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Abstract
Circular dichroism (CD) has been widely studied in recent decades because of its wide application in biomedical detection. Nanostructures with different heights (NDH) usually increase the transmission CD effect. To achieve such nanostructures, one needs to repeatedly perform the electron-beam lithography (EBL) method twice or more, layer-by-layer, which is a very complicated process. Here, we propose a method to prepare NDH by combining the EBL and oblique angle deposition (OAD) techniques. L-shaped planar silver nanostructures are prepared using EBL and normal electron beam deposition, and the OAD method is then used to partially cover one arm of the L-shaped nanostructure. Numerical simulations reveal that the height difference in the two arms of the L-shaped NDH (LSNDH) causes a difference in the polarization directions of the left- (LCP) and right-circularly polarized (RCP) incident light, thereby, generating CD effects. A 2D material is used to cover the LSNDH to further increase the charge polarization direction differences, which considerably increases the CD effect. These results are useful in simplifying and increasing the convenience of the preparation method of 3D chiral nanostructures. Furthermore, the proposed nanostructure may have potential application in biosensor, such as chiral enantiomer sensors.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
An object that is not the same as its mirror image is called chiral, and its mirror image cannot be superimposed on the original object [1]. Chirality is the order and organization of life. Many basic compositions of life, such as proteins, amino acids, and ribonucleic acids, are chiral [2]. Chiral metamaterials exhibit novel electromagnetic phenomena, such as circular dichroism (CD) [1–6], negative refractive index [7–10], and asymmetric transmission effects [11–14]. CD is the differential transmission between the left- (LCP) and right-circularly polarized (RCP) lights for a chiral nanostructure [15].
CD signals are generated using different types of chiral nanostructures, such as plasmonic planar chiral nanostructures, 3D nanostructures, and layer-by-layer structures, and their mechanisms in producing CD are also different. Planar chiral nanostructures achieve transmission CD under the illumination of oblique incident light because the phase difference of nonparallel electric dipoles in the different parts of planar chiral nanostructures are generated only in this case [16]. 3D chiral nanostructures, such as helical nanostructures, lead to larger CD effect because of the coupling of magnetic and electric dipoles due to the currents around the loop and along the nanostructure [17–19]. The layer-by-layer nanostructures also generate the CD effect because of the electromagnetic coupling between the two twisted layers [20,21]. For 3D and layer-by-layer nanostructures, the direct laser writing, polystyrene (PS) nanosphere template, and electron-beam lithography (EBL) techniques are usually used. The direct laser writing technology uses a variable-intensity laser beam to perform a variable dose exposure of the resistant material on the surface of the substrate for 3D structure fabrication. However, given the limitation of the wavelength of the laser, the prepared structure can only reach the order of millimeters [22,23]. The PS nanosphere template technique, which combines the molecular self-assembly and the oblique angle deposition (OAD) techniques, can fabricate concise 3D chiral nanostructure in a large area but is limited by poor accuracy and control on the dimensions of the nanostructure [24–26]. For increased accuracy, the EBL technique can eliminate small errors in the design and preparation results [27]. However, simple 3D and layer-by-layer nanostructures are fabricated by stacking the layered nanostructures by using EBL. Furthermore, each layer obtained by repeating a standard pattern of spin coating, lithography, developing, fixing, deposition, and lift-off processes relatively increases the overall fabrication time [28,29]. In order to avoid the complex preparation procedure of 3D nanostructure, the method of enhancing the CD effect of planar structures has attracted interests. For the thick planar chiral nanostructures with a certain height, magnetic resonance occurs at the lateral faces, and the coupling between the magnetic and the electric resonances increases the CD effect [30–32], which depends strongly on its height. When the height is less than or greater than the appropriate value, the intensity of CD decreases dramatically, thereby limiting their applications.
In addition, chiral nanostructures with different heights (NDH) at different parts have increased CD effect [33–36]. The 3D chiral nanostructure is prepared using the two-step electron beam exposure process. The process is complex, and the top layer nanostructure is difficult to align to a specific position of the bottom nanostructure. Furthermore, the process is cumbersome because the entire process of the first etching to the second layer with accurate alignment must be repeated. Therefore, chiral NDH with high precision, simple preparation method, and easy application to devices must be proposed.
In this paper, the combination of the EBL and the OAD techniques is proposed to prepare chiral nanostructures with different heights. By taking L-shaped NDH (LSNDH) as the example, we show the preparation process and the physical mechanism of increasing the CD signal. L-shaped NDH (LSNDH) are prepared to increase the CD effect. One arm of the LSNDH is covered with another layer of material (Ag) by using the OAD technique. Numerical calculations show that the different heights of the two arms provide a difference of polarization direction between LCP and RCP light illuminations at the bottom surface, thereby increasing the CD effect. Increasing the thickness of one arm of the LSNDH and the coverage area of the top layer results in enlarged CD intensity. In addition, the MoS2 layer is added to the top surface of the LSNDH to increase the polarization direction difference between the top and the bottom surfaces of LSNDH, which can increase the intensity of CD.
2. Results and discussion
2.1 Preparation of LSNDH
The LSNDH arrays are prepared using the EBL and the OAD techniques, and the specific preparation process is shown in Fig. 1. Figures 1(a)–1(d) show the preparation process of the template. Figures 1(e)–1(g) show the deposition process of nanostructure, and Fig. 1(h) shows the final LSNDH arrays obtained. The first step (Fig. 1(a)) is to prepare the ITO glass substrate. The second step (Fig. 1(b)) is to spin–coat the poly methyl methacrylate (PMMA) with a thickness (h) of 270 nm on the ITO glass substrate. The third step is to design the L-shaped template with different arm widths and lengths by using the graph generator (Fig. 1(c)). The fourth step is the formation process of the template, which is an L-shaped hole of LSNDH with electron beam exposure, development process, and fixing process (Fig. 1(d)). For the fabrication of the nanostructure, an electron beam evaporation apparatus is used to deposit the metal. The fifth and sixth step are the deposition process of the LSNDH by normal electron beam deposition method, including the deposition of the Ti (Fig. 1(e)) and Ag (Fig. 1(f)) layers with thicknesses of t1 = 5 nm and t2, respectively. This step forms the L-shaped nanostructure, and the arm in the x- and y- directions have lengths of l1 and l2, respectively, and widths of w1 and w2, respectively. The layer of Ti is used to ensure the adhesion between the ITO glass substrate and the metal nanostructure. The seventh step is the deposition process of the coverage layer by using the OAD method, where deposition direction is θ2 and φ = 0° (Fig. 1(g)). Here, θ is polar angle; it is defined as the angle between the normal of the substrate plane and the direction of vapor deposition. φ represents azimuth angle. The coverage layer only exists on one arm of the LSNDH with t3, l3, and w3. In this process, only the arm in the x-direction can be deposited. However, in the y-direction of the hole, the evaporated metal can only be deposited on the wall of the hole. Thus, the thickness of the metal film at the bottom of the hole remains unchanged. The l3 of the coverage layer is determined by θ, and the relationship between θ and l3 is $\theta = \arctan (h/{l_1} - {l_3})$, where h is the thickness of PMMA, and l1 is the length of the arm of LSNDH in the x-direction. In the final process, PMMA is removed from the glass substrate by using the lift-off process, and the LSNDH arrays are obtained (Fig. 1(h)).
Fig. 1. Schematics of the fabrication process of LSNDH. The process includes eight steps: (a) preparing ITO substrate, (b) spin-coating of PMMA layer, (c) designing L-shaped geometry, (d) electron beam exposure to make L-shaped template, (e) normal deposition of Ti, (f) normal deposition of first layer Ag, (g) titled deposition of second layer Ag and (h) lift-off the PMMA layer.
The LSNDH is characterized using SEM and AFM. Figures 2(a)–2(b) show the SEM images of the left- (LH) and the right-handed (RH) LSNDH arrays. The dimensions of the LSNDH are determined by averaging several representative areas obtained from different spots on the sample. The obtained parameters are l1 = l2 = 530 nm, w1 = 390 nm, w2 = 230 nm, l3 = 280 nm, and Px = Py = 1000 nm. AFM is used to ensure the height of LSNDH. Figure 2(c) shows a 3D plot of LSNDH extracted from the AFM data, which confirm the height difference between the different arms of the L-shaped structures. Figure 2(d) shows a height profile along the horizontal arm of LSNDH, and the height with (t3) and without (t2) the coverage layers are 60 and 40 nm, respectively. A height difference of approximately 20 nm is observed in the LSNDH.
Fig. 2. SEM images and three-dimensional AFM measurement of LSNDH. (a)−(b) SEM images of LH- and RH-LSNDH arrays; (c) Three-dimensional AFM image of RH-LSNDH arrays; (d) Height profile of LSNDH in panel along the cross-section line shown in the bottom left corner. The height of the arm with coverage layer height is around 60 nm.
2.2 CD effect of LSNDH
The transmission and CD spectra of LSNDH arrays are measured using the in-house spectroscopy for micron area. Figures 3(a)–3(c) show the transmission and CD spectra of the LH- and RH-LSNDH arrays. The transmission spectra show four resonance modes denoted as I, II, III, and IV. The transmission and CD spectra of LH- and RH-LSNDH are simulated using the finite element method. In the simulation, the parameters of chiral nanostructure are obtained using the SEM or the AFM images. The corners of the LSNDH are rounded to acquire similar nanostructures with the prepared nanostructures. The four evident valleys, namely, λI = 580 nm, λII = 630 nm, λIII = 840 nm, and λIV = 900 nm, in the transmission spectra are denoted as I, II, III, and IV, respectively. The different values of T-- and T++ lead to the CD effect. The CD peaks or valleys are observed around the four modes mentioned above. In addition, the planar L-shaped nanostructure array with t3 = 0 nm is prepared as a referent sample. It can be seen from both experimental and calculated data that when t3 = 0 nm, the maximum value of CD signal is only 0.3% (Figs. 3(c) and 3(f)). There is almost no CD signal, which agrees with the published results [5,16]. However, obvious CD signals appear in the LSNDH, and the maximum value of CD signal can reach 3%. The size of particle affects refractive index, which may cause the difference between experimental and simulated transmission spectra.
Fig. 3. Transmittance and CD spectra of LH- and RH-LSNDH under RCP and LCP light illuminations. CD spectra of planar L-shaped nanostructure (t3 = 0 nm). (a)−(c) measured in experiment; (d)−(f) calculated during numerical simulations. The insets show the corresponding models of the nanostructures designed in experiment or simulated numerically.
To understand the physical mechanism of the CD effects of the proposed nanostructure, the surface charge distributions under LCP and RCP light illuminations at the resonant modes are presented in Fig. 4. Four resonant modes exist, and their coupling mechanism at the resonant wavelength is presented in Figs. 4(a)–4(d). Red and blue dots represent the positive and the negative charges, respectively, and green arrows represent the coupling between the left and the right parts. The charge distributions of the top and the bottom surfaces are shown in Figs. 4(e)–4(t). The red and blue areas represent the distribution of positive and negative charges and marked with “+” and “−”, respectively. The dotted green arrow represents the direction of polarization.
Fig. 4. Resonance model and charge distributions on the top and bottom surface of LSNDH. (a)−(d) Resonance model at four resonant wavelengths. The red and blue dots represent the positive and negative charges, respectively. (e)−(l) Charge distributions at four major resonant wavelengths under LCP light illuminating. (m)−(t) Charge distributions at four major resonant wavelengths under RCP light illuminating.
At mode I, as shown in Figs. 4(e) and 4(m), a quadrupole and an octupole are formed on the top surface of the coverage layer and the vertical arm, respectively, and form the bonding mode. On the top surface, the resonance is formed in the x direction by the coupling between coverage layer and the vertical arm. The polarization directions change due to the height difference on the bottom surface. The difference in the polarization directions of the LCP and the RCP light illuminations at the bottom surface of the LSNDH leads to the CD signal. This resonant wavelength is mainly determined by the electron oscillation in the y-direction. At mode II, two quadrupoles are formed on both the coverage layer and the vertical arm, respectively. Two quadrupoles form the bonding mode. Similar to mode I, in mode II, the difference in the polarization directions of the LCP and the RCP light illuminations at the bottom surface of the LSNDH leads to the CD effect. This resonant mode is mainly due to the electron oscillation in the x-direction. At mode III, quadrupoles are also formed on the coverage layer and the vertical arm. However, two quadrupoles form the antibonding mode. At mode IV, a dipole is formed on the top surface of the coverage layer, and a quadrupole is formed on the vertical arm on the top surface. The dipole and the quadrupole form the bonding mode. In summary, different resonant modes appear on the top surfaces of the covered arm and another arm, and the difference in the polarization directions of the LCP and the RCP light illuminations occur on the bottom surface of the LSNDH due to the height difference, which leads to CD effects.
The structural parameters of the coverage layer affect the CD effect of LSNDH arrays dramatically. The t3 and l3 of nanostructure are changed while the other parameters (l1 = l2 = 530 nm, w1 = 390 nm, w2 = 230 nm, t2 = 40 nm, Px = Py = 800 nm) are maintained constant to explore the influence of coverage layer on the CD signal.
Figures 5(a) and 5(c) show the schematic of the t3 and l3 of the coverage layer. Figures 5(b) and 5(d) show the numerically calculated CD map with different t3 and l3, respectively. Red and green bars represent positive and negative CD signals, respectively. With increasing t3, the CD signals at the four modes do not evidently shift as resonances occur at the surface of LSNDH, as shown in Figs. 4(e)–4(t). Given that the parameters of the surface do not change, the peaks or valleys do not shift obviously. However, at increased thickness of the coverage layer, the height difference increases. Moreover, difference of polarization direction between LCP and RCP light illuminations increase at the bottom surface, thereby increasing the CD intensity at the four resonant wavelengths. With increased l3, no evident shift is observed in all four CD modes as the resonance on the top surface is excited by the resonance in the vertical arm and the coverage layer (Figs. 4(e)–4(t)). The total electron oscillation lengths of the four resonances do not change. Thus, the peaks or valleys of CD signal also do not shift. However, with increasing l3, the CD signal first increases and then decreases. The CD signal reaches its maximum at l3 = 300 nm, that is, the coverage layer occupies the whole horizontal arm of the LSNDH. When l3 increases and occupies the whole horizontal arm of the LSNDH, the overall thickness of the coverage layer increases, which increases the height difference. Thus, given the increase in height difference, the difference in the polarization directions of the LCP and the RCP light illuminations increases on the bottom surface of the LSNDH, which enhances the CD signals. With increasing length, the coverage layer also covers some part of the vertical arm of the LSNDH. This phenomenon increases the overall thickness of the vertical arm but decreases the height difference. Thus, given the decrease in height difference, the difference in the polarization directions of the LCP and the RCP light illuminations decreases on the bottom surface of the LSNDH, thereby decreasing the CD signal. At l3 = 530 nm, the coverage layer occupies the whole LSNDH, the height difference disappears, and the CD signal is lost. The same results are also confirmed in experiments. When the coverage is deposited on the vertical arm of the LSNDH, CD signals also is observed.
Fig. 5. The CD spectra of LSNDH for different parameters. (a) Schematic diagram of changing t3. (b) The CD spectra of LSNDH is simulated t3 from 0 to 40 nm. (c) Schematic diagram of changing l3. (d) The CD spectra of LSNDH simulated when l3 changes from 80 to 520 nm.
2.3 CD effect of LSNDH with MoS2 layer
As discussed above, the difference in the polarization directions of the LCP and the RCP light illuminations on the bottom surface of the proposed nanostructure arrays lead to the CD effect. A 2D material is used as another cover layer for the proposed nanostructure arrays to tune the electron oscillation at the top surface and increase the CD effect. MoS2, which is used in our study, is transferred on the LSNDH through the PMMA nanotransfer method [37,38].
Figure 6(a) shows the SEM image of the LSNDH with MoS2 layer, and the scale bar represents 3 µm. From the gray contrast of SEM image, the coverage and the MoS2 layers can be distinguished using the dashed line box. The white dotted box is the spectral acquisition area for transmission spectra in Fig. 6(b). Four resonance modes exist in the transmission spectra, and the transmission spectra under the LCP and the RCP light illuminations are different at the resonant wavelength in Fig. 6(c). The intensities of CD signals are twice as those without the MoS2 layer at the Mode II and III, and the position of the CD signal does not evidently shift. Comparing transmission spectra of LSNDH and LSNDH with MoS2 layer, the intensity of resonant valleys decreases by 8% and 4% under LCP and RCP light illuminations, respectively, which causes a doubling of CD intensity. The stronger coupling between the MoS2 layer and the LSNDH [39] causes this increase in the CD intensity. When the MoS2 layer is covered, the coupling between the MoS2 layer and the LSNDH is enhanced largely under LCP light illumination, but the coupling is slightly enhanced under the RCP light illumination. Based on the polarization direction of LSNDH, the polarization direction under LCP light illumination is considerably enhanced with the addition of the MoS2 layer. However, the polarization direction under the RCP light illumination is weakly enhanced. The phenomenon increases the CD signals. This finding proves that the 2D material can be used as coverage to increase the polarization direction difference between the LCP and the RCP illuminations and increase the intensity of the CD signals.
Fig. 6. The characteristic of RH-LSNDH with MoS2 layer. (a) The SEM images for RH-LSNDH with MoS2 layer. (b)−(c) The transmittance and CD spectra for RH-LSNDH with MoS2 layer under the circular polarized light illumination.
3. Conclusions
In summary, a highly precise and simple method is used to prepare the LSNDH and increase the CD signal. The height difference is achieved by covering part of the nanostructure that can control the deposition angle and time of OAD. The height difference nanostructure is prepared by the EBL and OAD technique to increase CD signal. Using EBL and OAD technique, the L-shaped nanostructure with different heights arrays are prepared, and the LSNDH arrays show optical chirality in visible and near-IR regions. The simulations reveal that the height difference causes the difference in the polarization directions of the LCP and the RCP light illuminations to increase the CD signal at the resonance wavelength. The effects of the different parameters of the coverage layer are investigated. Furthermore, the CD reaches the maximum when the coverage layer occupies the whole horizontal arm of the LSNDH. With increasing thickness and length of the coverage layer, the CD signal increases because difference of polarization direction between LCP and RCP light illuminations increase at the bottom surface. In addition, the 2D material layer is added to LSNDH, and the intensity of the CD spectrum is also enlarged due to the polarization direction under LCP light illumination is considerably enhanced based on the proposed nanostructure but under the RCP light illumination is weakly enhanced. Thus, various templates can be produced, and the deposition angle and time may be changed to produce other complex 3D nanostructures. This study provides a concise and precious method to fabricate chiral plasmonic nanostructures and contributes to the understanding of the mechanism of chirality enlargement.
4. Methods
4.1 Fabrication
The E-beam evaporator (DE 400) is used to deposit Ag nanostructure with the chamber pressure of ∼1e−7 torr. The evaporation rate of the electron beam is 0.05 nm/s. The Ag and Ti slug were purchased from Alfa Aesar, 99.99% and 99.98% purity, respectively.
We transfer the MoS2 on the PCMCAs by the PMMA nanotransfer method. First, the PMMA is spin-coated on the Si substrate where grown the MoS2 layer. The speed of spin-coating is 800 rpm and 3000 rpm in the first and second period, respectively. After spin-coating PMMA, the Si substrate was baked under 800℃ for 3 min. 3M tape with the window stick on the surface of PMMA film, and then put that into the 70℃ KOH solution to make the MoS2/PMMA film fall off from the Si substrate. The PMMA film was put into the pure water to remove residual KOH, and the pure water needs to be changed every half hour. After the PMMA membrane was cleaned in ultrapure water, it was attached to a copper mesh with carbon membrane and baked at 90℃ for 3 h. After the PMMA was removed with acetone vapor, the MoS2 membrane was transferred.
4.2 Characterization
SEM is performed using a FEI Nova Nano SEM 450. Electron acceleration is set to 2 kV or 3 kV and the images are acquired in secondary electron mode. Atomic force microscope (AFM) measurements are performed with a Bruker Dimension ICON with a RTESP-300 tip. The transmission spectra are obtained with an optical system which is setup by ourselves. The broadband light is directed to a collimator. A linear polarized plate (Throlab, AHWP05M) and a quarter-wave plate (Throlab, AQWP05M) are then used to generate the circularly polarized light.
4.3 Simulation
Transmission spectra and charge distributions of LSNDH are simulated by the three-dimensional finite method software COMSOL Multiphysics. Regarding the grid size in calculation, the minimum grid size of nanostructure arrays is set as 0.1 nm, and the maximum grid size is set as 10 nm. The perfectly matched layers are set up on the input and output port for absorbing light. The transmittance is defined as the ratio of output power to incident power, and the incident electric field is set as 1 v/m. The Chirality is represented by $CD = {T_{ -{-} }} - {T_{ +{+} }}$, where ${T_{ -{-} }}$ and ${T_{ +{+} }}$ are the transmittance spectrum of LCP and RCP light, respectively.
Funding
National Natural Science Foundation of China (51972204, 61575117); Fundamental Research Funds for the Central Universities (2019TS122, GK201601008).
Disclosures
The authors declare no conflicts of interest.
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