dots are utilized in numerous applications in
nano- and biotechnology. In device applications, where several different material components are
involved, quantum dots typically need to be assembled at explicit locations for enhanced
functionality. Conventional approaches cannot meet these requirements where assembly of
nanocrystalsisusuallymaterial-nonspecific,therebylimitingthecontroloftheirspatialdistribution.
Here we demonstrate directed self-assembly of quantum dot emitters at material-specific locations
inacolor-conversionLEDcontainingseveralmaterialcomponentsincludingametal,adielectric,and
a semiconductor. We achieve a spatially selective immobilization of quantum dot emitters by using
the unique material selectivity characteristics provided by the engineered solid-binding peptides as
smart linkers. Peptide-decorated quantum dots exhibited several orders of magnitude higher
photoluminescence compared to the control groups, thus, potentially opening up novel ways to
advance these photonic platforms in applications ranging from chemical to biodetection.
KEYWORDS: directed assembly.self-assembly.material selectivity.inorganic
binding peptides.quantum dots.LEDs.optoelectronics
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operation of the device. Immobilization of QDots only
on the targeted optical windows on a LED chip is,
therefore,essentialsimplybecauseonlytheQDotsself-
assembled at the desired locations can be efficiently
pumped. The QDots assembled on the metal or the
semiconductor regions out of the emission cone are
not pumped and, therefore, do not contribute to the
overall luminescence of the hybrid device. As a con-
sequence, in addition to avoiding the excess use of
QDots unnecessarily accumulated in undesired areas
ofthedevice,another,moreimportant,problemisalso
eliminated. In fact, the particles, assembled on metal
contacts, could hinder the connection between con-
tacts and wire or probe to the electric supply, thereby
making it difficult to drive the LED if their metal
connections are made after the immobilization of
QDots. Eliminating such a problem in real devices, at
large dimensional scales, is impractical during both
fabrication and performance, requiring additional
steps including the use of the traditional patterning
techniques involving multiple lithography steps and
cleaning procedures, or always necessitating the wir-
ingofthedevicesbeforehand.17-19Toovercomethese
limitations, our unique approach utilizes inorganic
solid-binding peptidesthataremolecularlinkers,func-
tionalizing the QDots, and have specific material selec-
tivity property, essential for selective spatial assembly.
To demonstrate how this approach works, we show
here the directed assembly of peptide-functionalized
QDots only on the targeted optical windows of a
nanocrystal-based color-conversion LED. Since the
fabrication approach is straightforward via a simple
patterning, potentially, it could be easily adapted to
other, and more sophisticated, chip configurations
such as modulators,20photovoltaics,21,22photodetec-
tors,23and optical memories.24
The genetically engineered peptides for inorganics
(GEPI) have been biocombinatorially selected using
phage or cell surface display approaches.25-28These
short amino-acid-containing sequences are then char-
acterized for their binding affinity and selectivity to-
wardtheirrobustutility.Thesolid-bindingpeptidesare
increasingly becoming ubiquitous molecular elements
frequently utilized inthe synthesis ofinorganic materi-
als and as linkers and assemblers of nanoscale
objects.29-32It is now also possible to in silico design
a new generation of peptides with enhanced binding
and material selectivity characteristics.33In this work,
we use such a designed peptide, the silica-binding
peptide (QBP1) as silica-specific linkers for QDots. The
QBP1hasa12aminoacidsequence,PPPWLPYMPPWS,
displaying binding affinity and free energy of binding
values of 7.0 ? 105M-1and 8.0 kcal/mol, respectively
(see Supporting Information). When used alone, QBP1
isfoundtohaveasurfacecoverageofabout80%under
equilibrium conditions. We used biotinylated peptide
(i.e., QBP1-bio) as a molecular linker for streptavidin-
functionalized QDot (i.e., SA-QDot) immobilization on
the targeted regions on the device. This way, the
peptide now features bifunctionality: silica binding
coupled with molecular recognition of QDot through
SA. In this study, we first explored the selectivity and
specificity of the peptide-decorated QDot hybrids on
silica, GaN, and gold surface to ensure their suitability
for our optoelectronic device application. The specifi-
city and affinity of these nanohybrids were found
satisfying. Further we employed the nanohybrid struc-
tures of QDots to furnish LED devices with high
selectivity. For the first time, we demonstrated the
utilization of peptide-QDot nanohybrids on a real
chip application. The results promise significant inter-
estforopeningnewavenuesindesigningandbuilding
up novel nanostructure-based devices.
RESULTS AND DISCUSSION
We first explored the process of spatially selective
assembly of QDots with high precision and surface
coverage. We used two approaches to build a layer of
QDots on the silica regions of the device (Figure 1).
First, the peptides were assembled by incubating
them on a polished 7 mm ? 7 mm silica substrate
(with a rms roughness of <2 nm) overnight in an
incubation chamber. The procedure involved a thor-
ough washing step that removed nonspecifically
bound and unbound peptides from the surface. At
this stage, the QBP1-bio-decorated surface was ready
for the assembly of SA-QDots, which was carried out
by the drop-casting technique. Again, the sample was
washed to remove any nonspecifically bound QDots
to ensure stable and homogeneous QDot film on the
surface. A negative control sample was also prepared
by assembling the SA-QDot layer on the silica sub-
strate directly in the absence of QBP1-bio (Figure 1a).
The photoluminescence (PL) measurements of the
samples, prepared with and without the QBP1-bio as
the linker, reveal that they are not significantly differ-
entfromeachother.Furthermore,theopticalbehavior
ofthesesamples,whichwerepreparedbythesequen-
tial assembly approach, was not reproducible. As we
show in Figure 1b, the contrast variation in the fluor-
escence image exhibits a nonhomogeneous spread-
ing of the SA-QDot on silica, with some of the regions
on the chip exhibiting as low a PL signal as in the
control group (Figure 1d). The problems revealed by
these preliminary observations are related to the
limited mass transfer of the QDots in solution. It,
therefore, takes a substantial amount of incubation
time for QDot-SA to diffuse and reach the QBP1-bio-
functionalized silica surface regardless of the number
of the available molecules on the target surface. The
sequential way for targeted assembly of QDots on a
silica substrate is a mass transfer-limited (or diffusion-
limited) process as there is no further mixing involved
to accelerate the mobility of the QDots reaching and,
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then, binding to the surface through SA-bio molecular
recognition.
To overcome the mass transfer problem described
above, in the second approach, the QBP1-bio solution
was first mixed with the QDot colloidal solution before
the drop-casting step. This would allow QDot colloids
andQBP1-bio molecules tobeinclose proximity inthe
buffer solution before the resultant homogeneous
mixtureisbroughtintocontactwiththesilicasubstrate
(see Supporting Information for detailed procedures).
The samples prepared via the peptide-assisted assem-
bly of the QBP1-QDot nanohybrids on the silica sub-
strate resulted in a significant enhancement in the PL
intensity compared to the control experiments
(Figure 1d). Furthermore, particle assembly was highly
uniform across hundreds of micrometer-wide regions
on the sample surface (Figure 1c). This hybrid ap-
proach, as a consequence of the close control of the
colloid mixture as well as the directed assembly of the
QDots, resulted in a 2 orders of magnitude increase in
PL intensity compared to the negative control and
more than 1 order of magnitude more intensity over
the sequential approach described above (Figure 1d).
In Figure 1, there is an obvious red shift in the peak
emission wavelength of QDots decorated with QBP1
compared tothe originalpeak emission wavelength of
theQDots.Thisredshiftiscommonwhenthedielectric
constant of the environment around the QDots in-
creasesduetotheconjugationofmolecules.34,35There
is also a broadening in the emission spectrum of
nonspecifically adsorbed streptavidin-coated quan-
tum dots on the silica surface. This is attributed to
the change of the hydrodynamic radius, which may be
triggered by denaturation of streptavidin upon non-
specific binding on silica surface. This process may
yield random change in the hydrodynamic radius of
each QDot, causing a collectively broadened emission
over a slightly wider range of wavelength.
In the next step, we demonstrate the binding spe-
cificity of QBP1-bio on an actual optoelectronic micro-
chipsurface,whichwasfabricatedforsolid-state-based
lighting applications.5On such an optoelectronic chip,
there are at least three different types of patterned
materials: dielectric (silica), metal (gold), and semicon-
ductor (GaN) (Figure 2). The multimaterial character-
istics of the chip, therefore, necessitate material-
selective binding during the QDot assembly process.
To target quantum dot assembly on specific regions of
the patterned dielectric surface (silica regions), one
mustdirecttheQDotsspatiallyandselectivelyontothe
targeted regions, while avoiding both the metal and
the semiconductorregions entirely.Toensurematerial
specificity of the solid-binding peptide used in this
work, we first characterized the binding specificity of
the QBP1-bio construct alone on the silica-patterned
gold substrate as well as gold-patterned silica sub-
strate, where silica was used as the dielectric and gold
as the conductor (Figure 2a). Preparation of the two
different architectures allowed us to investigate the
feasibility of decorating small features of patterned
device mesas in complementary geometry in typical
optoelectronic chips. Using the successful hybrid ap-
proach developed above, we incubated the samples
with each geometry using the hybrid (QBP1-bio)/(SA-
QDot) colloid solution, as described. Next, the PL
intensitiesofthesamplesfromthemetalanddielectric
regions in both geometries were recorded as a func-
tion of wavelength (Figure 2b). Since the QBP1-bio is
only selective to silica, but not to gold, the PL intensity
peak from the silica regions of the chip is 9 times
Figure 1. (a) Schematics of the assembly of QDots on silica: (i) without the use of silica (negative control), (ii) with QBP-bio
bound to the silica surface, and (iii) with the QBP-bio bound to the (SA-QDot). The panels in (b) and (c) correspond to
fluorescence images (at 610 nm) and PL spectra, respectively. The image in b (i) reveals minimum coverageand c (i) very low
CL signal. The approach in (ii), i.e., directing SA-QDot to QBP-bio-covered silica surface, has partial success, revealing
nonhomogeneous, patchy coverage, shown in b (ii) and fairly low CL signal in c (ii). Finally, colloidal mixture of QBP-bio and
SA-QDotleadstohomogeneouscoverageofthesilicasurfacewithcontinuousfluorescenceinc(iii)andcorrespondinglyhigh
CL signal in c (iii).
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stronger than that from the gold regions. Similar to
the gold/silica material selectivity tests above, we
also carried out experiments to test the material
selectivityofQBP1 between
(Figure2c),thelatterbeingthecriticalsemiconductor
component of LED chips. As shown in Figure 2, the
(QBP1-bio)/(SA-QDot) mixed colloid exhibited low
affinity toward GaN compared to the silica, as evi-
denced in the fluorescence images in both geome-
tries and the corresponding differences in the PL
signals (Figure 2d). These results demonstrate speci-
fic binding of the peptide-decorated QDots on the
silica surface but not on the GaN or gold regions of
the chip (see the control experiments in Supporting
Information, Figure S1).
In conventional methods, a number of different
masking steps are necessary to pattern the QDot film
in specific regions on an optoelectronic chip.36-39
These procedures, involving additional photolitho-
graphic patterning and subsequent removal of the
QDot film by etching, add additional masking steps,
thereby making the fabrication process laborious. Si-
milarly, the chemical immobilization methods involve
many different surface modification steps,40,41again
resulting in lengthy procedures. In our approach, we
silica and GaN
use an actual device, in somewhat complex architec-
ture, containing the three separate material regions of
silica, gold, and GaN, as summarized in Figure 3.
Following the targeted assembly of the QDots using
the silica-binding peptide as the molecular linker, we
next demonstrated device performance via the mea-
surement of the PL excitation. It can be claimed that
one can create a binary pattern using nonspecific
(hydrophobic-hydrophilic) interactions, but for the
real chip applications, a more complex selectivity is
desired(e.g.,inourmicrochipwhereweshouldbeable
to differentiate gold, silica, and GaN). This can be only
achieved by utilizing the specific binding where the
specificity implies material selectivity, here brought
about by the silica-binding peptide.
The simple LED device is made of InGaN/GaN quan-
tum wells embedded in a vertical p-i-n diode archi-
tecture with one metal contact to the p-side of the
diode, at the top, and another metal contact, to the
n-side of the diode, at the bottom. The top view of the
fabricated LED showninthe optical imageinFigure 3A
reveals fairly complex but distinct regions of the metal
contacts (yellow), the semiconductor mesa (gray), and
the silica film on the top (brownish), where the QDots
wouldbeassembled.ThehybridQDot-LEDisoperated
Figure 2. Schematics of QDot assembly on the silica region in a multimaterial containing mesa architecture (either ring or
island), containing either (a) Au/SiOxor (b) GaN/SiOxusing the (QBP1-bio)/(SA-QDot) hybrid molecular construct. The
corresponding fluorescence images of peptide-assembled QDots are shown below the sketches. The reversal of color to red
indicates that the dots assemble only on the silica regions in all cases. The photoluminescence spectra in (c) and (d)
correspond to (a) and (b), revealing a 9-fold stronger signal (red) from the silica regions in all cases, compared to those from
either gold or GaAs regions, respectively.
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by using the LED as the pump light source and
assembled QDot layer as the photoluminescent layer.
TheLEDemitsinbluewhenelectricallydrivenatroom
temperature (Figure 3B) and optically pumps as-
sembled QDots. The performance of the device is
demonstrated in Figure 3C, showing the electrolumi-
nescence spectra (EL) emitted from the LED at a peak
wavelength of 462 nm in blue. The EL signal is
parametrized with respect to the various levels of
current injection, ranging from 0.1 to 1.3 mA, at room
temperature. Here we observe that, as the current
injection is increased, the electroluminescence of the
LED is expectedly increased. The purpose of the next
step is to self-assemble the QDots selectively on the
targeted silica regions of the device. To accomplish
this task, we used the procedure of mixing QBP-bio
and functionalized the SA-QDot in the colloidal solu-
tion, which, as demonstrated above and illustrated in
Figures1and2,providesthemostsuccessfultargeted
assembly while distinguishing one material region
from another because of the presence of the silica-
binding peptides that are material-selective with
preference to silica. This is demonstrated in the con-
focal microscopy image in Figure 3D, which confirms
that the red-emitting, peptide-decorated QDots are
assembled fairly uniformly on the silica film of the
LED, leaving anywhere else black, including the Au
and GaN regions. These results are also schematically
illustrated in Figure 3E for clarity, outlining the device
architecture; the QDots assemble in the silica regions
(light blue), leaving the gold (yellow) and GaN (violet)
regions untouched.
In all of the assembly experiments to investigate
comparatively the optical quality of the resulting as-
semblies using these different approaches, we used
commercial silica slides with very low surface rough-
nesspurchasedfromthevendorSPI(Pennsylvania).On
the other hand, Figure 3D shows the application of the
hybrid assembly approach to real LED chip surfaces.
During the fabrication of these LEDs, we coated silicon
oxide films using plasma-enhanced chemical vapor
deposition, which were subsequently patterned using
standard photolithography and reactive ion etching.
As a result, the film quality of the silica coating on our
LEDchipsisworsethanthatofthesilicaslidesacquired
fromSPI.ThesilicafilmsonourLEDssufferhighsurface
roughness and irregularities, which in turn give rise to
non-uniformity in the assembled QDot film on these
LEDs.
It is also possible to optically excite the QDots
directly with the electroluminescence of the LED for
color conversion by electrically driving it since the red-
emitting QDots are intimately integrated on the sur-
faceoftheLED.Wecarriedoutsuchanexperimentand
obtained the resulting PL spectra from the device, as
demonstrated in Figure 3F. In the spectra, the inte-
grated QDots emit in red (at 620-625 nm) upon their
optical excitation with the LED electroluminescence
(shown Figure 3B). In these PL spectra, the parametri-
zation was carried out at various levels of current
Figure 3. (A) Plan view of the fabricated LED device with microarchitectured metal, semiconductor, and dielectric regions
(lightopticalmicroscopy).(B)Imageand(C)profilesofelectroluminescenceofthemicrofabricatedLEDdeviceat462nmand
atvariouslevelsofdrivingcurrentatroomtemperature.(D)SchematicrepresentationoftheLEDshownin(A)withthe(QBP1-
bio)/(SA-QDot) hybrid construct targeted assembly only on the silica regions of the device. (E) Fluorescence microscopy
imagecontrasted bylightemittinginred at 620nm,confirming directedself-assemblyof the(QBP1-bio)/(SA-QDots)onlyon
thetargetedsilicasurfaceofthedevice.(F)Photoluminescenceofthehybridconstructtargetedassemblyonthesilicathatis
optically pumped by the electrically driven LED at room temperature.
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