Large-Area Nanofabrication of Partially Embedded Nanostructures for

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Large Area Nanofabrication of Partially Embedded Nanostructures for Enhanced Plasmonic Hot Carrier Extraction Charlene Ng, Peng Zeng, Julian Andreas Lloyd, Debadi Chakraborty, Ann Roberts, Trevor A. Smith, Udo Bach, John Elie Sader, Timothy J. Davis, and Daniel E. Gomez ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01988 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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ACS Applied Nano Materials

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Large Area Nanofabrication of Partially Embedded

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Nanostructures for Enhanced Plasmonic Hot Carrier

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Extraction

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Charlene Ng,*,† Peng Zeng, ‡ Julian A. Lloyd, ¶ Debadi Chakraborty, § Ann Roberts, ∥ Trevor

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A. Smith, ‡,⊥ Udo Bach, ¶,⊥ John E. Sader, §,⊥ Timothy J. Davis, ∥ Daniel E. Gómez *,#, †,⊥

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†CSIRO, Manufacturing, Private Bag 33, Clayton, VIC, 3168, Australia

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‡School of Chemistry, The University of Melbourne, VIC 3010, Australia

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¶Department of Materials Science and Engineering, Monash University, Clayton, VIC 3800, Australia

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§School of Mathematics and Statistics, The University of Melbourne, VIC 3010, Australia ∥School of Physics,

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The University of Melbourne, VIC 3010, Australia

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⊥ARC Centre of Excellence in Exciton Science

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#RMIT University, Melbourne, VIC, 3000, Australia

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 Current Address: Leibniz Institute for Polymer Research, 01069 Dresden, Germany

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Email: [email protected] and [email protected]

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Abstract

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When plasmonic nanoparticles are coupled with semiconductors, highly energetic hot carriers

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can be extracted from the metal–semiconductor interface for various applications in light

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energy conversion. However, the current quantum yields for hot electron extraction are

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generally low. An approach for increasing the extraction efficiency consists of maximizing

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the contact area between the surface of the metal nanostructure and the electron accepting

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material. In this work, we developed an innovative, simple and scalable fabrication technique

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that partially embeds colloidal plasmonic nanostructures within a semiconductor TiO2 layer

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without utilizing any complex top-down nanofabrication method. The successful embedding

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is confirmed by scanning electron microscopy and atomic force microscopy imaging. Using

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visible pump, near-infrared probe transient absorption spectroscopy, we also provide

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evidence that the increase in surface contact area between the nanostructures and the

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electron-accepting material leads to an increase in the amount of hot electron injection into

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the TiO2 layer.

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Keywords: plasmonic, hot electrons, Schottky barrier, large scale fabrication, partially embedded structures, plasmonic photocatalysis

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Nanoscale heterogeneous structures consisting of metal-semiconductor junctions have great

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potential in applications related to photonic energy conversion. When plasmonic metal

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nanostructures are in direct contact with a semiconductor, a Schottky barrier can be formed at

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the metal–semiconductor interface1 [Figure 1(A)]. Hot charge carriers that result from non–

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radiative plasmon decay can be emitted across this interface, leading to charge–separated

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states that are useful for various applications, including photodetectors2, photovoltaics3 and

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photochemistry.4 The reported collection efficiency of these electrons is low, typically
20%.16 Another approach to increase the charge–

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separation quantum yield is to increase the contact area between metal and semiconductor

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[Figure 1(C)].8 This in principle allows for a larger fraction of the excited hot electron

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population to be extracted into the semiconductor material (shaded area in the momentum–

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space sphere shown in Figure 1(C)). Recently, an 11-fold increase in the incident photon-to-

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current conversion efficiency and enhanced water splitting were observed in structures,

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where gold nanoparticles were partially inlaid in a TiO2 layer.18 In addition, three-

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dimensional tandem plasmonic Au/TiO2 nanodiodes19 and photovoltaic cell with double

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Schottky barriers20 has also shown to extract excited carriers more efficiently. This further

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verifies the significance of partial coverage of the surface of plasmonic nanoparticles within

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an electron accepting layer. On the other hand, complete surface coverage is expected to be

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detrimental to hot carrier extraction, as it blocks the required charge flow to preserve charge-

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neutrality in the nanostructure (i.e. electron/hole flow).

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Whilst the concept of partial surface coverage has been also demonstrated in the past 8,18, the

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reported approach is of limited scalability due to the use of top–down nanofabrication

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techniques (such as electron-beam lithography). Furthermore, top–down fabrication

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approaches set a lower–limit to the minimum size of the metal nanostructures ( > 20 nm),

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which in turn sets the optical response of the nanostructures to the NIR region of the

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electromagnetic spectrum.21 This implies a low electrochemical potential of the charge–

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separated state, limiting the range of applications in hot–electron photo–redox catalysis.22



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Here, we describe a simple, large scale and low-cost fabrication process that yields partially

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embedded metal-semiconductor structures at macroscopic areas, and without the need of

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lithography and dry etching steps. The nanostructures consist of nanocrystalline colloidal –

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based Au nanorods as the building blocks, and due to the small size of these nanorods, the

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overall optical response of the metal-semiconductor structures remains in the visible range of

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the spectrum. Furthermore, we provide evidence via ultra-fast pump-probe spectroscopy that

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the embedment achievable with our fabrication technique leads to an increase in the quantum

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yield for hot electron injection into the semiconducting material.

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In Figure 2 we show a schematic of the “lift–off” process we have developed to create

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partially embedded metal–semiconductor structures over macroscopic areas. Firstly (step A,

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Figure 2), a sacrificial layer of methyl methacrylate (MMA, an acetone–soluble material) is

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spin–coated onto a cleaned Si wafer. Metal nanocrystals are subsequently self–assembled on

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the surface of MMA by immersing the MMA/Si wafer in an aqueous dispersion containing

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Au nanorods (Length: 50 nm, Diameter: 20 nm) colloid. The self-assembly process is based

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on electrostatic interactions, where the N,N,N-trimethyl-(11-mercaptoundecyl)ammonium

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chloride (TMAC)-coated Au nanorods exhibit a positive charge and adsorb readily onto the

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negatively-charged MMA/Si substrate.

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After the Au nanorods are self–assembled onto the MMA/Si wafer, a layer of 50 nm titanium

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dioxide (TiO2) is deposited onto the structure via a standard electron beam evaporation

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process (step B, Figure 2). The thickness of this layer must exceed the diameter of the Au

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nanorods (estimated to be ∼ 20 nm, supporting information figure S1) in order to ensure

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almost complete coverage of the nanostructures. At this point, more layers could be added,

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including a mirror, which would result in the creation of metal–semiconductor–metal

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structures that can exhibit increased light absorption23,24 or plasmon coupling to Fabry-Pérot

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resonances18 (supporting information Figure S4 – S6). 6 ACS Paragon Plus Environment

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An epoxy adhesion layer is then used to bond the complete structure to a glass substrate (step

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C, Figure 2) which acts as a rigid support. Next (step D, Figure 2), the sample is immersed in

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acetone for 3 hours to dissolve the MMA sacrificial layer and release the Si wafer (i.e. “lift-

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off”), resulting in structures where Au nanorods are partially buried within TiO2 (step E,

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Figure 2). For the experiments to be described below, we have also prepared a reference

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sample consisting of non-embedded Au nanorods, which are self-assembled on the top

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surface of the TiO2 film. In this case, the self–assembly of Au nanorods is performed on the

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surface of TiO2 after the acetone lift–off step (step E, Figure 2). Although the self-assembly

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process of the nanorods is performed on different surfaces, an analogous particle density of

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2.63 x 109 nanorods/cm2 and 2.65 x 109 nanorods/cm2 is achieved for the partially embedded

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and non-embedded structures respectively. To further affirm the equivalence between these

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structures, the average interparticle distance between the nanorods of partially embedded and

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non-embedded strucures are 172.95 nm and 160.51 nm respectively, as shown in Figure S1.

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With these, we can assure that all of the characterizations and measurements performed on

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these structures are a result of the embedment, as opposed to the difference in the number of

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plasmonic nanoparticles. The aspect ratios of the Au nanorods before and after electron beam

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evaporation are also calculated to be 1.9±0.2 and 2.0±0.4 respectively, confirming that the

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morphology of the nanorods are not damaged during the electron beam evaporation process.



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Figure 2. Nanofabrication. (A) Methyl methacrylate (MMA) is spin–coated on a Si wafer

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and Au nanorods are self–assembled electrostatically with a random orientation on the MMA

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layer. (B) Deposition of 50 nm TiO2 via electron beam evaporation. (C) The sample is

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adhered to a glass substrate via an epoxy layer and (D) subsequently immersed (upside-

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down) in acetone to remove the MMA layer and release the structure from Si wafer to form

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the final partially embedded structure.

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One of the most attractive features of this fabrication method is that it is readily applicable to

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a wide range of nanoparticles possessing different shape, size and composition, including

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complex nanostructures such as Janus or alloyed nanoparticles. Figure S1 demonstrates the

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assembly of TMAC coated silver cuboids onto the MMA sacrificial layer via similar

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electrostatic interactions, where deposition of TiO2 can be subsequently performed onto the

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surface to achieve partial embedment of the plasmonic nanostructures. To date, the


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incorporation of such complex nanostructures is not possible via other nanofabrication

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techniques.

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Figure 3(A) and (B) show surface topography images obtained with Atomic Force

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Microscopy (AFM) for the resulting embedded and non–embedded structures. In Figure

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3(A), the Au nanorods are embedded within the TiO2 and the AFM topography image

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consists of a random collection of pits denoting the location where the Au nanorods are

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embedded. In fact, the nanorods are shown to be buried ∼ 2 nm below the TiO2 surface as

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depicted in Figure 3(A) as well as in supporting information Figure S3. On the contrary, the

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surface topography of Figure 3(B) clearly reveals the geometrical outline of the Au nanorods

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that rest on top of the surface of the TiO2 film. Figures 3(C) and (D) show scanning electron

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microscopy images of both kinds of structures [3(C): embedded, and 3(D): non–embedded]

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where the presence of Au nanorods on both samples is clearly visible. Together, the images

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of Figure 3 indicate that the process described in Figure 2 leads to structures where Au

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nanorods are partially buried inside TiO2 and retain their geometrical shape throughout the

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process.

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Figure 3(E) shows the measured optical extinction [defined as ― ln(𝑇), 𝑇 is the transmission

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spectrum] for the structures of Figure 3, along with the measured absorption spectrum for the

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Au nanorods in solution. The spectrum in solution is dominated by a single band that

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corresponds to the longitudinal localised surface plasmon resonance of the Au nanorods. The

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extinction spectra of the Au nanorods when placed on top and when partially buried inside a

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thin film of TiO2 show redshifts, which are attributed to the presence of the high refractive

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index of the TiO2. Numerical full wave simulations were carried out to identify the observed

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extinction bands, as shown in Figure 3(F). In Figure 3(F) we show the calculated absorption

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spectra for idealised structures consisting of spherically–capped Au nanorods resting on top

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of a thin film with refractive index 2.4, and for a similar structure embedded to a variable 9 ACS Paragon Plus Environment


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extent in the film. By immersing the Au nanorod in the semiconductor, the localised surface

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plasmon resonances (both longitudinal and transverse) redshift in proportion to the amount of

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surface coverage.

Partially embedded structures

Non-embedded structures

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Figure 3. Structural characterisation and Optical properties. (A) and (B) are atomic force

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microscopy topography images of the embedded and non–embedded structures respectively.

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(C) and (D) are the corresponding scanning electron images. (E) Measured extinction [

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―𝑙𝑛(𝑇)] spectra of the embedded, non–embedded structures along with the absorption

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spectrum of the Au nanorods in solution as control. (F) Calculated absorption spectra of

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embedded structures based on depth and non-embedded structures using a full–wave

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numerical solution of Maxwell equations.


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We now turn our attention to the effect of increased metal-semiconductor contact area on the

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possible hot-carrier decay of the metal nanostructures. Following Landau (non–radiative)

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damping of the particle surface plasmon resonances, the resulting hot–electrons have a finite

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probability of injection into the semiconductor material that is in contact with the

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nanoparticle surface.15 Once injected, these carriers rapidly relax to the lower states of the

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conduction band of the semiconductor, and can thus absorb both visible and near–infrared

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light, enabling the detection of these charge–transfer processes through visible pump (to

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excite the particle plasmons) near–IR probe (to detect the injected electrons) spectroscopy, as

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we describe next.25,26 (A)

(C)

(B)

Decay and injection

pump @ 732 nm

Eg>hν

probe hν

pump

e-

Energy

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trap states

h+ Au

pump @ 615 nm

cb EF vb

TiO2

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Figure 4. Ultra-fast pump probe spectroscopy. (A) Pump pulses excite localised surface

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plasmon resonances in the Au nanorods which, after non-radiative decay, can result in the

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injection of hot-electrons into the conduction band (cb) of TiO2. Intraband excitation of these

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electrons is possible with NIR probe pulses. Also shown are the relevant energy levels of the

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system: 𝐸𝐹: Fermi level of the metal, vb: valence band and 𝐸𝑔: semiconductor bandgap. (B)

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and (C) show the transient absorption ΔA spectra as a function of probe wavelength and

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pump-probe delay time (Δt) for the (B) partially and the (C) non-embedded structures. The

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pump wavelengths are indicated in the figures.

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As shown in the diagram of Figure 4(A), we photo–excited the samples using ultra–short

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light pulses, spectrally tuned to the localised surface plasmon resonance of the Au nanorods, 11 ACS Paragon Plus Environment


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which according to the data of Figure 3(E), occurred at 634 nm for the non–embedded

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structures and 732 nm for the embedded structures. These wavelengths are significantly

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below the band-gap of TiO2 and consequently, no direct excitation of electron-hole pairs in

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this material is possible. After non–radiative relaxation of the localised surface plasmon

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resonances, injection of electrons into the TiO2 can take place, leading to a transient

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population of electrons at the bottom of the conduction band of the semiconductor [Figure

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4(A)]. These electrons can absorb near–infrared light (via intraband electronic transitions)

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and are thus detectable through the changes in the absorption of time–delayed near–IR light

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probe pulses.26 In fact, this method for detecting plasmonic hot-electron injection into TiO2

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has been applied in numerous occasions in the past.27,28

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Figure 4(B) and (C) show the results of these measurements as a function of probe

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wavelength and pump–probe time delay. In general, the measured transient absorption (𝛥𝐴)

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spectra indicate excited state absorption (𝛥𝐴 > 0), where the amplitudes of the measured

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signals are larger for the samples where the Au nanorods are partially embedded in TiO2.

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This indicates a larger amount of hot electrons are being injected into the TiO2 layer, which

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result from an enhancement in hot carrier extraction within the partially embedded structures.

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In the time domain, the detected signals consist of a fast rise (sub–picosecond time–scale)

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and a rapid decay (𝛥𝑡 < 5 ps) which is followed by a slower, multi–exponential decay (𝛥𝑡 >

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5 ps). It has been documented that the electron population in TiO2 decreases with time due to

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an interplay of processes such as: recombination with the positive holes present in Au29,

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surface electron trapping (𝛥𝑡 < 200 fs, Figure 5 shows a representation of such states)30 and

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charge transfer to ambient oxygen molecules (10 ―6 ― 10 ―3s)31, processes with marked

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differences in time scales, which can account for the multi–rate decay. The transient

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absorption changes measured for partially embedded Au nanorods in metal–semiconductor–

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nanorod structures (Figure 5), exhibit damped oscillations with a strong Fourier component at 12 ACS Paragon Plus Environment

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∼ 20 GHz (supporting information Figure S7). We assign this oscillation frequency to

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mechanical (breathing mode) oscillations of the Au nanorods. These oscillations are part of

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the non–radiative decay channels available to localised surface plasmon resonances in the Au

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nanorods14, and occur in competition with electron injection across the Au–TiO2 interface.

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In Figure 5(A), we show a direct comparison of the transient absorption changes for Au

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nanorods that are partially embedded, non–embedded and a control sample where the

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nanorods are deposited directly on a glass substrate. For the rods on a glass substrate, no

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transient signal is observed, which indicates that no electron injection events take place, a

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result consistent with the expectation that few or no electron–accepting states exist in a glass

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support. On the contrary, for the other two cases there is a non–negligible transient signal,

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which is largest (in magnitude) for the partially embedded samples. We argue that this

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difference in signal magnitude is largely due to a higher probability of electron injection

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afforded by the larger number–density of Au–TiO2 interfaces in the partially embedded

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structures, as shown in the comparison in Figure 5(B) and (C). More details on the ultrafast

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pump-probe configuration and spectra of pump and probe light can be found in Figure S8-9.

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We would also like to emphasize the magnitude of the measured transient absorption

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increases linearly with pump power, as shown in Figure S10, which indicates that the

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experiments were performed in the small signal limit region. It is also noteworthy to mention

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that there is a possibility that the ligands on the surface of Au nanoparticles could be

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deteriorated during the electron beam evaporation of TiO2 and the removal of these ligands

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could have an impact on the efficient of hot carrier injection. Further investigations, which

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will not be covered in this work, are thus required to address this.

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Figure 5. Amplitude of transient absorption signal. (A) Time evolution of the transient

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absorption 𝛥𝐴 signal measured at 1000 nm for samples consisting of Au nanorods on glass,

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on TiO2 and partially–embedded in TiO2. Also shown are diagrams of the idealised cross-

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section of cylindrical Au nanorods of radius 𝑅 and length 𝐿 (B) on TiO2 and (C) partially

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embedded in it. The arrows indicate interface normal vectors. In (B), the Au–TiO2 contact

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area is 𝑑𝑆, whereas this area increases to 𝑆 in the case shown in (C).

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In summary, we have demonstrated a novel and simple nanofabrication method to partially

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embed Au nanorods into TiO2. Our developed process primarily requires physical vapour

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deposition techniques and does not involve any complex processes such as electron beam

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lithography or reactive ion etching, making it highly attractive for large scale fabrication.

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Furthermore, the incorporation of complex nanostructures such as Janus nanoparticles or

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alloyed nanoparticles that is unattainable in the current lithography method is feasible in the

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introduced fabrication technique. In addition, we have shown that the embedment of

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plasmonic nanoparticles leads to a higher efficiency of hot–electron injection into the

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semiconductor using ultrafast pump-probe spectroscopy. In particular, we envisage that

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contrary to other approaches32,33, our method can enable the construction of near–perfect

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absorbers of light that incorporate partially–buried metal nanostructures.23 We also foresee 14 ACS Paragon Plus Environment

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that these substrates can be used as active elements in novel sensors34, photo–detectors35 or in

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plasmonic photochemistry.4,36–41

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Acknowledgements

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This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the

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Victorian Node of the Australian National Fabrication Facility (ANFF). C. N. was supported

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by an OCE Fellowship from CSIRO. D.E.G. acknowledges the ARC for support through a

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Future Fellowship (FT140100514). D.E.G and U.B. acknowledge the ANFF for the MCN

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Technology Fellowships. The authors acknowledge use of facilities within the Monash

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Centre for Electron Microscopy A.R., T.J.D. & D.E.G. acknowledge the ARC for support

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through a Discovery Project (DP160100983).

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acknowledge support from the Australian Research Council Centre of Excellence in Exciton

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Science (CE170100026).

T.A.S., D.E.G. U.B. and J.E.S. also

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Supporting Information Available: Detailed experimental section, further material

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characterization, mirror-semiconductor-nanorod structures, Assignment of mechanical

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oscillations and Kinetic model.

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ORCID Charlene Ng: 0000-0001-5381-3855 Daniel E. Gómez: 0000-0002-7871-4068 Udo Bach: 0000-0003-2922-4959 John Elie Sader : 0000-0002-7096-0627 Timothy J. Davis : 0000-0002-7299-4900

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References 15 ACS Paragon Plus Environment


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(1) (2) (3) (4)

(5) (6)

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