Nanoparticle Imprint Lithography: From Nanoscale Metrology to Printable Metallic Grids

Abstract

Large scale and low-cost nanopatterning of materials is of tremendous interest for optoelectronic devices. Nanoimprint lithography has emerged in recent years as a nanofabrication strategy that is high-throughput and has a resolution comparable to that of electron-beam lithography (EBL). It is enabled by pattern replication of an EBL master into polydimethylsiloxane (PDMS), that is then used to pattern a resist for further processing, or a sol-gel that could be calcinated into a solid material. Although the sol-gel chemistry offers a wide spectrum of material compositions, metals are still difficult to achieve. This gap could be bridged by using colloidal nanoparticles as resist, but deep understanding of the key parameters is still lacking. Here, we use supported metallic nanocubes as a model resist to gain fundamental insights into nanoparticle imprinting. We uncover the major role played by the surfactant layer trapped between nanocubes and substrate, and measure its thickness with subnanometer resolution by using gap plasmon spectroscopy as a metrology platform. This enables us to quantify the van der Waals (VDW) interactions responsible for the friction opposing the nanocube motion, and we find that these are almost in quantitative agreement with the Stokes drag acting on the nanocubes during nanoimprint, that is estimated with a simplified fluid mechanics model. These results reveal that a minimum thickness of surfactant is required, acting as a spacer layer mitigating van der Waals forces between nanocubes and the substrate. In the light of these findings we propose a general method for resist preparation to achieve optimal nanoparticle mobility and show the assembly of printable Ag and Au nanocube grids, that could enable the fabrication of low-cost transparent electrodes of high material quality upon nanocube epitaxy.

Keywords: PVP lubrication; gap-plasmon metrology; nanocube sliding; nanogrids; nanoimprint; printable nanopattern.

Figure 1

Summary of nanocube assembly using a commercial NIL setup. In phase 1, nanocubes are deposited on a substrate using convective deposition in 1 mM SDS. In phase 2, nanocubes are imprinted by pressing the inflated nanopattern PDMS stamp on the substrate. In order to rewet nanocubes, a droplet of solution is placed on PDMS; we typically use 0.01 wt % Triton X-45 in ethanol. Final PDMS position (height) and PDMS inflation are tuned to control the force exerted on the substrate.

Figure 2

35 nm PVP-capped Ag nanocubes deposited on glass by convective deposition in SDS with different PVP concentrations and imprinted at 0.4 mm/s, upon rewetting the substrate with 3 μL of Triton-X45 0.01 wt % in ethanol, showing the crucial role of PVP as an additive to the convective assembly solution (a) 0.125 mg/mL PVP, (b) 0.5 mg/mL PVP, (c) 1 mg/mL PVP, (d) 2 mg/mL PVP in the deposition solution. Inset shows the corresponding SEM image. Scale bars are 1 μm unless stated otherwise.

Figure 3

Probing PVP layer thickness using cavity plasmon resonances. (a) Schematic representation of the optical setup used to probe gap-plasmon resonances. (b) Electromagnetic field distribution (Ez) in a dielectric cavity formed between a silver mirror and a silver nanocube separated by 1 nm Al₂O₃ and 5 nm PVP (simulation performed on Lumerical (FDTD)). (c) SEM image of 40 nm Ag nanocubes deposited from a 0.5 mg/mL PVP solution on a Al₂O₃-silver substrate. (d) Averaged spectra for 0.5, 1, and 2 mg/mL PVP before nanoimprint. Filled regions show standard deviations between measurements (more than 50 spectra) for each sample at each wavelength. Points and bars indicate mean resonance values as well as standard deviations calculated by fitting each individual spectra with a Lorentzian function. (e) Extinction cross-section (nm²) map at 65° excitation for various PVP thickness. Points and bars correspond to the mean experimental cavity plasmon resonances (same as (d)) and mean peak widths for the three samples (0.5, 1, and 2 mg/mL), obtained by fitting spectra with a Lorentzian function around 500–550 nm. For the 0.5 mg/mL sample, a blue square indicates the position of the long wavelength mode. Resonances from the reference sample shown in (f) are indicated in pink. (f) Reference sample realized by LB and corresponding averaged spectra for 10 different 20× BF measurements. Shadow areas represent standard deviations calculated for each point, dots indicate the average resonance values obtained by fitting each spectrum with a Lorentzian function, and bars corresponding to standard deviations. (g) Corresponding SEM image.

Figure 4

Impact of PVP layer thickness on nanocube mobility. Sample 1 (a), 2 (b), and 3 (c) are imprinted upon rewetting the substrate with 3 μL of Triton-X45 0.01 wt % in ethanol; (d) averaged spectra for the 3 samples 0.5 mg/mL, 1 and 2 mg/mL PVP before (bottom) and after (top) imprinting. Filled regions show standard deviations between measurements (more than 50 spectra) for each sample. (e) Cavity plasmon resonances mean values (circles) and standard deviations (bars) obtained by fitting each spectrum with a Lorentzian function and confronted to values obtained prior to imprinting. (f) Variation in PVP thickness estimated by comparing values from (e) to the simulations, taking into account standard deviations.

Figure 5

NIL mechanism summary. (a) Force required to overcome nanocube-substrate interaction and displace nanocubes plotted for different friction coefficient (μ) values with respect to the approximative Stokes force exerted on nanocubes by solvent. (b) Schematic representation of solvent squeezed out of the substrate and exerting increasing lateral forces on nanocubes as well as dissolving part of the PVP chains. (c) Nanocubes gradually losing their PVP spacer layer as they are displaced on the substrate as a result from VDW attraction and PVP chains solubility in the rewetting solvent, which eventually results in permanent trapping.

Figure 6

Optical images of (a) Ag grid with 4 μm pitch, (b) Ag grid with 2 μm pitch. SEM images of (c) Ag grid with 4 μm pitch (d) Au grid with 4 μm pitch. Ag and Au were deposited by convective assembly in 1 mM SDS in the presence of 1 and 0.5 mg/mL PVP respectively (imprinting parameters described in the Methods section).

Figure 7

(a) Schematic of the process, from curing liquid PDMS on top of nanocubes to peeling PDMS off and deposition on a new substrate by contact printing. (b) SEM images of grids in PDMS. Inset on the bottom left shows the flexibility of the PDMS substrate supporting the grids. (c) Grids deposited on silicon upon contact printing.