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Journal ArticleDOI

Combinatorial approaches toward patterning nanocrystals

21 Oct 1998-Journal of Applied Physics (American Institute of Physics)-Vol. 84, Iss: 7, pp 3664-3670

AbstractA scheme for generating complex, spatially separated patterns of multiple types of semiconducting and/or metallic nanocrystals is presented. The process is based on lithographic patterning of organic monolayers that contain a photolabile protection group and are covalently bound to SiO2 surfaces. The process results in spatially and chemically distinct interaction sites on a single substrate. Nanocrystal assembly occurs with a high selectivity on just one type of site. We report on the production of binary, tertiary, and quatemary patterns of nanocrystals. We highlight and discuss the differences between nanocrystal/substrate assembly and molecule/substrate assembly. Finally, we investigate the assembled structures using photoluminescence and absorption spectroscopy.

Topics: Nanocrystal (53%)

Summary (2 min read)

Introduction

  • The process is based on lithographic patterning of organic monolayers that contain a photolabile protection group and are covalently bound to SiO2 surfaces.
  • The particles and to prevent formation of bulk material, any p terning approach must be carried out at low temperatures in nonchemically hostile environments.
  • The chemistry involved the selective assembly of nanocrystals is different than the case of molecules, and these differences are highlig and discussed.

A. General information

  • All chemicals used were of analytical grade or of high purity available and obtained from Aldrich, Fisher, Acro Gelest, and Boehringer Mannheim.
  • Spectra were taken with a home-built spectrometer using 473.6 nm line of an argon–krypton laser~coherent! for excitation.
  • Images were taken with a Cambridg 360 scanning electron microscope operated at 15 kV.
  • B. Preparation of nitroveratryloxycarbonyl–glycine „NVOC–GLY… Synthesis of the compound NVOC–GLY has been m tioned before12 but has not been described in detail.

D. Pt nanocrystals

  • The volume of the black ganic layer was reduced to about 10 mL by rotary evapo tion.
  • The particles were precipitated by adding 400 mL a etone and storing at24 °C overnight.
  • Images of the samples revealed average metal core diameter of 2.560.5 nm.
  • The concentra tion used in experiments with the lithographically pattern substrates corresponded to an optical density of 0.5 at 1 path length at 700 nm.

E. Epitaxially grown CdSe/CdS-core/shell structured nanocrystals

  • The synthesis of dodecylamine stabilized CdSe/C core/shell structured NCs has been described in literature.
  • Co responding to their emission maxima the particles had a age CdSe core diameters of 2.4 and 3.7 nm and a CdS thicknesses of about 1.5 and 1.0 ML, respectively.
  • As co pared to Rhodamine 560, the fluorescence quantum y was around 18% for the YNC solution and 8% for the RN solution.
  • The particle concentrations of the solutions the authors u for treating the substrates corresponded to an optical den of 0.5 at 350 and 430 nm for the YNCs and the RNCs a cm path length, respectively.

F. Preparation of photosensitive glass and Si/SiO 2 substrates

  • Glass slides and Si wafers were cleaned according to procedure described by Linfordet al.15 Briefly, microcover slides and pieces of silicon wafers were cleaned by trea them first with a mixture of concentrated H2SO4 and 30% H2O2 ~70:30 v/v! at 100 °C for 1.5 h.
  • The substrates were wash extensively and stored under 18.2 MV cm water until needed.
  • All prepa tions with the light sensitive NVOC group were carried o in a dark-room environment.

G. Lithographic fabrication of patterned nanocrystal arrays

  • This long exposure time is due to the low-power UV em sion from the lamp.
  • Th amplification of particle binding, where the initially as sembled particles themselves served as a selective subs was repeated once.
  • The whole slide was then immersed in solution of YNCs for 45 min~step 2!.
  • An amino-functionalized substrate that had been protected with photolabile NVOC group was partly deprotected by irradiation throug mask in the near UV~I!.
  • The substrate was then treated with a solution of amine bilized nanoparticles, which assembled on areas of deprotected a groups~II !.

A. Preparation of photosensitive glass and Si/SiO 2 substrates

  • Under the conditions the authors chose to functionalize glass Si/SiO2 substrates with amino groups it was found that trialkoxysilane 3-aminopropyltriethoxysilane polymerize and precipitated on the substrate.
  • Even the best slides, when probed by S were often characterized by a small amount of submic particulate matter on the substrate.
  • This matter appare originated from the surface-functionalization step.
  • As m sured by the efficiency of particle binding, the dialkoxys lanes generated a higher coverage of amino sites than monoalkoxysilanes, and were used for most of the exp ments discussed here.
  • Solution, the authors could not reac NVOC–Cl with surface-bound amino groups.

B. Lithographic fabrication of patterned nanoparticle arrays

  • This pattern could be dev oped with fluorescent markers.
  • Selective assembly of particles onto amine-patter substrates works well if the particle surfactants readily dergo ligand exchange with the surface amino groups.
  • After the red luminescent particles had been bound, NVOC half of the slide was photodeprotected and the wh substrate was immersed in a solution of YNCs~Fig. 2, step 2!.
  • ~corresponding to the similarly labeled quadrants of Fig. 2! the absorption spectra of the quadrants Y/Au and R/Au clearly reveal presence of Au NCs as indicated by the absorption cent at 530 nm.
  • U these data, and taking into account the different PL quan yields, the authors calculated that the binding selectivity is arou 93%.

IV. CONCLUSIONS

  • Amino functionalized substrates that have been bloc with the photolabile protection group NVOC can be used prepare complex, two-dimensional nanoparticle arrays lithographic masking techniques.
  • The technique for prod ing these patterns occurs under conditions that are both bient and chemically mild.
  • Other techno logically relevant substrates, such as semiconducting p mers, should be relatively straightforward extensions of procedures developed here.
  • The authors are currently working on and other aspects relevant to integrating nanocrystal ass blies into microelectronic test devices.

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Combinatorial approaches toward patterning nanocrystals
T. Vossmeyer, S. Jia, E. DeIonno, M. R. Diehl, and S.-H. Kim
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569
X. Peng and A. P. Alivisatos
Department of Chemistry, University of California, Berkeley, California 94720-1460
J. R. Heath
a)
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569
~Received 7 April 1998; accepted for publication 7 July 1998!
A scheme for generating complex, spatially separated patterns of multiple types of semiconducting
and/or metallic nanocrystals is presented. The process is based on lithographic patterning of organic
monolayers that contain a photolabile protection group and are covalently bound to SiO
2
surfaces.
The process results in spatially and chemically distinct interaction sites on a single substrate.
Nanocrystal assembly occurs with a high selectivity on just one type of site. We report on the
production of binary, tertiary, and quatemary patterns of nanocrystals. We highlight and discuss the
differences between nanocrystal/substrate assembly and molecule/substrate assembly. Finally, we
investigate the assembled structures using photoluminescence and absorption spectroscopy.
© 1998 American Institute of Physics. @S0021-8979~98!06719-X#
I. INTRODUCTION
During the past few years, solution-phase synthetic
schemes for the generation of semiconducting and metallic
nanocrystals ~NCs! have advanced to the point where these
particles are now of similar quality to the high-grade mate-
rials used by the modern microelectronics industry. Concur-
rent with this development have been a number of ‘‘proof-
of-principle’’ demonstrations in which NCs have been
integrated into various electronic
1–3
and photonic
4
devices or
have been shown to catalyze various reactions.
5
These dem-
onstrations have shown that, indeed, NCs may play an im-
portant role in future technologies such as photovoltaics,
switches, phosphors, light-emitting diodes ~LEDs!, elec-
tronic data storage systems, and sensors. For all of these
applications, particle size and stoichiometry both play impor-
tant roles. For photonics-based applications, these variables
determine color, and for electronic-based devices, particle
size determines switching or charging voltages. Most test-
case devices have been fabricated as either single-particle
devices ~serial fabrication!, or in such a fashion that the spa-
tial positioning of the NCs is not important. However, most
of the photonics and electronics applications will eventually
require parallel schemes for the control of spatial positioning
of the NCs. Examples might include multiple color LEDs,
color pixels for field-emission displays, or multichannel
chemical sensors. Standard patterning techniques, such as
laser ablation of the material and deposition through a
shadow mask, do not work well for NCs since most metal
and semiconductor NCs have covalently bound organic sur-
factants that tend to desorb at temperatures around 100 °C.
Because the surfactants are usually necessary to stabilize the
particles and to prevent formation of bulk material, any pat-
terning approach must be carried out at low temperatures and
in nonchemically hostile environments.
Only a couple of methods for the preparation of spatially
resolved assemblies of NCs have appeared in the recent lit-
erature. Whitesides and co-workers
6
prepared micropatterns
of palladium colloids, which in turn served as a catalyst for
electroless deposition of copper. They used a micropatterned
poly~dimethylsiloxane!~PDMS! stamp soaked with Pd-
colloid solution and stamped patterns onto functionalized
Si/SiO
2
, polyimide, and glass substrates. Microcontact print-
ing for particle assembly has several advantages: it is fast,
simple, cheap, and capable of surprisingly high spatial reso-
lution. The primary disadvantage is that the technique does
not employ a mask, thus making it incompatible with other
patterned processing steps that are common to many device
fabrication schemes. We have recently described how a strat-
egy for combinatorial solid-phase peptide synthesis devel-
oped by Fodor and co-workers
7
can be extended to light-
direct assemblies of Au, Pt, or CdSe nanoparticles onto solid
supports.
8,9
In that approach, photolabile protection groups
@nitroveratryloxycarbonyl ~NVOC!# coupled to an amino
functionalized substrate were selectively removed by irradia-
tion through a mask in the near UV. The resulting pattern of
protected and unprotected amino groups was then exposed to
the nanoparticle solution and selective particle assembly
~.85% selectivity! was achieved on areas with exposed
amino groups. A similar approach had previously been used
to create patterned arrays of biomacromolecules.
10,11
In this article we describe how clean patterns of multiple
types of metal and semiconductor NCs can be produced, in
which each type of nanocrystal is spatially separated from
every other. As an example, we describe the preparation of a
substrate characterized by spatially distinct luminescence
properties, using different sizes of highly luminescent core/
a!
Author to whom correspondence should be addressed.
JOURNAL OF APPLIED PHYSICS VOLUME 84, NUMBER 7 1 OCTOBER 1998
36640021-8979/98/84(7)/3664/7/$15.00 © 1998 American Institute of Physics
Downloaded 19 May 2006 to 131.215.225.175. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

shell structured CdSe/CdS NCs. The chemistry involved in
the selective assembly of nanocrystals is different than for
the case of molecules, and these differences are highlighted
and discussed. Finally, we characterize the efficiency of our
selective patterning process, as well as the properties of the
patterned NCs, by interrogating the photophysical signatures
of the various patterned regions on the substrate.
II. EXPERIMENTAL SECTION
A. General information
All chemicals used were of analytical grade or of highest
purity available and obtained from Aldrich, Fisher, Acros,
Gelest, and Boehringer Mannheim. Photoluminescence ~PL!
spectra were taken with a home-built spectrometer using the
473.6 nm line of an argonkrypton laser ~coherent! for ex-
citation. Absorption spectra were taken with a Hewlett Pack-
ard 8451 A diode array spectrophotometer. Scanning elec-
tron microscope ~SEM! images were taken with a Cambridge
360 scanning electron microscope operated at 15 kV. Optical
micrographs were taken with a remodeled ReichertJung
Polyvar Infrapol microscope operated in reflection mode and
equipped with a charge-coupled device camera or with a
ReichertJung MET fluorescence microscope equipped with
an RCA silicon intensified camera and an argon ion laser for
fluorescence excitation.
B. Preparation of nitroveratryloxycarbonylglycine
NVOCGLY
Synthesis of the compound NVOCGLY has been men-
tioned before
12
but has not been described in detail. 0.27 g
~3.6 mmol! glycine was dissolved in 14 mL 10% ~w/v! so-
dium carbonate solution followed by addition of 10 mL di-
oxane. The mixture was stirred in an ice bath and 1.0 g ~3.6
mmol! nitroveratryloxycarbonyl chloride was added slowly.
The mixture was allowed to warm to room temperature and
stirred for several hours. After this, the mixture was poured
into 200 mL water and then washed three times with 80 mL
diethylether. Under vigorous stirring, a pH of 23 was ad-
justed by slowly adding 5 M HCl. The orange-yellow pre-
cipitate obtained was refrigerated overnight, and subse-
quently, filtered and then recrystallized two times from a
water/methanol ~50:50 v/v! mixture. The melting point was
180 °C and the yield 70%.
C. Au nanocystals
Preparation of the dodecylamine stabilized gold nanoc-
rystals has been described in the literature.
13
We used reac-
tion scheme 1 given in Ref. 13 to prepare particles of an
average core size of 2.6 nm. In order to react the particles
with the lithographically patterned substrates, we dissolved
them in toluene to a concentration corresponding to an opti-
cal density of 0.5 at 0.5 cm path length at 550 nm.
D. Pt nanocrystals
Preparation of the Pt nanoparticles is analogous to the
preparation of gold nanoparticles. 152 mg ~0.445 mmol! of
PtCl
4
was dissolved in 15 mL deionized water. To this stir-
ring solution was added 247 mg ~0.445 mmol! N~C
8
H
17
!
4
Br
in 15 mL toluene, resulting in two-layer separation. Next, a
solution of 82 mg ~0.445 mmol! C
12
H
25
NH
2
in 15 mL tolu-
ene was added, which immediately turned the solution milky
yellow. Then, 185 mg ~4.89 mmol! NaBH
4
dissolved in 15
mL water was added. The milky solution immediately turned
black and was allowed to stir overnight. The aqueous layer
was separated and discarded. The volume of the black or-
ganic layer was reduced to about 10 mL by rotary evapora-
tion. The particles were precipitated by adding 400 mL ac-
etone and storing at 24 °C overnight. The particles were
then filtered and redissolved in toluene. Transmission elec-
tron microscope ~TEM! images of the samples revealed an
average metal core diameter of 2.560.5 nm. The concentra-
tion used in experiments with the lithographically patterned
substrates corresponded to an optical density of 0.5 at 1 cm
path length at 700 nm.
E. Epitaxially grown CdSe/CdS-core/shell structured
nanocrystals
The synthesis of dodecylamine stabilized CdSe/CdS-
core/shell structured NCs has been described in the
literature.
14
Here, we used yellow (l
max
5560 nm) and red
(l
max
5610 nm) luminescent NCs ~YNCs and RNCs! dis-
solved in a 0.1 M solution of octylamine in chloroform. Cor-
responding to their emission maxima the particles had aver-
age CdSe core diameters of 2.4 and 3.7 nm and a CdS shell
thicknesses of about 1.5 and 1.0 ML, respectively. As com-
pared to Rhodamine 560, the fluorescence quantum yield
was around 18% for the YNC solution and 8% for the RNC
solution. The particle concentrations of the solutions we used
for treating the substrates corresponded to an optical density
of 0.5 at 350 and 430 nm for the YNCs and the RNCs at 1
cm path length, respectively.
F. Preparation of photosensitive glass and Si/SiO
2
substrates
Glass slides and Si wafers were cleaned according to the
procedure described by Linford et al.
15
Briefly, microcover
slides and pieces of silicon wafers were cleaned by treating
them first with a mixture of concentrated H
2
SO
4
and 30%
H
2
O
2
~70:30 v/v! at 100 °C for 1.5 h. Then, the substrates
were rinsed with deionized water and immersed into a mix-
ture of 29.2% ammonium hydroxide solution and 30% H
2
O
2
~70:30 v/v! for half an hour. The substrates were washed
extensively and stored under 18.2 MV cm water until
needed. To react the substrates with aminoethoxysilanes we
used the experimental setup described by Haller.
16
A rack
holding the dried substrates was immersed into 250 mL of
dry toluene into which 1.5 mL ~7.2 mmol! 3-
aminopropylmethyldiethoxysilane were injected. When us-
ing 3-aminopropyldimethylethoxysilane only 0.6 mL ~3.2
mmol! were injected. As described in the results section, we
also tried to use 3-aminopropyltriethoxysilane at similar con-
centrations. After refluxing the solution overnight under ar-
gon atmosphere, the substrates were transferred into dry tolu-
ene and stored under argon until needed.
3665J. Appl. Phys., Vol. 84, No. 7, 1 October 1998 Vossmeyer
et al.
Downloaded 19 May 2006 to 131.215.225.175. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

In order to react the photolabile compound NVOCGLY
with the surface bound amino groups, the substrates were
washed with dry CH
2
Cl
2
and put into a closed reaction ves-
sel containing 2 mL of dry CH
2
Cl
2
. A solution of 30 mg ~95
m
mol! NVOCGLY in 100
m
L dry dimethylformamide
~DMF! was injected followed by 20
m
L ~128
m
mol! 1,3-
diisopropylcarbodiimide ~DIC!. After standing for1hat
room temperature the NVOCGLY/DIC treatment was re-
peated once. The substrates were then put into a solution of
200
m
L ~2.1 mmol! acetic anhydride and 350
m
L ~2.0 mmol!
diisopropylethylamine in 3 mL of dry DMF in order to block
any amino groups that had not reacted. After washing the
slides with ethanol and water and drying them in an argon
stream, they were stored under argon at 218 °C and used for
the lithographic experiments within one week. All prepara-
tions with the light sensitive NVOC group were carried out
in a dark-room environment.
G. Lithographic fabrication of patterned nanocrystal
arrays
The strategy is illustrated in Fig. 1. Photosensitive sub-
strates were irradiated for 30 min through a mask in the near
UV/Vis ~l.360 nm! with a 1000 W tungstenhalogen lamp.
This long exposure time is due to the low-power UV emis-
sion from the lamp. We have demonstrated that if a 364 nm
Ar
1
ion laser is used for irradiation, the exposure time is
reduced to just a few seconds. The infrared radiation from
the lamp will quickly heat up a substrate, so it was removed
by passing the light beam through a water filter. For prepar-
ing macroscopic patterns, the substrates were immersed in a
55.0 mM solution of semicarbazide hydrochloride in metha-
nol and partly covered by the mask during irradiation. Mi-
cropatterns were prepared by sandwiching the semicarbazide
hydrochloride solution between the substrate and a stripe-
patterned chrome-on-glass mask. The resulting NH
2
/NVOC
patterns of deprotected and still protected amino groups
could be tested by fluorescence microscopy after labeling the
free-amino groups with either ATTO-TAG,
17
or fluorescein
by reaction with fluorescein isothiocyanate.
7
For preparing binary micropatterns consisting of metal
~Pt or Au! and CdSe/CdS-core/shell NCs, the metal particles
were first selectively assembled by immersing the
NH
2
/NVOC-patterned substrate into a solution of the par-
ticles for about 1 h. When using Au NCs, the substrate was
subsequently treated with a 0.1 M solution of 1,8-
octanedithiol in methanol for 10 min and then again im-
mersed into the Au particle solution for half an hour. This
amplification of particle binding, where the initially as-
sembled particles themselves served as a selective substrate,
was repeated once. When Pt NCs were assembled, buty-
lamine was added to the toluene/Pt colloidal solution up to a
concentration of 0.1 M. The same procedure was used to
amplify particle binding with the exception that the dithiol
solution was replaced with a 0.1 M solution of 1,6-
hexanediamine in methanol. Here, the amplification was not
repeated.
After the selective assembly of the metal particles onto
the substrate, a second deprotection was performed with the
stripe mask oriented perpendicular to the lines of the metal
particle assemblies. The substrates were then exposed to a
solution of RNCs for about half an hour, washed with sol-
vent, and dried.
The macroscopic patterns illustrated in Fig. 2 were pre-
pared as follows. Half of a glass slide was deprotected, and
the whole slide was immersed in a solution of RNCs for 30
min ~step 1!. The other half of the slide was then deprotected
with a 0.2 M solution of butylamine in toluene sandwiched
between the substrate and the mask ~instead of the methan-
olic semicarbazide solution!, because alcohols were found to
irreversibly quench the luminescence of the substrate bound
core/shell NCs. The whole slide was then immersed in a
solution of YNCs for 45 min ~step 2!. After treating the slide
for 10 min with a 0.1 M solution of 1,8-octanedithiol in
toluene, it was washed with toluene and only one half of the
slide ~step 3A! was dipped into Au-nanoparticle solution for
45 min to give a quaternary pattern. In a similar experiment
the whole slide was dipped into a solution of RNCs for 30
min after treating the binary pattern with the dithiol ~step
3B!. After each assembly step these patterns were investi-
FIG. 1. Reaction scheme for the stepwise preparation of multiple particle
arrays. An amino-functionalized substrate that had been protected with the
photolabile NVOC group was partly deprotected by irradiation through a
mask in the near UV ~I!. During deprotection the substrate was covered with
a solution containing a reagent for scavenging reactive side products of the
photoreaction. The substrate was then treated with a solution of amine sta-
bilized nanoparticles, which assembled on areas of deprotected amino
groups ~II!. The deprotection was repeated ~III! and another type of amine-
stabilized nanoparticles was assembled onto areas of freshly deprotected
amino groups, yielding a binary nanoparticle array ~IV!. The cartoon depicts
the assembly of patterns only a few nanocrystals wide, and was drawn this
way for the sake of clarity. In fact, the written domains were actually a few
microns (10
2
–10
3
particles! wide.
3666 J. Appl. Phys., Vol. 84, No. 7, 1 October 1998 Vossmeyer
et al.
Downloaded 19 May 2006 to 131.215.225.175. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

gated by UV/Vis absorption and/or fluorescence spectros-
copy.
III. RESULTS AND DISCUSSION
A. Preparation of photosensitive glass and Si/SiO
2
substrates
Under the conditions we chose to functionalize glass and
Si/SiO
2
substrates with amino groups it was found that the
trialkoxysilane 3-aminopropyltriethoxysilane polymerized
and precipitated on the substrate. The use of the dialkoxysi-
lane 3-aminopropylmethydiethoxysilane and the monoalkox-
ysilane 3-aminopropyldimethylethoxysilane usually re-
sulted in substrate surfaces that were totally transparent and
could not be distinguished from untreated slides by eye.
Sometimes, however, parts of the slides showed a white pre-
cipitate when the dialkoxysilane was used, and these slides
were discarded. Even the best slides, when probed by SEM,
were often characterized by a small amount of submicron
particulate matter on the substrate. This matter apparently
originated from the surface-functionalization step. As mea-
sured by the efficiency of particle binding, the dialkoxysi-
lanes generated a higher coverage of amino sites than the
monoalkoxysilanes, and were used for most of the experi-
ments discussed here.
Although NVOCCl reacts readily and in high yield
with primary amines in ~basic! solution, we could not react
NVOCCl with surface-bound amino groups. Therefore, we
first reacted NVOCCl with the amino group of glycine and
then coupled NVOCGLY to the surface-bound amino
groups to prepare the photosensitive substrates.
B. Lithographic fabrication of patterned nanoparticle
arrays
The procedure is illustrated in Fig. 1. Irradiation of the
photosensitive substrates through a mask in the near UV re-
sulted in patterns of deprotected and protected amino groups
of the surface bound glycine. This pattern could be devel-
oped with fluorescent markers. If deprotection was done in
air, the defined pattern was barely discernable by fluores-
cence microscopy. A higher yield of free-amino groups was
achieved by wetting the substrate with a solution of semicar-
bazide hydrochloride in methanol or butylamine in toluene
for the photodeprotection step. The solvates were added as
scavengers for an aldehyde by-product that is presumably
formed during the photodeprotection step,
18
and which might
rereact with the free-amino groups.
When the NH
2
/NVOC-patterned substrates were dipped
into NC colloidal solutions, the NCs preferentially as-
sembled on photodeprotected areas, thus undergoing a ligand
exchange with the substrate-bound amino groups. By step-
wise repetition of the deprotection and assembly process,
micropatterns consisting of different particle arrays could be
prepared. In Fig. 3, three examples of binary particle assem-
blies are shown, including Au- and strongly luminescent
CdSe/CdS-core/shell NCs on a glass substrate, and Pt- and
CdSe/CdS-core/shell NCs on a Si/SiO
2
substrate. The
reflection-mode micrograph @Fig. 3~A!# only reveals stripes
of the Au NCs, which absorb much more strongly than the
semiconductor NCs. In contrast, the semiconductor NCs ap-
pear as bright stripes in the fluorescence micrograph @Fig.
3~B!#, while the strongly absorbing Au NCs appear as dark
stripes. For the SEM micrograph @Fig. 3~C!#, both the Pt NCs
~vertical lines! and the semiconductor NCs ~horizontal lines!
appear with similar contrast.
In order to investigate the selectivity of the method we
prepared macroscopic patterns that could be analyzed by
fluorescence spectroscopy. As shown in Fig. 2, we first ex-
posed a half-photodeprotected glass slide to a solution of
RNCs ~step 1!. The fluorescence intensities measured on this
particular slide indicated a greater than 10/1 selectivity ratio.
In other experiments, this selectivity ratio ranged from 100/1
to 8/1. The binding selectivity of the NCs depended on the
FIG. 2. Scheme for the preparation of macroscopic patterns for spectros-
copy. First, half of a glass slide is photodeprotected to produce a substrate
with amine interactions on one half, and NVOC on the other. Step 1: The
slide is immersed into a solution of red-luminescent particles ~RNCs!, cre-
ating the pattern as shown. Step 2: The remainder of the slide is photodepro-
tected, and the slide is immersed into a solution of yellow-luminescent par-
ticles ~YNCs!, creating spatially separated regions of red and yellow
particles. Step 3A: Dithiols are exchanged for the alkylamine nanocrystal
surfactants, and half the slide is immersed into a solution of Au nanocrys-
tals, producing a quaternary pattern. Step 3B: After a similar ligand ex-
change, the whole slide is immersed into a solution of RNCs, producing the
binary pattern. The optical properties of these slides are discussed in the text
and shown in Figs. 4 and 5.
FIG. 3. Three different microscopy images of assembled and spatially sepa-
rate metal and semiconductor NC arrays. ~A! Optical micrograph ~reflection
mode! of a binary pattern of Au- and highly luminescent CdSe/CdS-core/
shell NCs on a glass substrate. Only the Au-NC arrays are seen because of
their strong absorbance. ~B! Fluorescence micrograph of the same slide
shown in part ~A!. Here, the arrays of luminescent NCs are clearly seen as
bright horizontal stripes. Again, the strongly absorbing Au-NC arrays show
up again as dark vertical stripes. ~C! SEM micrograph of Pt-NC arrays
~vertical stripes! and CdSe/CdS-core/shell NC arrays ~horizontal stripes!
assembled on a Si/SiO
2
substrate. The scale bar in part ~A! is 100
m
m and is
valid for all images.
3667J. Appl. Phys., Vol. 84, No. 7, 1 October 1998 Vossmeyer
et al.
Downloaded 19 May 2006 to 131.215.225.175. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

type of particle, the particle concentration, the chemical
composition of the NC solution, and the exposure time of the
slide to the particle solution. This complexity highlights the
differences that characterize selective assembly of particles
rather than molecules, and is one of the major findings of this
article. Selective assembly of particles onto amine-patterned
substrates works well if the particle surfactants readily un-
dergo ligand exchange with the surface amino groups. How-
ever, nanocrystalsubstrate interactions and molecular
substrate interactions can be quite different. For example,
spatially resolved libraries of peptides or oligonucleotides
can be prepared by simply taking advantage of strong cova-
lent chemical interactions between solution-phase molecules
and substrate-bound molecules. This is not always the case
for NCs. Such materials are characterized by strong interpar-
ticle and particlesubstrate dispersion attractions that scale
geometrically with the size of the particle.
19
Such interac-
tions can compete quite effectively with ligating particle
substrate interactions, and thus decrease binding selectivity.
This can become a major problem when trying to stepwise
assemble different kinds of particles onto the same substrate.
Thus, the matrix of conditions for achieving spatially selec-
tive binding of multiple types of NCs to substrates is quite
complex. Prolonged exposure times and high particle con-
centrations eventually gave strong unselective binding. On
the other hand, addition of a ligating reagent ~octylamine for
the semiconductor NCs; butylamine for the Pt NCs! to the
particle solution usually increased the particle solubility, and
thereby decreased unselective binding to the substrate. The
optimum conditions for selective particle binding varied
strongly from solution to solution, and thus, had to be deter-
mined for each individual particle sample by stepwise con-
trolling the assembly process.
After the red luminescent particles had been bound, the
NVOC half of the slide was photodeprotected and the whole
substrate was immersed in a solution of YNCs ~Fig. 2, step
2!. This immersion step was optimized by stopping it every
15 min, and monitoring the progress by measuring the yel-
low particle photoluminescence. When the PL intensity was
in the same range as for the RNCs ~after 45 min!, the cou-
pling reaction was stopped.
The complexity of the binary pattern was increased by
coupling 1,8-octanedithiol to the substrate-bound particles
and redipping only one half of the slide into Au-NC solution
~Fig. 2, step 3A! or a solution of RNCs ~Fig. 2, step 3B!.
Consider the metal/semiconductor pattern prepared accord-
ing to Fig. 2, step 3A. This slide had four fields of different
particle assemblies. Each quadrant is characterized by a
unique set of optical properties, as shown by UV/Vis absorp-
tion and photoluminescence spectroscopy. Compared to the
absorption spectra Y and R in Fig. 4~B!~corresponding to
the similarly labeled quadrants of Fig. 2! the absorption
spectra of the quadrants Y/Au and R/Au clearly reveal the
presence of Au NCs as indicated by the absorption centered
at 530 nm. For comparison the UV/Vis absorption spectra of
a Au-NC colloidal solution (Au
(S)
) and solutions of the yel-
low (Y
(S)
) and red (R
(S)
) luminescent core/shell NCs are
given in Fig. 4~A!. The PL of these various layers can be
understood qualitatively within the context of a Forster cou-
pling model for energy transfer. When the PL spectra Y and
Y/Au or R and R/Au are compared, a dramatic quenching of
the luminescence is observed, indicating energy transfer
from the semiconductor NCs to the metal NCs. The quench-
ing is stronger for the Y/Au pattern, consistent with the fact
that the PL spectrum of the YNCs overlaps almost directly
with the absorption spectrum of the Au NCs. The higher
luminescence quantum yield exhibited by the yellow NCs
will also lead to a slightly larger characteristic length scale
for energy transfer for these particles to the Au NCs.
Now consider the luminescence properties of the
semiconductor-only quadrants ~Y and R! of the slide pre-
pared according to Fig. 2, step 3A. These PL spectra are
presented in Fig. 4~B!, and the solution-phase PL of those
same particles are presented in Fig. 4~A!. Notice that the PL
from the substrate-bound particles is asymmetric when com-
pared to the solution-phase case. We would like to utilize
this information to quantify the binding selectivity of binary
patterns, but we also need to consider the role that energy
transfer might play in modifying these observed lumines-
cence intensities. In other words, if YNCs unselectively bind
to RNCs, then the PL spectrum of the RNC quadrant could
be affected in two ways. First, the YNCs could contribute
luminescence intensity, thus skewing the RNC spectrum to
slightly higher energies. In this case, the observed asymme-
try in the PL spectrum gives a measurement of the nonselec-
tivity of the second assembly step ~Fig. 2, step 2!. However,
nonselectively bound YNCs could also transfer their excita-
tion to the RNCs, thus enhancing the observed RNC quan-
tum yield. Thus, to separate out the effect of energy transfer,
we have prepared a bilayer ~YNC on RNC! structure ~Fig. 2,
FIG. 4. ~A! Left: UV/Vis-absorption spectra of the red (R
(S)
) and yellow
(Y
(S)
) luminescent CdSe/CdS-core/shell NCs dissolved in chloroform, and
of Au NCs dissolved in toluene. Right: corresponding luminescence spectra
of the red (R
(S)
) and yellow (Y
(S)
) luminescent NC solutions. ~B! Left:
absorption spectra of the quaternary pattern depicted in Fig. 2 ~step 3A!.
Right: corresponding luminescence spectra.
3668 J. Appl. Phys., Vol. 84, No. 7, 1 October 1998 Vossmeyer
et al.
Downloaded 19 May 2006 to 131.215.225.175. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jsp

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TL;DR: High-density arrays formed by light-directed synthesis are potentially rich sources of chemical diversity for discovering new ligands that bind to biological receptors and for elucidating principles governing molecular interactions.
Abstract: Solid-phase chemistry, photolabile protecting groups, and photolithography have been combined to achieve light-directed, spatially addressable parallel chemical synthesis to yield a highly diverse set of chemical products. Binary masking, one of many possible combinatorial synthesis strategies, yields 2n compounds in n chemical steps. An array of 1024 peptides was synthesized in ten steps, and its interaction with a monoclonal antibody was assayed by epifluorescence microscopy. High-density arrays formed by light-directed synthesis are potentially rich sources of chemical diversity for discovering new ligands that bind to biological receptors and for elucidating principles governing molecular interactions. The generality of this approach is illustrated by the light-directed synthesis of a dinucleotide. Spatially directed synthesis of complex compounds could also be used for microfabrication of devices.

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Frequently Asked Questions (1)
Q1. What have the authors contributed in "Combinatorial approaches toward patterning nanocrystals" ?

A scheme for generating complex, spatially separated patterns of multiple types of semiconducting and/or metallic nanocrystals is presented. The authors report on the production of binary, tertiary, and quatemary patterns of nanocrystals. The authors highlight and discuss the differences between nanocrystal/substrate assembly and molecule/substrate assembly. Finally, the authors investigate the assembled structures using photoluminescence and absorption spectroscopy.