AVS2012 Session NS+SP-MoA: Nanopatterning and Nanolithography

Gold nanostructures were fabricated by interferometric lithography (IL) with a view to their application for optical bio-sensing based on localized surface plasmon resonance. This approach relies upon very modest instrumentation, high processing speed and capacity for fabrication of dense arrays of nanostructures over macroscopic areas. The dimensions and morphology of nanostructures obtained were characterized by AFM/SEM. The arrays of 65-200nm wide and 125-400nm long gold nano-dots and nano-rods with spacing of 120-220nm were fabricated by IL.

UV-vis absorption spectra of gold nanostructures showed a characteristic peak at ca. 520nm associated with localized surface plasmons. A spectroscopic ellipsometry study of the gold nanostructures was carried out. Raman spectra of a thin film of phathlocyanine adsorbed on gold nanostructures showed an enhancement of Raman scattering of up to 200 times compared with the same film deposited on continuous gold layer.

Post-lithographic processing by annealing was found to improve the optical properties of the nanostructure arrays still further. Annealed samples appear higher and smaller compared to as-prepared samples. AFM images show that all the samples annealed at between 450-470°C for 50−120min retain a regular pattern. XRD analysis of annealed gold nanostructures evinces formation of crystal clusters on the surface.

In order to demonstrate the biosensing capabilities of annealed and as-prepared nanostructures, the LSPR response of nanostructures to changes in the bulk refractive index (RI) was investigated. The RI sensitivity of the annealed samples is found to be 145nm/RIU. Compared to the RI sensitivity value of the as-prepared sample (52.5nm/RIU) this value is ~2.7 times higher due to smaller size of nanostructures and larger spacing between them.

A model biosensing of streptavidin-biotin binding and BSA were successfully performed on as-prepared and annealed gold nanostructures. Measurements were based on the LSPR shift induced by local RI change when proteins are immobilized on the nanostructures. Streptavidin concentrations were washed over the nanostructured surface in cumulative succession ranging from 0.1ug/ml to 0.1mg/ml, followed by an identical protocol with biotin. Characteristic increases and saturation are noted through shifts in the absorption of the LSPR peak at ca. 635nm. Annealed nanostructures demonstrated a higher detection limit (DL) compare to as-prepared one: the DL for BSA is 1.5fM and 50pM for annealed and as-prepared nanostructures, respectively. Our results demonstrate the potential of interference lithography for the application of plasmon-based biosensing devices.

[1] D. Pires, J. L. Hedrick, A. De Silva, J. Frommer, B. Gotsmann, H. Wolf, M. Despont, U. Duerig, and A. W. Knoll, Science 328, 732 (2010).

[2] A. W. Knoll, D. Pires, O. Coulembier, P. Dubois, J. L. Hedrick, J. Frommer, U. Duerig, Adv. Mat. 22, 3361 (2010).

[3] P. C. Paul, A.W. Knoll, F. Holzner, M. Despont and U. Duerig, Nanotechnology 22, 275306 (2011).

[4] F. Holzner, C. Kuemin, P. Paul, J. L. Hedrick, H. Wolf, N. D. Spencer, U. Duerig, and A. W. Knoll, NanoLetters, 11, 3957 (2011).

We introduce a laser assisted electron beam induced deposition (LAEBID) process which is a nanoscale direct write method that implements a synchronized electron beam induced deposition (EBID) process with a pulsed laser step for thermal desorption of the reaction by-product. The pseudo-localized (~ 100 mm spot), pulsed laser enables thermal desorption of the reaction by-product without the issues associated with bulk substrate heating, which shortens the precursor residence time and distort pattern fidelity due to thermal drift. Current results show a significant purification of platinum deposits (~35% Pt) with the addition of synchronized laser pulses as well as a significant reduction in deposit resistivity. Measured resistivity from platinum LAEBID structures (1.2x10⁴ mW-cm) are more than 3 orders of magnitude lower than standard EBID platinum structures (2.2x10⁷ mW-cm) from the same precursor and are lower than the lowest reported EBID platinum resistivity with post-deposition annealing (1.4x10⁴ mW-cm).

Understanding and controlling of intermolecular interaction in highly organized π-conjugated molecules is crucial for the development of the high performance organic devices. Molecule-molecule interaction through frontier orbital states is assumed to be substantially varies with the packing pattern and intermolecular distance. H-terminated Si(100) surface has appeared as an ideal template for controlled stacking of π-conjugated molecules, where the intermolecular distance is defined by the dimer-dimer distance on the surface. To date, a number of studies have been reported on the controlled growth of different types of molecular assemblies and their junctions on the surface. It is expected that intermolecular interaction between such assemblies should strongly varies with substituent and extent of π-conjugation of the molecules. Here we report various nanopattern formation with some simple acene molecules such as 9-vinyl anthracene, 2-vinyl naphthalene and styrene on H-terminated Si(100) using scanning tunneling microscope (STM) and ultraviolet photoelectron spectroscopy (UPS).

Controlled stacking of styrene molecules on H-terminated Si(100) surface through chain reaction mechanism is well known. Self-directed growth of molecular stacking is exclusively directed along the dimer row on the surface, where the phenyl rings are stacked in parallel along the dimer row. Unlike styrene molecule, 9-vinyl anthracence forms both orderly stacking pattern along the dimer row and disorderly nanopattern on the surface. The relative appearances of these orderly and disorderly nanopatterns in STM images depend on the applied sample bias while scanning. At lower bias, the orderly pattern appears much brighter than that of the disorderly pattern. This contrast difference between two structures relates to the enhanced π-π interaction in the case of orderly stacked nanopattern. We also observe that the orderly nanopattern of anthracence molecules undergoes reversed chain reaction very frequently compared to that of styrene molecule.

Atomic Layer Epitaxy (ALE) is a fundamental process in the Atomically Precise Manufacturing (APM) of nanoscale devices. Si2H6 has been shown to be a good precursor molecule for patterned growth on Si(100) using Chemical Vapor Deposition (CVD). For ALE of silicon, three parameters have to be considered: the incident Si2H6 flux, the temperature, and partial H coverage of the surface. These parameters have been investigated experimentally using in-situ infrared absorption spectroscopy to characterize the nature and coverage of species on a Si (100)-(2x1) surface in a well-controlled environment (2x10-10 Torr base pressure).

The flux dependence was studied over two orders of magnitude and temperature dependence from 173 K to 473 K for varying fluxes. It was found that the nature of SiHx species formed at the surface was strongly dependent on the flux, with higher silanes (e.g. x>1) for higher fluxes. Furthermore, the time required for saturation also decreased with increasing flux and temperature.

The impact of partial H coverage was also investigated with H coverage from 0.01 To 1ML Monolayer, achieved by partial desorption of a H saturated surface. On the substrate’s surface has been also investigated with FT-IR under UHV conditions. It has been found that even a very small amount of Hydrogen (~1% ML) substantially reduces the chemisorption of disilane on the clean parts of Si(100)-(2x1).

This complex dependence on flux and temperature arises from the complex and highly temperature dependent adsorption/dissociation behavior of disilane on Si(100). The dependence on partial H coverage highlights the spatial requirements of the dissociation products, as uncovered in our earlier work.1

1 J.-F. Veyan, et al., Journal of Physical Chemistry C 115, 24534.

We are using Scanning Probe Microscope (SPM) to write patterns directly on a single crystal silicon surface that was reconstruted and hydrogen-passivated in the ultra high vacuum (UHV) chambers, and are followed by the process of reactive ion etching to transfer the patterns into the silicon bulk. The patterns were written either by an atomic force microscope (AFM) in the ambience or by a scanning tunneling microscope (STM) in UHV chambers and was immediately proceeded by an oxidation process in a seperate vacuum chamber. AFM or STM patterned samples were immediately transferred to a RIE chamber via an argon back-filled inflatable glove box.

In terms of patterning, the AFM in the ambient enviroment and the STM in the UHV both can perform lithograph; however, the mechanism are quite different: the AFM is adding species, OH^-, to the surface, and the STM is removing species, H, from the silicon surface.

The features patterning by the AFM prior to RIE process were 1~2nm in height, and were 15~20nm height after RIE and 6nm wide (FWHM) that were measured and confirmed by a critial dimension AFM (CD-AFM). For the features patterning by STM, it was conjectured the height were 0.2~0.3nm prior to RIE. After the RIE process, the height was measured to be 0.7nm.

This paper presents the nanoparterning of the hydrogel-gold nanocomposite. We synthesized water based colloidal suspension of covalently bound gold nanoparticles (5‐10nm) on the backbone of Poly(Allyl amine) by reduction of gold salt on the amine groups of polymeric chain. Poly(N-isopropylacrylamide) was dissolved in Poly(Allyl amine) based gold colloidal solution. This solution then spun-coated on Hexamethyldisilazane primed Si wafer for nanolithography. The hydrogel precursor was dried in air. Resulting thickness of the hydrogel precursor was approximately 1 mm. The samples were then exposed to 110-180 μC/cm² e-beam. The exposure does was optimized for different thickness of precursor. Different characterization techniques such as UV-vis spectroscopy, differential scanning calorimetry, FTIR, scanning electron microscopy, and atomic force microscopy were employed for characterization. Thus, gold nanoparticles were incorporated into a three dimensional, cross‐linked, polymeric hydrogel network by nanopatterning technique. With the ability of hydrogels to expand or contract with changes in pH and temperature, the spacing between nanoparticles can be controlled, allowing a single material being able to resonate at different frequencies. The ability to perform lithography to form a patterned hydrogel with homogeneous nanoparticles opens the door to development of nanoswitches, sensors, MEMS as well as drug delivery devices.