Manipulation of hot electron flow on plasmonic nanodiodes fabricated by nanosphere lithography

To briefly explain nanodiodes, plasmonic Au is on top of n-type TiO₂ (n-TiO₂), and n-TiO₂ is on the framework insulator TiO₂ (f.i-TiO₂), using nanosphere lithography with reactive ion etching. The scheme of the fabrication process of plasmonic Au/TiO₂ nanodiodes is illustrated in figure 1. First, polystyrene (PS) beads—from Sigma Aldrich—in the nanopattern of hexagonal close-packing, of which the diameter is 460 nm, were coated on the quartz substrate (10 × 20 mm²) with the self-assembly method of nanosphere lithography [28, 47]. Ethanol dilutes the PS beads solution to 50%. Then, 170 μl of a 2% sodium dodecyl sulfate solution was dropped on the water contained in the petri dish. 5 μl of the PS solution was slowly dropped on a sliding glass laid obliquely in the petri dish, while arranged in a monolayer. The PS beads monolayer on the water surface was lifted off with the quartz substrate. Second, it was etched with oxygen plasma in the reactive ion etching chamber to reduce the size of the PS beads. Third, 90 nm of TiO₂ thick film was deposited on the sample through the electron beam evaporator. This TiO₂, named f.i-TiO₂, acted simply as a framework insulator. This process was prepared with a 3.5 × 6.5 mm² stainless steel mask and 0.4 Å s⁻¹ deposition rate under a vacuum of 8.5 × 10⁻⁷ Torr. Fourth, the sample was sonicated for 20 min in toluene solution (Sigma Aldrich) to remove PS beads and for 10 min in water to rinse the toluene. Fifth, 100 nm thickness of Ti film was deposited on f.i-TiO₂ film using the electron beam evaporator with a 3 × 6 mm² stainless steel mask and 0.8 Å s⁻¹ deposition rate under the vacuum of 2.0 × 10⁻⁶ Torr. Sixth, the sample was annealed at 400 °C in the air for 3 h to make n-TiO₂, named n-TiO₂. Seventh, two Au/Ti electrode pads were deposited. The Ti was 50 nm of thick film, and Au was 100 nm of thick film using the electron beam evaporator with a 3 × 6 mm² stainless steel mask and 1.0 Å s⁻¹ deposition rate under the vacuum of 2.0 × 10⁻⁶ Torr. The Ohmic contact electrode pad was on n-TiO₂/f.i-TiO₂ film, and another electrode for connecting the Schottky contact was on the quartz. Finally, to build the Schottky junction between plasmonic Au and n-TiO₂, 30 nm of thin Au film was deposited on the position between the n-TiO₂/f.i-TiO₂ film and the Au/Ti electrode pad using the electron beam evaporator with a 0.5 × 10 mm² stainless steel mask and a deposition rate of 0.5 Å s⁻¹ under the vacuum of 8.5 × 10⁻⁶ Torr. The Schottky junction area of plasmonic Au on n-TiO₂ was 0.5 × 0.5 mm². Therefore, changing the etching time and size of standardized PS beads facilitates control of the hole size.

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The optical and electric properties of the plasmonic nanodiode were carried out as follows: current density–voltage curves and short-circuit photocurrent density curves were measured in air conditions at room temperature using a source meter (Keithley Instrumentation 2400). Current density–voltage curves and short-circuit photocurrent density were obtained with the sweep mode and the ammeter mode, respectively. For the short-circuit photocurrent density of the light source, a tungsten-halogen lamp of 9 mW cm⁻² was used perpendicular to the nanodiodes. The spectrum of the lamp is displayed in figure S1 (available online at stacks.iop.org/NANO/32/225203/mmedia). This contains light in the visible region. Lumerical Inc. offers FDTD simulations to characterize absorbance spectra and electric field distribution. IPCE was measured to detect the hot electrons flow from plasmonic Au to n-TiO₂ depending on photon energy, with a tunable xenon arc lamp source (Newport, TLS-300XU). Absorbance spectra were measured with the ultraviolet-visible-infrared spectrometer (Shimadzu, UV3600). To characterize them, 1 mm thick quartz plate was used as a substrate of plasmonic Au/TiO₂ nanodiodes, and another quartz plate was used as a reference.

Figure 2(a) shows a schematic of hot electron generation on the Au/n-TiO₂ Schottky diode under illumination. Hot electron generation is amplified on the side of the hole area, and hot electrons can be transferred to n-TiO₂. The energy band diagram of the Schottky junction is shown in figure 2(b). Hot electrons induced by LSPR excitation of plasmonic metals can overcome the Schottky barrier and be found as photocurrent with n-TiO₂. Also, hot electrons can be directly transferred with interfacial plasmon decay. Cross-section images of the active area for corrugated hole arrays plasmonic Au/n-TiO₂/f.i-TiO₂ nanodiodes (CHAP Au/TiO₂) fabricated by nanosphere lithography with reactive ion etching are shown in figures 2(c) and (d). The real cross-sectional morphology of corrugated hole arrays is confirmed through scanning transmission electron microscopy, a high-angle annular dark-field image (figure 2(c)), and an elemental mapping image with energy-dispersive x-ray spectroscopy (EDS) (figure 2(d)). When deposited, 30 nm Au thin film could become spontaneously nanostructured as a corrugated structure with the nanostructured framework. Under illumination, LSPR occurs in nanostructured Au, and hot electrons can be generated. These hot electrons are injected into n-TiO₂. N-TiO₂ is an n-type semiconductor, which has high crystallinity, whereas f.i-TiO₂, which is the simple framework, is an insulator and amorphous. Crystallinity information about those TiO₂ is shown in x-ray diffraction (figure S2). Bare quartz and f.i-TiO₂ do not have x-ray diffraction peaks. N-TiO₂ has peaks for anatase and rutile. The nanostructured Au consists of the hole areas that correspond to the pit parts of Au. The 140 nm n-TiO₂ thick film is located on the 90 nm f.i-TiO₂ thick film, and the 30 nm Au thin film is on top of the n-TiO₂ thick film. Au thin film is ordered and securely connected as the nanostructure of corrugated hole arrays. The actual structure of the upper area is a slightly rounded, not vertical shape. Finally, Au thin film is deposited 30 nm on corrugated hole arrays and 20 nm on the side between the upperparts and the pit parts.

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To verify the surface morphology and size of corrugated hole arrays depending on etching time, scanning electron microscope images were taken (figure 3). As etching time increases, the hole diameter decreases. This is because the PS gets smaller during oxygen plasma etching (figure 3(a)). At 60 s etching time, the hole diameter is much larger, and at 150 s, the hole diameter is far shorter. With nanosphere lithography and reactive ion etching, the size of plasmonic Au can be easily controlled. Here, we designate the structure at 60 s etching time as the large-hole-shaped structure (figure 3(b)) and the structure at 150 s etching time as the small-hole-shaped structure (figure 3(c)). The large-hole-shaped structure has 280 ± 2 nm holes, and the gap between the holes is 122 ± 2 nm. The small-hole-shaped structure has 115 ± 4 nm holes, and the gap between the holes is 270 ± 4 nm. To aid in the understanding of the structures, cross-sectional schematics are depicted in figures 3(d) and (e). F.i-TiO₂ is deposited in a honeycomb shape between the reduced-sized PS gaps. Then the hole area naturally occurs because n-TiO₂ and Au are sequentially deposited. The small PS gaps lead to the large hole area, and the large PS gaps lead to the small hole area.

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The Schottky junction between plasmonic Au and n-TiO₂ is formed, as seen in figure S3. Here, the input bias is on the Schottky electrode, and the output bias is on the Ohmic electrode. When a forward bias is applied, the built-in potential of n-TiO₂ becomes flat banding, and electrons of n-TiO₂ flow to the plasmonic Au. On the other hand, when a reverse bias is applied, electrons of plasmonic Au cannot move to the n-TiO₂ due to the Schottky barrier. This Schottky barrier acts as an energy barrier and electron filter. Therefore, only hot electrons with high energy can cross the Schottky barrier and flow to n-TiO₂. In addition, hot electrons can be collected via direct charge transfer. Obtained fitted J–V curves and Schottky barrier heights are also displayed in figure S3(a). The Schottky barrier heights for the large-hole-shaped structure and the small-hole-shaped structure are 0.923 eV and 0.861 eV, respectively. J–V curves in the dark and light are shown in figure S3(b). The Schottky barrier height can be calculated through the fitting of current density–voltage data to the thermionic emission equation [48]:

$$\begin{eqnarray}&&I=AA* * {T}^{2}{\rm{\exp }}\left(-\displaystyle \frac{q{\varphi }_{b}}{{k}_{b}T}\right)\left[{\rm{\exp }}\left(\displaystyle \frac{q\left(V-I{R}_{s}\right)}{\eta {k}_{b}T}-1\right)\right],\end{eqnarray} \tag{ 1 }$$

where A is the active area, A** is the Richardson constant, T is the room temperature, q is the elementary electric charge, k_b is the Boltzman constant, ${\varphi }_{b}$ is the Schottky barrier height, $\eta$ is the ideality factor, and R_s is the series resistance of the nanodiodes.

The optical properties of CHAP Au/TiO₂ nanodiodes, depending on the hole size, are displayed in figure 4. Figure 4(a) shows the short-circuit photocurrent density with and without illumination. Essentially, the photocurrent density is measured as the reverse current. The photocurrent density values are set to the positive values for convenience. The photocurrent density is enhanced under the light, and plasmon-driven hot electrons occasion this enhancement. For the small-hole-shaped structure, the photocurrent density is higher. Structurally, the hole area of plasmonic Au is surrounded by n-TiO₂. The direction of the hot electron momentum corresponds to the direction of the electric field of the incident light. In other words, hot electrons are generated perpendicular to the incident light [13, 26], and the momentum of hot electrons generated in the hole area is directly towards n-TiO₂. Accordingly, the hole area collects many hot electrons, and hot electron collection depends on the size of the hole area. Also, in general, the smaller the plasmonic metal, the more plasmonic decay occurs via absorption [5, 49, 50]; therefore, the structure with the small hole area exhibits higher hot electron collection efficiency. Figure 4(b) presents the IPCE spectra of CHAP Au/TiO₂ nanodiodes depending on the hole size. Hot electrons are actively generated from 1.8 eV to 2.6 eV of photon energy as a peak in the IPCE spectrum. This can be assigned to the intraband excitation of hot electrons from the sp band of plasmonic Au. The generation of hot electrons between 2.8 eV and 3.2 eV is assigned to the interband excitation of hot electrons from the d band of plasmonic Au. Above 3.2 eV, IPCE increases dramatically because band-to-band excitation in n-TiO₂ arises. Each intensity of IPCE depending on the hole size accords with a trend of the photocurrent density. The point here is that the IPCE peak can be manipulated depending on the hole size. The large-hole-shaped structure has a red-shifted IPCE peak (2.2 eV). Whereas, the small-hole-shaped structure has a blue-shifted IPCE peak (2.3 eV). In general, the smaller the size of plasmonic nanostructures, the plasmonic excitation peaks tend to be shifted to the blue [30, 37, 39]. As described above, the hot electron collection of CHAP Au/TiO₂ nanodiodes depends on the size of the hole area. Therefore, the IPCE peak of the small-hole-shaped structure is shifted to the blue, and the IPCE peak of the large-hole-shaped structure is shifted to the red. Figures 4(c) and (d) show the absorbance spectra of experimental data and simulated data from FDTD analysis, depending on the hole size. We observe a peak of intraband excitation in each spectrum. Like the IPCE spectrum, this peak is shifted along with the size of the hole area and matches the tendency of the IPCE peak well. Both experimental and simulated absorbance have the plasmonic peak at same position and have the same peak position shift tendency. The small-hole-shaped structure has an additional peak of the interband excitation at 2.9 eV of photon energy. This leads to a linear increase in the IPCE spectrum from the interband excitation of the small-hole-shaped structure, and the IPCE intensity of the small-hole-shaped structure in the interband excitation region is much higher than the large-hole-shaped structure. We note that the mechanisms of plasmonic absorbance are different from those of IPCE. Plasmonic absorbance is the concept of light absorbance where free electrons of metal oscillate at a specific wavelength of the incident light, while IPCE represents the spectrum of hot electron generation by plasmonic decay across the Schottky barrier. Hot electrons generated on the surface of plasmonic Au have a very short mean free path of 20–40 nm [51], so a lot of hot electron energy decay occurs before crossing the Schottky barrier. Therefore, the number of hot electrons that can cross the Schottky barrier is further reduced as they move to the interface between Au and n-TiO₂. As higher energy photons are incident, higher energy hot electrons are actively generated [52], so even if the energy of the hot electrons collapses due to the short mean free path, the number of hot electrons across the Schottky barrier increases. As a result, IPCE becomes the most active when photon energy higher than the existing LSPR peak is irradiated; thus, the IPCE peak is blue-shifted more than the absorbance peak. However, the important point is that for each spectrum, as the size of the hole area decreases, both the IPCE peak and the absorbance peak become blue-shifted. These absorbance peaks only appear in nanostructures, and thin Au film does not have any plasmonic absorbance peaks (figure S4). As such, the intensity and peak position of optical and electrical properties can be easily manipulated through control of the size with the particular shape.

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FDTD electric field distribution was carried out to elucidate LSPR excitation and the hot electron generation of CHAP Au/TiO₂ nanodiodes depending on the hole size, and the result is shown in figure 5. The light enters Au/TiO₂ perpendicularly, and the electric field distribution images are expressed as Au/TiO₂ interfaces in the x−z plane of a cross-section view. The border is represented by the white dotted lines. FDTD simulation models for the electric field distribution were based on the hole size and the photon energy of the absorbance peaks. The main point is to have strong electric field confinement at Au/TiO₂ interfaces for efficient collection of hot electrons. Figures 5(a) and (b) are for the large-hole-shaped structure. Under the 1.7 eV photon energy, the peak absorbance position, the strong electric field confinement phenomenon occurs (see inside the yellow circles). On the other hand, the small-hole-shaped structure has much stronger electric field confinement at Au/TiO₂ interfaces under the 1.9 eV photon energy, which is the absorbance peak position (figures 5(c) and (d)). Those FDTD electric field distributions, which could also help visualize trends of absorbance and IPCE, were consistent with the peak shift observed on the large-hole-shaped and the small-hole-shaped structures on plasmonic nanodiodes.

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Earlier studies of charge separation at plasmonic-semiconductor interfaces have shown strong evidence that the dominant charge transfer mechanism is interfacial plasmon decay (chemical interface damping). Our electromagnetic field enhancement specifically occurs at the interface, further supporting this concept. Overall, the IPCE measurement combined with FDTD simulation indicates that LSPR excitation and hot electron generation can be controlled depending on the hole size. This strategy of changing the morphology of plasmonic nanostructure on the Schottky diode can give rise to the advanced scheme of plasmonic catalysis and photocatalysis due to the ability to control surface plasmon-driven hot electron generation [53, 54].