Supporting Information for

Large-Area Nanoimprinted Colloidal Au Nanocrystal-Based

Nanoantennas for Ultrathin Polarizing Plasmonic Metasurfaces

Wenxiang Chen,1 Mykhailo Tymchenko,

4 Prashanth Gopalan,

2 Xingchen Ye,

3 Yaoting Wu,

3

Mingliang Zhang,1,3

Christopher B. Murray,2,3

Andrea Alu,4 and Cherie R. Kagan

1,2,3,*

1Department of Electrical and Systems Engineering,

2Department of Materials Science and

Engineering, 3Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104,

USA

4Department of Electrical and Computer Engineering, The University of Texas at Austin, Texas

78712, USA

*To whom correspondence should be addressed. E-mail: kagan@seas.upenn.edu

Phone : (215) 573-4384. Fax : (215) 573-2068

Materials: Hydrogen tetrachloroaurate (III) hydrate (99.9%) is purchased from Strem Chemicals.

Oleyamine (further referred to as OAm, 70%), tert-butylamine-borane (TBAB, 97%), chromium

etchant and (3-mercaptopropyl) trimethoxysilane (MPTS, 95%) are purchased from Sigma-

Aldrich. 1,2,3,4-tetrahydronaphthalene (tetralin) and ammonium thiocyanate (SCN, 99+%) are

purchased from Acros. Chromium pellets (Cr, 99.999%) are purchased from Kurt J. Lesker.

Poly(methyl methacrylate) (PMMA) 495 A4, PMMA 950 A2, methyl isobutyl ketone (MIBK),

and Remover PG are purchased from MicroChem. n-octadecyl trichlorosilane is purchased from

Gelest. Acetone (99.8%), isopropanol (IPA) (99.5%), hexane (99.9%), and microscope slides are

purchased from Fisher Scientific. NXR-1000 thermal imprint resist is purchased from Nanonex.

Gold Nanocrystal (NC) synthesis: 5 nm gold NCs are synthesized using a previously reported

method1 with some modifications. 200 mg of hydrogen tetrachloroaurate (III) hydrate, 10 mL of

OAm, and 10 mL of tetralin are mixed at room temperature. The mixture is magnetically stirred

under a nitrogen flow for 5 min. 1 mmol tert-butylamine–borane (Aldrich) is dissolved in 1 mL

OAm and 1 mL tetralin by sonication, and then injected into the precursor solution. The solution

changes color to deep red in 5 s. The solution is allowed to react for 1 h at room temperature and

then 60 mL acetone is added to precipitate the Au NCs. The Au NCs are collected by

centrifugation (8000 rpm, 6 min), washed twice with ethanol and then dispersed in hexanes, and

an additional time just prior to nanoantenna fabrication.

Another method2 is also used to synthesize gold NCs: briefly, 0.08 g of AuCl3 is dissolved into a

solution consisting of 3 mL of OAm and 3 mL of o-dichlorobenzene. Under a nitrogen flow and

vigorous magnetic stirring, the mixture is then injected into a boiling solution (∼181 oC) of 0.4 g

of 1,2-hexadecanediol dissolved in 15 mL of o-dichlorobenzene. The reaction solution is

maintained at 165 oC for 2 min before being cooled to room temperature using a water bath. Au

NCs are isolated by ethanol precipitation and centrifugation. It is washed with IPA and then

dispersed in hexanes prior to nanoantenna fabrication.

Both synthetic methods produce similar Au NC materials for use in metasurface fabrication.

Master template fabrication: A 1.5 cm by 1.5 cm device grade silicon wafer with 250 nm

thermal SiO2 is cleaned by an O2 plasma (75 mTorr, 150 W) for 10 min. The electron-beam

lithography resist PMMA 495 A4 (diluted to A2 in anisole) is deposited by spin-coating at 3000

rpm for 1 min, is prebaked at 180 oC for 2 min, and is allowed to cool to room temperature for 2

min. A second resist layer of PMMA 950 A2 is then deposited following the same procedure.

The total resist thickness is about 120 nm. The pattern is then exposed in an Elionix electron-

beam lithography system operating at a 50 kV accelerating voltage. After exposure, the sample is

developed in a 1:3 solution of MIBK and IPA for 90 s. Residual resist is removed by

descumming in an O2 plasma (75 mTorr, 70 W) for 6 s. A 12 nm chromium mask is deposited

onto the sample using electron-beam evaporation at 0.3 Å/s and then the sample is soaked in

remover PG at 75 oC for 8 h to complete the lift off process. Reactive ion etching is used to etch

200 nm into the SiO2 layer to define the nanoantenna master templates, 250 nm into the SiO2

layer for the mid-IR quarter-wave plate master template, and 250 nm into the SiO2 layer and then

60 nm into the Si substrate for the extreme bandwidth quarter-wave plate master template. These

etching depths are optimized to yield the best nanoimprinting results. The chromium mask is

removed by soaking the sample in etchant for 2 min and then rinsed with deionized (DI) water

twice. The sample is then treated with an O2 plasma (75 mTorr, 150 W) for 10 min and then

soaked in a 1:1000 n-octadecyltrichlorosilane solution in hexane for 10 min. The template is

subsequently rinsed in hexane and DI water.

Nanoantenna/metasurface quarter-wave plate fabrication: Nanoantenna arrays and

metasurface quarter-wave plates are fabricated on microscopic glass slides. The substrate is

cleaned with acetone and IPA. NXR-1000 thermal imprint resist is deposited by spin-coating

onto the substrate at 3000 rpm for 1 min and is prebaked at 155 oC for 5 min. The master

template consisting of either nanoantenna arrays or the quarter-wave plate pattern is placed on

top of the substrate. The master template, thermal resist, and substrate stack is heated to 133 oC

and compressed at 350 psi for 5 min 30 s, and then cooled to room temperature in a Nanonex

NX-2600 nanoimprint tool. The template is then carefully detached from the substrate by hand.

An oxygen plasma descum process (75 mTorr, 70 W, 6 s) is performed to remove the residual

layer in the imprinted area. The Au NC dispersion (concentration varying from 10 mg/mL to 20

mg/mL) is deposited by spin-coating on the resist patterned substrate at 1000 rpm for 30 s. The

resist is lifted off in acetone with mild sonication for 30 s. Finally ligand exchange is carried out

by immersing samples into 1% NH4SCN in acetone for 2 min, followed by rinsing twice in clean

acetone for 2 min to remove unbound ligand.

Measurement and Characterization

AFM measurement: AFM measurements are performed using an MFP-3D-BIO microscope

(Asylum Research Corp.) with an AC240TS silicon cantilever (Olympus). The height of the

nanoantenna is defined as the distance between the center of the nanorod top surface to the

substrate.

SEM measurement: SEM measurements are performed using an FEI Quanta 600 ESEM. For

samples on glass substrates, environmental mode is used to characterize the surface. For samples

fabricated on silicon wafers, high vacuum SEM mode is used.

Ellipsometry measurement: Glass substrates are cleaned and coated with a self-assembled

monolayer of 3-mercaptopropyl-trimethoxysilane (MPTS) to improve NC adhesion to the

substrate, according to our previously published protocol and references.3 80 mg/mL dispersions

of Au NCs in hexane are spin-coated onto the glass substrate and ligand exchange in NH4SCN is

carried out as described above. Variable-angle ellipsometry is carried out using an M-2000

ellipsometer (J.A. Woollam Co.). The complex reflectance ratio is measured from 370 nm to

1680 nm at angles of 45 o, 55

o, 65

o, and 75

o. The optical properties of the Au NC thin film is fit

by a Gen-Osc layer which consists of a Drude oscillator, describing the oscillations of the free

electrons, and 3 Lorentz oscillators, describing the bound electron transitions, according to our

previously published protocol.3

FT-IR measurement: Transmittance FT-IR measurements are performed on a Nicolet 6700

(Thermo Scientific) spectrometer with a mercury cadmium telluride detector. The spectra are

reported after background subtraction using a blank substrate. In the transmittance spectra for

nanoantenna arrays, the magnitude of the transmittance is normalized to the density of nanorods.

For polarization-dependent measurements of the nanoantenna arrays, linearly-polarized

excitation is generated by passing the IR light through a linear IR polarizer (ThermoFisher

Scientific 0045-347). To characterize the mid-IR quarter-wave plate, we generate circularly

polarized light by passing the IR light first through a linear IR polarizer and then a mid-IR

quarter-wave plate (Altechna 2-IRPW-ZO-L/4-2800-C). A second IR linear polarizer is located

between the sample and the detector to record the intensity of the transmitted light at each

polarization angle. To characterize the extreme bandwidth quarter-wave plate, we generate linear

excitation at different polarization angles by passing the IR light through a linear IR polarizer.

This polarizer is rotated from 0 to 90 degrees in steps of 5 degrees. At each polarization angle of

the first polarizer, a second IR linear polarizer which is in between the sample and the detector

records the angular intensity of the transmitted light.

UV-VIS measurement: The transmittance spectrum of the 178 nm x 178 nm nanoantenna array

is measured using a Cary 5000 UV-vis-NIR spectrophotometer (formerly Varian Inc., now

Agilent Technologies). The spectra are reported after subtraction of a background consisting of a

blank substrate. For the polarization-dependent measurements of the 178 nm x 178 nm

nanoantenna array, we generate linear excitation by passing the excitation through a Vis-NIR

linear polarizer (Harrick PGT-S1V). This polarizer is rotated from 0 to 90 degrees to generate

linear excitation at different incident angles.

Figure S2 QWP Figure 3 Extreme bandwidth QWP Figure 4

Au NC nanorod array fabricated by e-beam

lithography

Nanorod direction

Horizontal Vertical Horizontal Vertical Horizontal Horizontal

Number of nanorods analyzed

8 8 8 4 16 12

Average edge roughness

2.9 nm 2.9 nm 2.9 nm 7 nm 4.5 nm 5.9

Misalignment 0.4° 0.2° 0.2° 0.1° 0.6° 1.5°

Table 1 Line-edge roughness (LER) and misalignment of nanoimprinted, colloidal Au NC

metasurfaces.

Figure S1 SEM image of the master template used to fabricate the sample in Figure 1(e). The

nanorods in the template have a length of 816 nm and a width of 242 nm.

Figure S2 SEM images taken from 8 different areas across a NC-based nanoantenna array [the

same sample as in Figure 1(e)] to determine the length and width and their variations.

Figure S3 SEM image of a Au NC nanorod array patterned by e-beam lithography.

Figure S4 Atomic Force Microscopy images of nanocrystal-based nanoantennas fabricated with

heights of (a) 71 nm, (b) 81 nm and (c) 119 nm. (d) Line cuts of the AFM images and (e)

transmittance measurements for the nanorod samples shown in (a-c).

Figure S5 The permittivity of a SCN-exchanged Au NC thin film.

Figure S6 Transmittance spectra for linearly polarized illumination of nanoantenna arrays with

nanorods of (a) 863 nm, (b) 680 nm, (c) 470 nm and (d) 178 nm lengths. The nanoantenna length

is oriented horizontally at zero degrees.

Figure S7 Normalized transmittance at resonance as a function of the angle of linearly polarized

incident light for the bulk Au nanoantennas with dimensions: (a) 868 nm x 188 nm, (b) 677 nm x

228 nm, (c) 492 nm x 250 nm, and (d) 185 nm x 185 nm.

Figure S8 Transmittance spectra for the Au NC-based quarter-wave plate excited by light

linearly polarized along the x and y directions. Dashed curves: Full wave simulation results;

Solid curves: experiment results.

Figure S9 An SEM image showing a larger area of the nanoimprinted, colloidal Au NC-based

mid-IR quarter-wave plate fabricated on a glass substrate.

Figure S10 A higher resolution SEM image showing the nanoimprinted, colloidal Au NC-based

mid-IR quarter-wave plate fabricated on a glass substrate.

Figure S11 An SEM image showing a larger area of the nanoimprinted colloidal Au NC-based

extreme bandwidth quarter-wave plate fabricated on a glass substrate.

References

(1) Peng, S.; Lee, Y.; Wang, C.; Yin, H.; Dai, S.; Sun, S. A facile synthesis of monodisperse

Au nanoparticles and their catalysis of CO oxidation. Nano Research, 2008, 1, 229–234.

(2) Ye, X.; Chen, J.; Murray, C. B. J. Am. Chem. Soc. 2011, 133, 2613–2620.

(3) Fafarman, A. T.; Hong, S. H.; Caglayan, H.; Ye, X.; Diroll, B. T.; Paik, T.; Engheta, N.;

Murray, C. B.; Kagan, C. R. Nano Lett. 2013, 13, 350–357.