Single nanoflake-based PtSe₂ p–n junction (in-plane) formed by optical excitation of point defects in BN for ultrafast switching photodiodes

Abstract

Here, novel lateral PtSe₂ p–n junctions are fabricated based on the PtSe₂/BN/graphene (Gr) van der Waals heterostructures upon the illumination of visible light via the optical excitation of the mid-gap point defects in hexagonal boron nitride (h-BN). A stable photo doping effect was achieved for tuning the polarity of PtSe₂-based field-effect transistors (FETs). The constructed diodes display excellent rectifying performance, with a rectification ratio of up to ∼1.0 × 10⁵ and an ideality factor of ∼1.3. Distinctive self-biased photovoltaic behavior was detected, specifically in the positive open-circuit voltage (V_oc = 0.32 V) at zero source–drain current (I_ds), and also the negative short-circuit current (I_sc = 16.2 nA) at zero source–drain voltage (V_ds) generated for the p–n diode state upon the illumination of incident light (600 nm, 40 mW cm⁻²). Moreover, output V_oc switching behavior was achieved for the p–n diode state by switching the input light signal on and off, with a photoresponse over the broadband spectral range of 200–1200 nm. Various photovoltaic parameters were also measured. Also, using this elegant approach, homoinverters were fabricated that reached a maximum gain of ∼30 (V_DD = 2 V). These findings pave the way to developing self-biased photovoltaic devices by exploiting 2D noble metal dichalcogenide materials.

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1. Introduction

The outstanding progress made in transition metal dichalcogenides (TMDs) has led to their wide auspicious applications in nanotechnology due to them having astonishing electronic and semiconducting properties. TMDs have shown many favorable uses including in photodetectors,^1–6 DNA biosensing,⁷ band-to-band tunneling field-effect transistors (FETs),⁸ and spintronics FETs.⁹ As a result of van der Waals (vdW) forces, individual layers of TMDs are stacked together to form a multilayer. Various TMDs have a bandgap of around 1 to 2 eV.^10–14 Inspired by the attractive electronic and semiconducting properties of TMDs materials, researchers have now discovered other 2D materials such as noble metal dichalcogenides (NMDs). NMDs are air stable and show thickness dependence optical bandgaps, for example there exists a transition from the semiconductor to the metal in both PdS₂ and PtSe₂ upon a sharp modulation in thickness.^15–17

Among the NMDs, platinum diselenide (PtSe₂) is the most explored and has emerged as an exciting material with attracting properties. Theoretically, at room temperature the carrier mobility of PtSe₂ can be up to 40 000 cm² V⁻¹ s⁻¹. PtSe₂ can be exfoliated into a few layers or monolayer via mechanical exfoliation using adhesive tape or a micromechanical cleavage method and has a 0 eV bandgap for the bulk material while as a monolayer (bi-layer) material it shows a bandgap of 1.20 eV (0.21 eV).¹⁸ The crystal structure of PtSe₂ is made up of pentagonal-structured layers of Se–Pt–Se atoms, which are pentagonally packed with one sheet of Pt atoms and two sheets of Se atoms.¹⁹ PtSe₂ is air stable over a long period (one year), longer than black phosphorus (BP).¹⁵

Lots of studies have been accomplished to control or modulate the carrier type in 2D materials to obtain either p- or n-type conduction to make unipolar field-effect transistors (FETs) for upmarket p–n diodes and inverters for fundamental logic operations.^20–25 Various chemical doping treatments such as surface modification through interfacial charge transfer between the dopant and material have been widely demonstrated to control or tune the carrier type in 2D materials.^26–29 This traditional methodology is irreversible and non-programmable due to instability in an ambient environment.^26–29 Facilitating unipolar n- or p-type conduction in a similar nanoflake often needs a photoresist for chemical or elemental doping or otherwise to modulate carrier type via a gating effect, as the p–n diode state does not persist once the gating effect is removed.^20,30,31 These deficiencies hamper the intrinsic operations of a device.

Ju et al.³² reported the photo doping effect in heterostructures of graphene and boron nitride through the photosensitive excitation of mid-gap donor-like states in BN for nonvolatile and programmable functions. There is an absence of an intrinsic bandgap in graphene which hinders its applications in logic functional devices. In this work, we investigated carrier type variation in a PtSe₂ nanoflake via the optical excitation of mid-gap point defects in BN. We found that, upon illumination of visible light, p-type FETs based on PtSe₂ can be inverted to n-type. The PtSe₂/BN/graphene heterostructure shows both n–n and p–n diode states. Various photovoltaic parameters were measured in the p–n diode state. The devices were also measured in terms of their open-circuit voltage (V_oc) switching behavior at the p–n junction. For application in complementary electronic devices, the two FET channels of a single PtSe₂ nanoflake was selectively doped upon the illumination of light to fabricate homoinverters. We believe that this study will promote the development of electronic devices.

2. Experimental

PtSe₂ and BN nanoflakes were mechanically exfoliated from their bulk crystals using adhesive tape and a dry transfer technique was used to stack BN and PtSe₂ nanoflakes using a micromanipulator with a polydimethylsiloxane (PDMS) stamp. For the photo-induced effect, the heterostructures of BN and PtSe₂ were irradiated with visible light (λ = 450 nm, 30 mW cm⁻²) under negative gate voltage. The e-beam lithography process was performed to form the source and drain electrodes and Ni/Au (6/10 nm) metals were deposited on the PtSe₂ flake. The optical transparencies of the PtSe₂ and BN nanoflakes were altered with back brightness due to their different layer thickness and also the topography, morphology, and thickness of the PtSe₂ and BN flakes were examined using atomic force microscopy (AFM), optical microscopy, and Raman spectroscopy, respectively. Electrical transport characteristics were measured at (T = 300 K) in a vacuum box using a picoammeter (Keithley 6485) and source meter (Keithley 2400). Photovoltaic measurements were performed under visible light of various intensities with a broadband spectrum (200 to 1200 nm).

3. Results and discussion

A BN nanoflake with the help of a micromanipulator was stacked on graphene, checking that it was partially stacked on the graphene and on the substrate. Finally, the top PtSe₂ flake was transferred on BN (i.e., on the previously transferred one) by ensuring that it was well aligned with the graphene. In each step, samples were placed on a hot plate to remove water vapor from the interfaces or exterior surfaces and were also cleaned using acetone, methyl alcohol, and then dried under a flow of N₂ gas. Additionally, after the fabrication of devices, to obtain clean interfaces all devices were annealed for 4 h at a temperature of 200 °C in a furnace tube under a flow of 97.5% Ar/2.5% H₂ gas. A schematic illustration of the device is shown in Fig. 1a and the optical microscopy image is shown in Fig. 1b. Fig. S1a and b (ESI†) present the AFM images of PtSe₂ and BN flakes, respectively. The PtSe₂ and BN flakes show thicknesses of ∼2.71 and ∼51.2 nm in Fig. S1c and d (ESI†), respectively. The micro-Raman spectra of the PtSe₂, BN and graphene nanoflakes are shown in Fig. S1e (ESI†). Raman spectra of thin PtSe₂, multilayered BN nanoflakes, and monolayer graphene were recorded using a setup with an excitation laser (514 nm, 2.41 eV). Typically, Raman signatures of graphene (G peak at 1600 cm⁻¹ and 2D peak at 2700 cm⁻¹), PtSe₂(E_g mode at ∼200 cm⁻¹ and A_1g mode at ∼300 cm⁻¹), and BN (E_2g mode at ∼1365 cm⁻¹) were observed.

Fig. 1 (a) Schematic design of the PtSe₂/BN/Gr heterostructure on a Si/SiO₂ substrate. (b) An optical image of the device showing the formation of a lateral PtSe₂ p–n junction.

Initially, without any doping effect the (I_ds–V_ds) characteristics of the PtSe₂/BN/graphene van der Waals heterostructure were investigated thoroughly at back gate voltage (V_bg = 0 V) between probes (2, 3), as shown in Fig. 1a. As shown in Fig. S2 (ESI†), the device shows ohmic contact behavior. When the transport curve (I_ds–V_bg) of PtSe₂ was measured either between probes (1, 2) or (3, 4), it retains its original p-type conduction, as shown in Fig. S2 (ESI†). The PtSe₂ nanoflake reveals a transition from a semimetal to a semiconductor as the thickness is modulated.¹⁶ Here, p-type conduction was investigated in thin PtSe₂ nanoflakes (Fig. S3a, ESI†). In previous reports, devices based on thin PtSe₂ nanoflakes or thin films revealed various intrinsic semiconducting properties.^16,33–35 When the PtSe₂/BN/graphene heterostructure was illuminated with visible light under a negative photo-induced gate voltage (−V^light_gr) of −15 V for 5 minutes the on-current in the electron regime was strongly increased, which shows carrier type variation in the PtSe₂ nanoflake. Moreover, for the electron regime, the on-current nearly approaches a similar level to that of the hole regime, demonstrating a more symmetric and ambipolar type in the PtSe₂ nanoflake (Fig. S3b, ESI†). At a greater value of −V^light_gr (i.e., −30 V), the electron current is further enhanced, representing an electron dominant effect (Fig. S3c, ESI†). After the doping process, the (I_ds−V_bg) characteristics of PtSe₂ were measured at V_bg = 0 V between the aforementioned probes. The device shows strong diode-like rectifying behavior, further confirming carrier inversion in the PtSe₂ nanoflake (Fig. S2, ESI†). The mechanism of carrier modulation in the PtSe₂ nanoflake under −V^light_gr is similar to the doping effect in graphene/BN heterostructures.^36,37 In our case, taking advantage of −V^light_gr to graphene, the carrier type variation that occurred in the area of the PtSe₂ nanoflake partially positioned on the BN/graphene heterostructure is inverted to n-type as a result of the excitation of the mid-gap states in the BN flake, while the remaining area exhibited p-type conduction (Fig. 2a). Upon illumination, the electrons are excited from mid-gap point defects in BN into its conduction band (Fig. S4, ESI†). The photoexcited electrons enter into PtSe₂ under −V^light_gr, which was applied to graphene during the implementation of light (Fig. S4, ESI†). Fig. 2b displays a schematic diagram of the device structure of the PtSe₂/BN/Gr heterostructure under light illumination with the implementation of −V^light_gr aimed at a photo doping effect. It can be seen from Fig. 2c that after the photo-induced effect, positive defects (or charges) remain in BN after eliminating the effect of both −V^light_gr and light illumination. Fig. S5b (ESI†) shows that after the doping process the forward and backward transfer curves represent the weak hysteresis of the device, which is comparable to the pristine device because there is no trapping of charges at the surface of BN (Fig. S5a, ESI†). Moreover, this indicates that the positive charges close to the PtSe₂ channel are nearly eliminated after 5 minutes of the photo doping process, and also this further supports our conclusion that the positively charged defects are deep in the BN flake.

Fig. 2 (a) Transfer curves (I_ds–V_bg) of p-PtSe₂ and n-PtSe₂ at V_ds = 1 V. Representation of the PtSe₂/BN/Gr van der Waals heterostructure (b) under light illumination with the implementation of negative gate voltage and (c) after the photo-induced effect. (d) The (I_ds–V_bg)characteristics of PtSe₂ p–n diodes on a log scale at different V_bg. (e) The rectification ratio and ideality factor as a function of V_bg. Schematic of the energy band diagram showing the (f) n–n⁺ and (g) p–n junction states.

Many previous reports have described chemical p-type doping in MoS₂ with AuCl₃ to invert its polarity to go from n- to p-type FETs,^27,38 the electrostatic gating effect for modulation of the carrier type in WSe₂, and³⁹ elemental n-type doping in BP with Al to invert its polarity to go from p- to n-type FETs.⁴⁰ In the abovementioned approaches, stability of the treated devices over a long period in an ambient environment could not be guaranteed.^26,27,29 Through the implementation of a photo-induced doping technique, no breakdown in the transfer characteristics was observed after keeping the devices for a long retention time. PtSe₂ flakes show a steady n-type electron doping effect in air (Fig. S6a, ESI†). Moreover, the electron concentration n_(n-PtSe2) in the PtSe₂ flake can be described as follows:⁴¹

n_(n-PtSe2) = −C_g (V_bg − V_th)/e, (1)

where e is the value of charge on an electron, V_bg is the back gate value, V_th is the threshold voltage and C_g is the capacitance of the back gate per unit area (∼115 × 10⁻¹⁰ F cm⁻², for BN, C_g = ε₀ε_r/d, where ε_r = 4⁴² (value of the dielectric constant for BN), for a 51.2 nm nanoflake of BN, where the C_g is ∼6.91 × 10⁻⁸ F cm⁻², and after adding them the total capacitance is ∼9.85 × 10⁻⁹ F cm⁻²). e.g., at −40 V and for 0 days n_(n-PtSe2) was calculated to be ∼2.06 × 10¹² cm⁻². The mobility μ_(n-PtSe2) can be calculated using the following equation:^41,43Here, corresponds to the length/width of the PtSe₂ flake (2/6), represents the slope of the linear plot from the transfer curve. For example, the μ_(n-PtSe2) FET was valued to be ∼9.7 cm² V⁻¹ s⁻¹, e.g., at −40 V and for 0 days. Both n_(n-PtSe2) and μ_(n-PtSe2) as a function of retention time (in Fig. S6b, ESI†) showed almost n-type stable doping effect in PtSe₂ over a long period. Moreover, we also observed stability in the rectifying behavior of the diode in an ambient environment over a long period (Fig. S7, ESI†).

In our case, the thin PtSe₂ flakes have a bandgap of 0.21 eV and a work function of 4.62 eV.¹⁸ Since Ni has a work function of ∼5.2 eV, which is slightly deeper than the valence band maximum of PtSe₂, it makes ohmic contact to p-type PtSe₂, as shown in Fig. S8a (ESI†). Moreover, we observed the ohmic contribution of Ni contacts to n-type PtSe₂ flakes by observing the linear (I_ds–V_bg) characteristics, as shown in Fig. S8b (ESI†). According to the Schottky–Mott rule, the Schottky barrier height (Φ_B) is related to the difference between the work function of a metal and electron affinity of a semiconductor: Φ_B = Φ_metal − χ_semi. Nevertheless, some semiconductors do not comply with this rule due to the formation of metal-induced gap states,^44–46 which fill with electrons and accordingly pin the center of the bandgap closer to the Fermi level. This influence is well known as Fermi-level pinning.⁴⁷ We believe that for the above-mentioned reason the Ni/Au metals contacts may exhibit a strong ohmic contribution to PtSe₂. A further reason is that for thin 2D nanoflakes the metals electrodes diffuse into them which results in a low Schottky barrier or ohmic contribution.^48–50

The lateral PtSe₂ diode rectifying behavior was estimated at different gate bias (V_bg) from (−30 to +30 V) with a step of 10 V as shown in Fig. 2d. The PtSe₂ diode exhibits minor diode-like behavior from 0 to +30 V. As V_bg changes from −10 to −30 V, the junction current promptly declines in the reverse bias regime as compared to the forward bias. In Fig. 2e, a greater change in the rectification ratio, i.e., forward bias to reverse bias currents, at a constant V_ds value of ±2 V can be observed. The diode shows a rectification ratio of up to ∼1.0 × 10⁵ (Fig. 2e). The lateral PtSe₂ diode shows a better rectification ratio in contrast to those reported in previous studies.^28,51–55 The rectifying behavior of the PtSe₂ p–n diode initiates from the amendment of the built-in potential at the junction of p-PtSe₂ and n-PtSe₂ by variation in V_ds as a function of gate voltage. As V_bg changes from 0 to +30 V, the n−n⁺ junction state exists as a result of the movement of the Fermi level (E_F) close to the conduction band of p-PtSe₂ and n-PtSe₂ where the electrons become dominant in both of the channels, as shown in the band profile of Fig. 2f. Instead, as the V_bg changes from 0 to −30 V, the Fermi level moves near to the bandgap of p-PtSe₂ and n-PtSe₂ and consequently a p–n junction state (in Fig. 2g) occurs owing to the formation of built-in potential at the homointerface, causing a superior change in the rectification ratio. Fig. 2e shows the variation in the rectification ratio of the PtSe₂ homojunction p–n diode, which changes from ∼0.68 × 10¹ to ∼1.0 × 10⁵ when the gate voltage varies from −30 to +30 V. Furthermore, the ideality factor can be determined in the forward-biased regime using the interpretation form of the diode equation:^25,41,43

where I_D and I_S are the diode current and reverse-bias saturation current, respectively. V is the applied voltage across the diode, n is the ideality factor, q is the electronic charge, T represents the temperature, and K_B is the Boltzmann constant. The ideality factor of the PtSe₂ homojunction p–n diode changes from 1.3 to 4.2 as the back gate voltage changes from −30 to +30 V, as shown in Fig. 2e.

Under laser irradiation, the PtSe₂ homojunction p–n diode can separate the photoexcited charge carriers to advance the photonic devices. Our diode exhibits the p–n junction state under dark conditions at V_bg = 0 V. Fig. 3a shows the (I_ds–V_ds) characteristics of the attained diode in both the dark and upon the illumination of light (λ = 600 nm) with various intensities of laser light. It can be seen that a self-biased phenomenon exists upon the illumination of incident light, and a positive open-circuit voltage (V_oc = 0.32 V) is created, i.e., the voltage obtained at zero current and also a negative short-circuit current (I_sc = 16.2 nA) is produced, the current obtained at zero voltage bias, showing the distinctive self-biased photovoltaic behavior of the PtSe₂ homojunction p–n diode. Fig. 3b shows the electrical power (Pele.) as a function of the operating voltage at various intensities of laser irradiation, which is obtained from the product of I_ds and V_ds. The PtSe₂ homojunction p–n diode can operate as a photovoltaic device, i.e., it can generate a maximum electrical power of 1.87 nW at V_ds = 225 mV for 40 mW cm⁻² intensity of incident light. Using laser light with a greater intensity can generate more electron–hole pairs, causing the (I_ds–V_ds) curves to shift towards more negative values of the short circuit current, as shown in Fig. 3a, and the value of the electrical power also increases (Fig. 3b). Furthermore, the values of I_sc and V_oc are linearly reliant on the intensity of the laser light (Fig. 3c), which is comparable to previous reports.^56,57 A self-biased photoresponse of the PtSe₂ homojunction p–n diode was observed at a different intensity of incident light, as shown in Fig. 4a. It is clear that as the power of laser irradiation increased from 8 to 40 mW cm⁻² the photocurrent (I_ph = I_illumination − I_dark) improved from 8.3 to 14.4 nA. Furthermore, as shown in Fig. 4b, as the wavelength changes from 200 to 1200 nm (40 mW cm⁻²), the photocurrent decreases from 22.09 to 0.8 nA. We calculated the photoresponsivity (R) and the detectivity (d) using the following equations: R = J_p/P_in, D = R/J_d, where J_p and P_in are the photocurrent density and input power, respectively, and J_d is the dark current density. Our device shows a maximum high responsivity of 1487.9 mA W⁻¹ and a detectivity of 2.0 × 10⁹ Jones with a power intensity of 8 mW cm⁻² (λ = 600 nm) for a junction area of 112 μm². Fig. 4c shows the relationship between the responsively and detectivity as a function of the incident light intensity. The external quantum efficiency (EQE) can be found using the following equation:⁵⁸, where I_ph represents the photocurrent, P_Laser represents the laser power, λ = 600 nm represents the wavelength of light illumination, c represents the speed of light, and h represents Planck's constant.^25,43 We measured the EQE of the device at zero gate bias, which gave a value of 3068%, greater than the values previously reported.^59,60 The better photovoltaic performance of our PtSe₂ homojunction p–n diode could be due to the continuous bandaging of the p-PtSe₂ and n-PtSe₂ junctions with small-bandgap materials. Fig. 4d shows the EQE as a function of light intensity. Fig. 4e shows the relationship between the normalized photocurrent to dark current ratio (NPDR) and noise equivalent power (NEP), which can be estimated using the following equations NPDR = R/I_d, where R, I_d, and q represent the responsivity, the dark current, and electronic charge, respectively. The values of NPDR and NEP were calculated to be 1.2 × 10⁹ w⁻¹ and 0.41 × 10⁻¹² W Hz^−1/2, respectively. The NEP value reveals that the PtSe₂ p–n diode is capable of sensing power in the picowatt range, which is essential for photovoltaic devices that are used in disaster applications. The rise and fall time were estimated using the equations:⁶¹ respectively, where A and B are constants and λ₁ and (λ₂) are the rise and decay time constants. Subsequently, through the fitting of these equations, we estimated a rise time of 31 ms and a fall time of 33 ms. Fig. 5a shows an illustration of the PtSe₂ based p–n diode showing its mechanism as a switching photodiode. The maximum value of the open-circuit voltage (V_oc = 0.35 V) is achieved (as an output) directly as the incident light is switched on (as an input) and quickly disappears when the light is switched off, showing switching behavior of the PtSe₂ p–n diode state upon the switching on and off of light (40 mW cm⁻²), as shown in Fig. 5b. Besides a homojunction p–n diode, homogeneous PtSe₂ as an inverter was also achieved via carrier type variation in the PtSe₂ channel using this elegant photodoping technique. The inset of Fig. 5c shows a diagram of the PtSe₂ homoinverter in which the two channels are connected in series. Upon the illumination of light, the two FET channels can be doped selectively. One FET channel demonstrates n-type transport conduction after the use of the photoinduced doping technique, whereas the remaining one preserves its original p-type transport behavior. Three metal electrodes were deposited on PtSe₂ nanoflake consecutively to work as the V_DD, V_out, and GND, which represent the power supply, output signal, and ground, respectively. The input signal (V_in) was supplied to the graphene underneath BN, which also serves as a bottom gate electrode. The output characteristics (V_in–V_out) of the PtSe₂ homoinverter with a sweep of the input signal from −2 to +2 V are shown in Fig. 5c. To determine the performance of the PtSe₂ homoinverter, the gain was extracted from the slope of the (V_in–V_out) curve. Finally, this elegant photodoping technique was used to form homoinverters with a gain of almost ∼30 at V_DD = 2 V, as shown in Fig. 5c, which showed better performance than in previous reports. The stable doping effect in 2D PtSe₂ offers an easy way to develop NMD-based photovoltaic and complementary nano-electronic devices.

Fig. 3 (a) The (I_ds–V_bg)characteristics of PtSe₂ homojunction p–n diodes in the dark and the upon illumination of light with different intensities (λ = 600 nm). (b) The electrical power of PtSe₂ homojunction p–n diodes at different light intensities. (c) Self-biased created V_oc and I_sc as a function of light intensity.

Fig. 4 (a) Photoresponse of the p–n junction under the illumination of various intensities of light (λ = 600 nm) at V_ds = 0 V. (b) Photoresponse of the p–n junction under the illumination of different wavelengths of incident light (40 mW cm⁻²). The inset of b shows the enlarged photoresponse at 1200 nm. (c) Responsivity (mA W⁻¹) and detectivity (Jones), (d) EQE, and (e) normalized photocurrent to dark current ratio NPDR (W⁻¹) and noise equivalent power NEP (W Hz^−1/2) as a function of the intensity of light.

Fig. 5 (a) An illustration of a PtSe₂ based p–n diode showing its mechanism as a switching photodiode. (b) V_oc switching behavior in the p–n diode state upon the switching on and off of light (40 mW cm⁻²). (c) Output characteristics of p- and n-type PtSe₂ FET based inverters with extracted gains of the inverter as a function of the input voltage. The inset in c shows a diagram of a homoinverter based on PtSe₂.

4. Conclusion

In summary, novel lateral PtSe₂ p–n diodes were demonstrated based on a PtSe₂/BN/graphene van der Waals heterostructure upon the illumination of visible light via the optical excitation of mid-gap donor-like states in boron nitride. A stable photodoping effect was achieved for inverting the polarity of aPtSe₂ based FET from the p- to n-type. The PtSe₂ p–n diodes exhibit rectifying performance, with a rectification ratio of up to ∼1.0 × 10⁵ and an ideality factor of ∼1.3. The devices show good self-biased photovoltaic behavior upon laser irradiation and also show V_oc switching behavior in the p–n diode state upon the switching on and off of light. They also exhibit a broadband photoresponse over a wavelength range from 200 to 1200 nm and various photovoltaic parameters were examined. Also, p-PtSe₂ and n-PtSe₂ FETs were used to fabricate a homoinverter that reach a maximum gain of ∼30 (V_DD = 2 V). The work in this study can aid in the development of the application of 2D NMDs in future electronics and complementary electronic devices.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

This work is funded by the Higher Education Commission (HEC) of Pakistan under the National Research Program for Universities (NRPU) with project no. HEC/R&D/NRPU/2017/7876 and 5544/KPK/NRPU/R&D/HEC/2016.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc04642f

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