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The authors acknowledge the European Synchrotron Radiation
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authors declare no conflict of interest.
Materials and Methods
Figs. S1 to S6
Tables S1 to S3
3 March 2017; accepted 22 June 2017
Direct optical lithography of
functional inorganic nanomaterials
Yuanyuan Wang,1,2 Igor Fedin,1,2 Hao Zhang,1,2 Dmitri V. Talapin1,2,3*
Photolithography is an important manufacturing process that relies on using photoresists,
typically polymer formulations, that change solubility when illuminated with ultraviolet
light. Here, we introduce a general chemical approach for photoresist-free, direct optical
lithography of functional inorganic nanomaterials. The patterned materials can be metals,
semiconductors, oxides, magnetic, or rare earth compositions. No organic impurities are
present in the patterned layers, which helps achieve good electronic and optical properties.
The conductivity, carrier mobility, dielectric, and luminescence properties of optically
patterned layers are on par with the properties of state-of-the-art solution-processed
materials. The ability to directly pattern all-inorganic layers by using a light exposure dose
comparable with that of organic photoresists provides an alternate route for thin-film
Solution-processed colloidal nanocrystals (NCs) and quantum dots (QDs) have emerged as a versatile platform for building elec- tronic and optoelectronic devices (1). These materials enable nonepitaxial deposition
and low-temperature processing of, for example,
light-emitting diodes (LEDs), field effect transis-
tors (FETs), near- and mid-infrared photodetectors,
and solar cells. The transition from individual
devices to the level of electronic circuits, sensor
arrays, and high-definition QD LED displays re-
quires development of material-adapted patterning
methods. Depending on the resolution, throughput,
and defect tolerance, various patterning and dep-
osition techniques can be considered, includ-
ing photolithography and imprint lithography,
microcontact and inkjet printing, and laser or
electron beam (e-beam) writing (2). Among these,
photolithography evolves as a method of choice
for the electronics industry because of the com-
bination of high resolution and very low cost per
patterned element (3). The latter comes from the
parallel nature of the lithographic process; billions
of circuit elements can be defined simultane-
ously, in contrast to serial techniques such as
inkjet printing and e-beam writing.
We demonstrate a method that we call direct
optical lithography of functional inorganic nanomaterials (DOLFIN). This process combines multiple benefits of traditional photolithography and
is tailored toward efficient patterning of inorganic
nanomaterials and sol-gel chemicals without
diluting or contaminating them with organic
photoresists and other by-products. Almost any
inorganic functional material can be prepared
in the form of NCs by using a variety of available solution- and gas-phase techniques. Atoms
at NC surfaces easily engage in chemical bonding
with molecular species—so-called surface ligands—
that provide colloidal stability to NCs in desired
solvents (4). The ligand molecules can be constructed as ion pairs, Cat+X−, where X− is an
electron-rich nucleophilic group that binds to
the electron-deficient (Lewis acidic) surface sites,
typically metal ions. The negative charge on X− is
balanced by a cation, Cat+, as shown in Fig. 1A. In
polar solvents, cations dissociate from the surface
and form an ionic cloud responsible for colloidal
stabilization, whereas in nonpolar environments
or in films, the ion pairs stay tightly bound (5).
To implement DOLFIN, we designed photochemically active X− and Cat+ groups to enable
direct optical patterning of all-inorganic NCs.
In one approach, diphenyliodonium (Ph2I+) or
triphenylsulfonium (Ph3S+) cations, also known
as photoacid generators (PAGs) (3), were com-
bined with surface-binding inorganic anions, such
as Sn2S64−, CdCl42–, or MoO42− (Fig. 1B). Such
ligands provide colloidal stability to metals, semi-
conductors, and many other types of NCs (fig. S1).
Upon photon absorption, PAG molecules decom-
pose, releasing acidic protons (Fig. 1B). These
protons can react with the X− group or with the
NC surface in several different ways that alter
NC solubility in polar and nonpolar solvents (6).
As an alternative approach, the X− group itself
can be made photosensitive, as in the case of am-
monium 1,2,3,4-thiatriazole-5-thiolate (NH4CS2N3,
or TTT). The CS2N3− group photochemically de-
composes to surface-bound SCN− ions, N2, and
sulfur (Fig. 1, B and C) (7). The ligands with PAG+
or CS2N3− fragments nicely complement each
other: PAG-based ligands are extremely flexible
and versatile, whereas TTT ligands can be used
with pH-sensitive materials.
The photoactive anionic or cationic ligand components show strong UV absorption bands that can
impose on top of the absorption spectra of corresponding NCs, as in the case of CdSe QDs with
TTT ligands (Fig. 1C). These ligands provide excellent colloidal stability in N,N-dimethylformamide
(DMF), dimethylsulfoxide (DMSO), and other conventional solvents (Fig. 1D and fig. S2). However,
irradiation with UV light (l < 360 nm) triggers
decomposition of TTT ligands, making the NCs
insoluble in DMF. The photochemical transformation of CS2N3− to SCN– can be traced by using
characteristic IR absorption features of the CS2N3−
and SCN− groups shown in Fig. 1C, inset, and fig.
S3. Various patterns can be generated on rigid
and flexible substrates by illuminating NC films
through a mask and then washing off the unexposed NCs with DMF (Fig. 1, E and F, and fig. S4).
We observed a clear loss of colloidal stability after exposing the NC films to 254 nm, 6.3 m W/cm2
light for 20 s, which corresponds to an exposure
dose of 120 mJ cm−2. For reference, conventional
organic photoresists (such as Shipley S1800 series, Dow Chemicals) require a comparable 80 to
150 mJ cm−2 exposure dose (6). Similarly, NCs with
PAG-based ligands were patterned by using a 40
to 70 mJ cm−2 exposure dose (fig. S5). Moreover,
PAGs can be engineered to operate in different
spectral regions, ranging from the deep UV to
the visible (fig. S6). The decomposition of surface ligands does not cause changes in the size
1Department of Chemistry, University of Chicago, Chicago, IL
60637, USA. 2James Franck Institute, University of Chicago,
Chicago, IL 60637, USA. 3Center for Nanoscale Materials,
Argonne National Laboratory, Argonne, IL 60439, USA.
*Corresponding author. Email: email@example.com