Review on graphene-lithium niobate integration-based acoustoelectric, photonic, and optic devices

Abstract

Graphene-lithium niobate (G-LN) integration has emerged as a promising approach for advancing acoustoelectric, photonic, and optic devices. This hybrid integration leverages graphene’s remarkable optical transparency, excellent conductivity, high carrier mobility, tunable electronic properties, and compatibility with complementary metal oxide semiconductor technology, alongside LN’s high electro-optic, acousto-optic, and nonlinear-optic coefficients, creating a highly functional platform for novel devices. This mini-review comprehensively synthesizes the state-of-the-art and recent advancements in G-LN integration, summarizing its fundamental principles and processes of practical fabrication techniques, and exploring surface acoustic waves, graphene electrodes, surface plasmon polaritons, and graphene absorbers. This mini-review of G-LN integration could underscore its significance in supporting more robust, energy-efficient, high-performance, and uniquely diverse devices, implying its potential to drive breakthroughs across multiple disciplines, as well as inspire further advancements in G-LN integration-based device design and applications.

Graphical abstract

Keywords

graphene-lithium niobate integration / acoustoelectric devices / photonic devices / optic devices

1 Introduction

Given great exceptional electrical and optical properties, graphene has verified success in appealing for photonics and optoelectronics, especially as an ideal two-dimensional (2D) material integrated into Si photonics due to its broadband photodetection, large optical modulation, high-speed operation, and complementary metal oxide semiconductor (CMOS) compatibility [1−6]. Since its discovery by Novoselov et al. [7] in 2004, graphene-a monolayer of carbon atoms in a hexagonal lattice and the first true 2D material-has attracted significant scientific interest. As a zero-band-gap semimetal with a small valence-conduction band overlap, graphene exhibits intrinsic optical transparency, low white-light absorbance, and strong surface plasmon confinement in the terahertz (THz)/infrared spectral range (IR). Additionally, graphene offers remarkable thermal and electrical conductivity, with thermal conductivity up to 5300 W·m⁻¹·K⁻¹ and electron mobility exceeding 15000 cm²·V⁻¹·s⁻¹ at room temperature. Its resistivity, approximately 10⁻⁶ Ω·cm, is lower than that of most materials used for electrodes [7−9]. These unique optoelectronic properties make graphene highly promising for applications in acoustoelectric, photonic, and optic devices.

Lithium niobate (LiNbO₃, LN), also known as “silicon of optical materials”, is a purely artificial inorganic crystal with a wide transparency window, large electro-optic (EO), thermo-optic (TO), and acousto-optic (AO) coefficients, as well as strong nonlinear optical properties and stable chemical and physical characteristics. LN’s unparalleled properties have made it essential in high-performance photonic and optic devices [10−13]. As a hexagonal ferroelectric crystal with one of the highest spontaneous polarizations, LN is birefringent, with ordinary and extraordinary refractive indices (n_o = 2.211 and n_e = 2.138) at the 1550 nm telecommunication wavelength. It features a transparency range spanning from visible (400 nm) to mid-infrared (5 μm) wavelengths. Both bulk and thin-film LN(TFLN) platforms have been widely adopted in acoustoelectric, photonic, and optic devices, with TFLN emerging as a crucial platform for next-generation photonic integrated circuits (PICs). You et al. [14] gave their comprehensive review work about LN on insulators concerning the fundamental theory and various optic devices. Additionally, LN’s unique piezoelectric properties, first observed by Nassau et al. [15] at Bell Labs, have garnered attention in both theoretical and experimental research.

Dating back to 2010, researchers have explored G-LN integration, motivated by the potential for advanced study on surface acoustic waves (SAWs/SAW) response to ambient humidity. That is, to the best of our knowledge, the first G-LN integration-based device as an ambient humidity sensor was demonstrated and analyzed in experiments [16]. The G-LN integration-based device demonstrated carrier modulation in graphene through LN’s ferroelectric polarization, indicating potential applications in non-volatile memory, field-effect transistors (FETs/FET), flexible transparent electronics, sensors, and other devices with stable hysteresis [18−22].

Research topics of G-LN integration studied over the past years are almost all summarized here. This mini-review synthesizes recent developments in G-LN integration, beginning with its fundamental principles and fabrication processes. It is noted that the principles of SAWs will be elaborated in detail next in its dedicated section. The main content is structured to explore key topics-SAWs, graphene electrodes, surface plasmon polaritons (SPPs), graphene absorbers, and other devices-each with a dedicated section. A schematic diagram of G-LN integration is shown in Fig.1. In closing, we will give conclusions with our outlook and perspective shedding light on current challenges and potential opportunities in developing G-LN integration-based acoustoelectric, photonic, and optical devices, intending to inspire further advances in the field.

Fig.1 Schematic diagram of G-LN integration.

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2 Principles and processes

The fundamental principles underlying G-LN integration involve the interaction between graphene and LN, primarily through the modulation of graphene’s charge carrier density by the electrostatic field of the intrinsic or polarized LN substrate. Graphene’s electronic and optical properties can be tuned through charge carrier density adjustments via chemical, electrical, optical, and mechanical means. Carrier density values are derived from the Fermi level, where frequency-dependent conductivity is calculated, and DC conductivity is obtained by multiplying by a factor of (1−iωτ). The carrier density can then be determined by dividing DC conductivity by the electron charge and mobility [23]. As a zero-band-gap material, graphene’s unique properties are largely tied to its linear energy dispersion near the Dirac point. For practical applications, however, a non-zero-band-gap is often required to control carrier density. This can be achieved by interfacing graphene with ferroelectric substrates, which induces a non-zero-band-gap, reduces charged impurity scattering, and enhances carrier mobility. Ferroelectric substrates can also control carrier density and induce either p-type or n-type behavior due to hysteresis, manifesting in either normal hysteresis or antihysteresis effects. From this point of view, LN, including TFLN, is emerging as an ideal ferroelectric substrate for G-LN integration studies, not only given the rising study focusing on TFLN photonic devices in PICs.

Theoretical and experimental methods delve into and reveal the fundamental principles of G-LN integration. In 2013, Ding et al. [24] used first-principles calculations to investigate the interface interaction between graphene and the ferroelectric LN (0001) surface, finding that spontaneous polarization induced a band gap of approximately 311 meV. Subsequent studies by Ding et al. [25] used density functional theory (DFT) simulations to explore the structural and electronic properties of graphene on O-terminated LN (0001), revealing covalent bonding between graphene and LN. Baeumer et al. [26] demonstrated carrier density modulation in graphene by coupling it with adjacent ferroelectric polarization, creating spatially defined potential steps at 180° domain walls of periodically poled LN. Their first-principles DFT calculations indicated that graphene on the up-polarized (P+) surface gained electron density, while the down-polarized (P−) surface showed hole-doping, resulting in a carrier density difference of 1.35 × 10¹³ cm⁻² [Fig.2(a)]. Bidmeshkipour et al. [27] examined the screening effect of LN polarization on carrier mobility in top-gated graphene FETs, finding that LN polarization could reduce Coulomb scattering from charged impurities, significantly enhancing mobility below 1000 cm²·V⁻¹·s⁻¹. Additional studies by Salas et al. [28] and Richman et al. [29] further analyzed interface characteristics using DFT, with Salas focusing on reflectivity and absorption, while Richman examined lattice relaxation at the G-LN interface. More recently, Liu et al. [30], Yuan et al. [31], and Yue et al. [32] all used first-principles calculations to explore interfacial coupling and electrostatic doping, investigating phenomena such as band alignment, electrostatic doping effects, and tunable electronic structures in monolayer, bilayer, and magic-angle of graphene composited with LN.

Fig.2 Representative demonstration of principles. (a) Ferroelectrically driven spatial carrier density modulation in graphene [26]. (b) Nonvolatile all-optical control of the charge transport properties of graphene [40, 41]. (c) Coupling of graphene surface plasmons with LN waveguide [43, 44].

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Raman spectroscopy has proven invaluable in characterizing graphene’s properties, enabling analysis of layer number, orientation, edge quality, and response to external perturbations [33]. Papasimakis et al. [34] demonstrated that Raman spectra of G-LN integration structures reveal localized strain effects under continuous laser irradiation, with strain relaxation linked to electrostatic forces generated by charge accumulation in illuminated areas. Due to LN’s strong pyroelectric properties, increasing temperature causes charge accumulation at the surface. Sun et al. [35] studied interfacial stress states in G-LN integration systems under SAW coupling at critical temperatures, identifying tensile, stress-free, and compressive regimes relevant to integration processes. Fandan et al. [36] used Raman spectroscopy to observe phonon modulation in graphene via SAWs, revealing up to 15% intensity variation and phonon frequency shifts of 10 cm⁻¹ in both first- and second-order Raman modes. Sun et al. [37] further examined the thermodynamic properties of few-layer graphene on LN using SAW measurements, revealing interlayer lattice deformation and stick-slip friction at the G-LN interface. Irzhak et al. [38] investigated the effects of electric voltage on compression and stretching strain in graphene using Raman spectroscopy, observing phenomena such as graphene coating peeling. Drichko et al. [39] explored low-temperature electrical properties of monolayer graphene on LN, finding that variations in electrical conductivity, carrier mobility, and density were closely linked to SAW absorption and velocity changes.

Additionally, Gorecki et al. [40, 41] experimentally demonstrated the nonvolatile all-optical control of charge transport properties in graphene, whereby optically induced charge migration in LN locally modified graphene’s electrostatic environment, enabling spatial control over its resistivity. These charge distributions remain nonvolatile in the dark for years but can be reversed through uniform illumination or thermal annealing [Fig.2(b)]. Park et al. [42] investigated charge transport in graphene on periodically poled LN substrates using a dual-gate graphene transistor as a test platform. In this setup, a back-gate voltage clarified the effect of polarization on graphene’s electrical behavior. Liu et al. [43, 44] conducted experimental studies on the optical waveguide characteristics of G-LN integration in air, measuring LN waveguide refractivity and reflectivity through the prism coupling method. A significant increase in refractivity at 1540 nm was observed due to the generation of surface plasmons (SPs). For further research, they explored plasmonic optical absorption enhancement using attenuated total reflection measurements and simulations. Resonant absorption was attributed to the coupling of graphene SPs with optical modes in the LN-SiO₂ Fabry−Perot cavity and LN planar waveguide [Fig.2(c)]. They further improved optical wave coupling efficiency by introducing space light coupling into the TFLN waveguide via a prism. Theoretical results confirmed carrier accumulation on the graphene surface due to TFLN’s spontaneous polarization. Notably, an air gap of less than 0.25 nm exists between graphene and the TFLN surface due to van der Waals forces, which minimizes graphene absorption loss in TFLN waveguide propagation mode, underscoring graphene’s feasibility as an electrode [45].

To date, two primary steps typically involved in the G-LN integration processes are most common. Exceptionally, the chemical reduction method of reducing graphene oxide into graphene is just used and mentioned in Ref. [16]. The first step is growing graphene on copper foil via chemical vapor deposition (CVD), and the second step is transferring the graphene to an LN substrate using the poly(methyl methacrylate) (PMMA) transfer technique. This method releases graphene from the copper and positions it on the LN substrate, and it has been widely verified in reported studies [46].

The PMMA transfer technique is critical for the final device performance, with specific procedural examples illustrated in Fig.3. The processes are reported by Miseikis, a 5% PMMA solution in anisole is drop-cast onto 20 μm-thick copper foil and then baked for one minute at 180°C. The copper is subsequently etched away with a 0.0124 mol/L ferric nitrate solution, leaving graphene attached to the PMMA film, which floats on the surface of the etch solution. After rinsing in deionized water, the film is transferred onto the LN substrate. The PMMA is then dissolved by adding a drop of PMMA-in-anisole solution and finally removed in acetone [17]. In the procedure by Bidmeshkipour, monolayer graphene is synthesized by CVD in a low-pressure system (Black Magic, AIXTRON Nanoinstruments Ltd) on copper foil. The copper is first coated with a 4% PMMA solution in anisole and then baked for one minute at 170°C. Afterward, the copper is etched using a 0.2 M ammonium persulfate solution, and the graphene attached to the PMMA film floats on the etch solution’s surface. This stack is rinsed in deionized water, transferred onto the LN substrate, and the PMMA is removed in acetone [47]. In another approach reported by Liu et al. [48], a G-LN structure is fabricated by wet-transferring CVD-grown monolayer graphene onto TFLN (300 nm or 600 nm) on a SiO₂/Si(2 μm/500 μm) substrate. A 9 mm × 9 mm graphene sheet is released in deionized water and transferred onto a polished x-cut LN surface measuring 10 mm × 10 mm. After air drying for 20 minutes, samples are oven-dried at 70°C for 30 minutes and then soaked in acetone to completely remove the PMMA layer, forming the G-LN structure [Fig.3(a)]. However, in a lab, the PMMA transfer methods are commonly manual and require significant handling skills and time in above mentioned. Bosca et al. [49] demonstrated an automated system for transferring CVD graphene to target substrates. This automated method offers advantages, such as requiring no handling skills, saving time, ensuring reproducibility, and enabling large throughput and parallel processing. It has achieved 70% higher mobility, a 30% reduction in unintentional doping, and a 10% reduction in strain. This technique has been developed for laboratory-scale transfer and is scalable for industrial applications [Fig.3(b)].

Fig.3 Representative demonstration of processes. (a) A manual procedure of PMMA-transfer technique [48]. (b) An automated procedure of PMMA-transfer technique [49]. (c) Direct graphene synthesis on LN surface [50−54].

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In addition, a research group has recently focused on direct graphene synthesis processes on LN. In 2019, Liu et al. [50, 51] first demonstrated the synthesis of turbostratic graphene on LN via direct carbon ion implantation and investigated the thermal motion mechanisms during the annealing process. Carbon ions were implanted into LN at 30 keV with fluences of (2−10) × 10¹⁵ cm⁻², followed by annealing at 500−700°C to form an sp²-bonded hexagonal carbon structure. Turbostratic graphene was observed only in select regions on the LN surface [Fig.3(c)]. In another approach, Ni and Cu metal films were employed as catalysts to grow large areas of two-dimensional graphene on the LN surface. The annealing process was simulated using molecular dynamics to study thermodynamic behavior. Carbon ions were implanted into the Ni film at 80 keV with fluences of 1.5 × 10¹⁶ cm⁻², resulting in multilayer graphene formation after the Ni film was removed [Fig.3(c)] [52−54]. However, graphene synthesized directly on LN is not yet available for functional devices in practical applications.

3 Surface acoustic waves (SAWs)

SAWs much like liquid waves are elastic waves of low-frequency acoustic phonons confined and propagating on the surface of a solid. Since SAWs were successfully excited directly on the surface of piezoelectric crystals through interdigital transducers (IDTs) invented by Santos et al. [55], SAW devices have been widely used in communication, signal processing, quantum, and sensing areas. IDTs are a grating pattern whose fingers are alternatingly connected to a radio frequency (RF) signal voltage and the electric ground. The grating period determines the frequency of IDTs that mutually match the frequency of the RF signal. Then the inverse piezoelectric effect transforms a fraction of the electromagnetic energy applied to IDTs into SAWs [55]. Graphene is a good material choice for acoustoelectric (AE) amplification because it has high current capacity, electron mobility, and minimal physical dampening of the acoustic waves, which acts as the charge-carrying medium to the applied directly to the surface of LN. Herein, LN acts as a piezoelectric crystal substrate with a good piezoelectric parameter of d₁₅ = 74 pC/N. Moreover, LN, a 128° YX LN wafer, is readily chosen due to its high SAWs coupling coefficient k² = 0.056 for good AE coupling and large SAWs free-surface velocity v_SAWs = 3980 m/s for realizable photolithography resolution.

Due to the AE effect, the attenuation of SAWs that is the kinetic energy of acoustic wave transferring into charge carriers induces an electric current in graphene. Miseikis et al. [17] proposed their SAW device of G-LN integration and demonstrated AE current in graphene. Graphene directly deposited by CVD is transferred onto the surface of the LN substrate, and it directly integrates with IDTs used for SAW generation and detection. It resolved the problems of masking any AE current flow because the graphene film was processed before rectifying the RF signal. When an RF signal is applied to the interdigital transducers at the resonant frequency, a direct current flow is generated in graphene by the transport of p-type charge carriers, scales linearly with the applied RF signal power, and changes to a counter-flow current applied by a bias, which means the charge carriers in graphene can be controlled. Contrary to SAW generation, IDTs can also convert SAWs back into RF voltage, so SAW delay lines consisting of a pair of IDTs with the same grating periodicity are used as filters of RF signals. From the viewpoint of efficiency, IDTs made of graphene with exceptional lightness could be an advanced choice since minimized mass-loading effects. In the same research group, Mayorov et al. [56] verified the feasibility of using graphene as a conductive electrode for the generation and detection of SAWs. The insertion loss of −45 dB was obtained by a pair of IDTs made of graphene which was 17 dB lower than that of Cr/Au IDTs reported previously [Fig.4(a)]. Bandhu et al. [57] demonstrated the amplitude of the AE current as a linear function of both SAW intensity and frequency, as well as proportional to the attenuation of the acoustic wave caused by the charge transport. In addition, the commercially available CVD graphene grown on copper is transferred onto a 128° YX LN delay line device and the macroscopic acoustoelectric transport in graphene with a large electrode gap of up to 500 nm [Fig.4(b)]. For advanced study, they demonstrated a top gate voltage control of SAWs in the field-effect structure using a high capacitance ion-gel dielectric solution. A 0.1% velocity shift is achieved for a bias of approximately 1V by modulating the conductivity of the graphene. The charge carriers being trapped and transported in the graphene are also verified and the AE current can be reversed or switched off by using the gate voltage [Fig.4(b)] [58]. Tang et al. [59] investigated the AE properties of graphene via ionic liquid gating and revealed the AE attenuation as a function of the carrier density. The non-monotonic dependence of the AE current on the conductivity of graphene electron gas is predicted by the classical relaxation model. So to reveal the mechanism, the graphene carrier density has been modulated by optical and chemical means in experiments at the very beginning. Poole et al. [60] measured the AE photoresponse in graphene. The AE current increases with illumination, more than the measured conductivity changes of the graphene, whilst retaining a linear dependence on SAW intensity, which indicates mainly the effect of the illumination on the electronic properties of the graphene rather than the carrier density of determining the conductivity. Zheng et al. [61] further verified by modulating carrier density using a chemical doped graphene that is emerging into two different gases of NO₂ and NH₃ respectively. The experimental results agree with the classical relaxation model. The AE current initially increases proportionally with the increase of conductivity σ, and reaches a maximum when σ = σ_M, after that it decreases. Herein, σ_M is the character conductivity depending on the surrounding medium. A higher NO₂ concentration makes a higher carrier density meaning larger conductivity, so the conventional direct current becomes larger. However, the AE current becomes smaller since the conductivity is larger than σ_M. Surprisingly, the relative response changes of the AE current are much higher than the common drift current, which indicates its potential applications in chemical detections [Fig.4(c)] [61]. From a point of view, an ultrahigh working frequency should have been needed for the potential applications. In recent, Costanza et al. [62] designed SAW delay lines of G-LN integration at an operational frequency of 2.5 GHz. The AE current measured in the monolayer graphene gave a higher AE interaction in the GHz range than previously reported values with carrier absorption loss of 109 m⁻¹ and mobility of 101 cm²·V⁻¹·s⁻¹.

Fig.4 Representative demonstration of SAWs. (a) AE current flow and IDTs of graphene generating SAWs [17, 56]. (b) AE charge transport and controlling the properties of SAWs [57, 58]. (c) Acoustic charge transport in chemical doped graphene [61]. (d) SAW amplification with applied DC voltages [63−65]. (e) SAW propagation using SEM [69]. (f) A good performance of SAW resonators [74]. (g) A novel on-chip monolithic acoustic graphene transistor [75].

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Graphene charge carriers interacting with SAWs can also amplify SAWs. Reasonable high carrier mobility can amplify their drift velocity. If charge carriers in an electric field move with a drift velocity v_d in the same direction as SAWs exceeding v_SAWs, the carrier will transfer part of their kinetic energy to SAWs. Carmichael et al. [63] experimentally presented the narrowband SAW frequency responses at a center frequency of 1350 MHz with varying applied DC voltages on graphene, and a delay line asymmetry of 3 dB is given by the ratio of the forward versus reverse peak SAW response. For advanced research from the same group, Malocha et al. [64, 65] designed an embodiment of a monolithic coupled acoustoelectric amplifier (CAEA) with a 1.2 dB terminal gain using 138 mW DC power in continuous wave. It provides the direct coupling between the SAWs and the free carriers in graphene [Fig.4(d)] [63−65]. Then, Malocha [66] developed a novel theoretical and modeling approach for CAEA where the graphene overlays LN within or outside the path of SAWs. In comparison to previously published experimental results, it shows good agreement of yielding CW operation and a net terminal gain. Tynyshtykbaev et al. [67] analyzed the amplification of SAWs due to the appearance of an additional photoinduced AE current and a decrease of collisional scattering of photoinduced electrons in graphene. In addition, SAWs can also modulate the energy dispersion of electronic excitations in graphene. A kind of superlattice will be realized in graphene as SAWs moving typically three orders of magnitude slower than the graphene charge carriers. Kononenlo et al. [68] experimentally studied the photoresponse in multilayer graphene on LN under the conditions of SAWs. When the light irradiates the graphene, its AE current increases or decreases depending on the polarity of the potential applied to the graphene. Visualization of SAW propagation was performed using a scanning electron microscope (SEM), where SAWs made a periodic charge lattice occur in graphene that enhanced the mutual response between light and graphene. Roshchupkin et al. [69] experimentally studied SAW propagation using SEM too, in which the Fresnel diffraction pattern can be observed associating with SAW diffraction. Moreover, SAWs allow the collection of photo-stimulated charges and transfer them in graphene which will significantly improve the efficiency of solar cells in the future [Fig.4(e)].

Some unique SAWs-based devices of G-LN integration have been proposed for various applications based on the absorption and velocity of SAWs depending on graphene sensing to the mediums around it. Čiplys et al. [71] reported the SAWs, both in velocity and amplitude, responding to the ambient humidity. Rimeika et al. [16, 70, 71] from the same group monitored the fast changes in graphene dioxide properties by SAWs for sensing humidity. Guo et al. [72] demonstrated a relative humidity sensing device based on SAWs using graphene on 128° YX LN. It reveals the interaction mechanism between moisture and graphene, where the two-stage response is a low relative humidity range due to the absorption of water molecules and a high relative humidity due to the discrete moisture layer. Kuznetsova et al. [73] experimentally measured the changes in density and elastic modulus of graphene oxide film at room temperature due to water vapor absorption, which indicates the main principle in the electric conductivity of graphene. Furthermore, the sensor is also completely selective towards different gases. Zhou et al. [74] demonstrated a good performance of SAW resonators with massless IDTs of graphene for human breathing monitoring. A good performance of the device is finally investigated with 4-layer graphene and 80 pairs of IDT fingers [Fig.4(f)]. In addition, Liang et al. [75] demonstrated a novel on-chip monolithic acoustic graphene transistor on LN, which is a paradigm for more-than-Moore technology. A tunable SAW graphene transistor with a larger on-off ratio is realized by using LN [Fig.4(g)]. Furthermore, in the research direction of quantum oscillations of two-dimensional electron systems and topological materials, Mou et al. [76] carried out AE transport in a dual-gated AE device of G-LN integration at gate voltages and magnetic fields where the quantum oscillations can be detected. The AE transport rarely explored could serve as an important complement to electric and thermoelectric transport. Especially PMMA-transfer technique greatly enhances the accessibility of AE transport in the research of van der Waals materials [76].

A comparison of various SAWs-based devices based on G-LN integration is summarized in Tab.1. Among these, 128° YX LN stands out as a highly suitable choice for SAW devices due to its high coupling coefficient k², approximately 5.6%. Although the corresponding and experimentally measured values K² which depend on the specific types of acoustic waves, show higher values and significant variation across references. To improve performance, graphene IDTs are designed to be virtually massless, reducing mass loading effects. These devices typically operate in the MHz frequency range. Additionally, the interaction between SAW velocity and atmospheric media significantly impacts attenuation, making SAW devices particularly sensitive to environmental conditions. This sensitivity is especially advantageous for humidity sensing applications, where SAW sensors demonstrate high sensitivity. Furthermore, SAW gas sensors incorporating graphene oxide films have been experimentally validated for detecting gases such as H₂, CO, CH₄, NO, and O₂, suggesting promising directions for further research and development [73].

Tab.1 Comparison of different SAWs-based devices based on G-LN integration.

Device type	LN type	IDTs	Frequency (MHz)	K2 (%)a	Sensitivity	Ref.
Humidity sensor	YZ LN	Al	108	Null	0.5°/1%RH@10%−80%	[16]
Humidity sensor	YZ LN	Al	287	Null	3.4°/1%RH@RH<50%	[70]
Humidity sensor	YZ LN	Al	86.6	4.8	0.1°/1%RH@30%−80%	[71]
Humidity sensor	128° YX LN	Al	248.5	Null	1.38 kHz/1%RH@RH<50%;2.6 kHz/1%RH@RH>50%	[72]
Humidity sensor	128° YX LN	Al	23.06	10.3	0.4°/1%RH@RH<50%	[73]
Human breathing	128° YX LN	Graphene	154	3.78	Null	[74]
Transistor	TFLN	Au	287	11.5	Null	[75]

Note: a K2 is the experimentally measured electromechanical coupling coefficient of SAWs, and herein 10.3 for Lamb waves and 11.5 for Rayleigh waves.

In conclusion, the fundamental principles of SAWs concerning the interaction of graphene electronic excitations and SAWs in G-LN integration are primarily illustrated through the modulation of charge carriers in various ways. The generation mechanisms of AE currents are comprehensively reviewed, and the effects of attenuation and amplification on SAW propagation velocity due to the AE effect, when applying a DC voltage, are discussed in detail. Additionally, the periodic elastic deformations induced by SAWs can function as diffraction grating, thereby enhancing the efficient coupling of light and plasmons in graphene. Finally, the discussion concludes with an overview of unique SAWs-based devices and their potential applications in practical scenarios.

4 Graphene electrodes

In electronic devices, graphene serves as an exceptional electrode material due to its low sheet resistance, high optical transparency, and superior mechanical properties. These attributes have led to the widespread adoption of graphene electrodes across diverse applications, including transistors, memory devices, molecular junctions, touch screens, LCDs, LEDs, and solar cells. As a next-generation conductive medium, graphene holds significant promise for advancing the performance and integration of electrodes in electronic devices [77, 78].

According to the unique findings of graphene electrodes of waveguide-based LN EO devices, that is, graphene strongly attenuates the propagating optical wave whose major electric field is parallel to its surface [79]. Usually, for the x-cut LN platform and graphene overriding on its surface, the transverse electric (TE) field is attenuated significantly, while the transverse magnetic (TM) field has little loss. Because graphene, as a single layer of carbon atoms, allows no electric current to flow in the normal direction, and there are no dissipations of TM. Moreover, a monolayer graphene is simply and easily patterned with the fabrication process. In practice, graphene is probably used as electrodes in almost all devices if being characterized by electrical modulation. Chang et al. [80] demonstrated a mode converter of G-LN integration EO devices. Compared to using traditional Al metal electrodes, the half-π voltage is reduced by almost three times with graphene electrodes, as shown in Tab.2. A monolayer graphene is directly placed on the surface of the LN waveguide that is formed by the proton-exchange process, and the buffer layer is avoided at the same level of a low optical propagation loss which makes the fabrication process much simpler [Fig.5(a)] [80]. For advanced research from the same group, Jin et al. [81] proposed a complete reconfigurable EO three-mode converter based on the cascaded long-period grating of graphene in theory and experiment [Fig.5(b)], and the advanced characteristic parameters are shown in Tab.2. Graphene has exceptional chemical, mechanical, and thermal stability, as well as high electrical conductivity and low reactivity. Chaudhary et al. [82] demonstrated a low-voltage domain-wall LN memristor with a top graphene electrode, and even more layers of graphene are used to ensure their stability at a high-voltage domain-wall writing condition, avoiding burned or delaminated by high-density currents, which proves that graphene electrodes are more robust than legacy metal or oxide electrodes. Graphene electrode evolves realistic functional memristive devices on a low-voltage Operation [Fig.5(c)] [82].

Tab.2 Devices of different graphene electrodes based on G-LN integration.

Device type	LN type	Length (mm)	Voltage (V)	Loss (dB/cm)	Rolea	Ref.
Mode converter	Bulk LN	15	Vπ/2 = 24	0.3	Grating	[80]
Mode converter	Bulk LN	26	Vπ/2 = 37	0.8	Grating	[81]
Memristor	Bulk LN	Null	33	Null	Capacitor plate	[82]
EO modulator	TFLN	1.5	Vπ = 0.2	4.58	Null	[86]
EO modulator	TFLN	1.14	Vπ = 0.49	Null	Null	[87]
EO modulator	TFLN	Null	Vπ = 1.12 V·mm/Lb	0.88	Heater	[88]

Note: aGraphene also acts as another role; bL represents “Length”.

Fig.5 Representative demonstration of graphene electrode. (a) Graphene electrodes for LN electro-optic devices [80]. (b) Reconfigurable three-mode converter based on cascaded EO long-period gratings [81]. (c) Low-voltage domain-wall LN memristors [82]. (d) G-LN integration-based phase modulators [86]. (e) Scalable and reconfigurable continuously tunable TFLN delay line using graphene electrodes [88].

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In recent years, the waveguide-based TFLN EO modulator has gained recognition as a promising candidate for high-performance modulation, enabling the conversion of RF electronic signals into the optical domain. Chip-scale, high-performance EO modulators are essential for applications in modern telecommunication networks and microwave photonic systems, including PICs, and should operate at voltages compatible with CMOS technology [83−85]. Mao et al. [86] presented two types of graphene electrode designs for Si₃N₄ and TFLN hybrid waveguide-based EO modulators. The electrodes are vertically arranged of type I and laterally arranged of type II configuring for z-cut and x-cut TFLN respectively. As for type I, the relative position of TFLN waveguides and graphene edge is a key parameter for the propagation losses of TFLN waveguides, which is studied by simulation methods and the lowest loss of the simulation results is about 4.58 dB/cm, as shown in Tab.2. Because the plasma resonance induces a coupling effect between guiding modes in TFLN waveguides and plasmonic modes in graphene, additional propagation loss is caused. The interval between the left electrode and right electrode in type II is the related parameter for the propagation loss, and the larger the interval, the lower the propagation loss. What’s more, the EO integral factors indicating the EO modulation efficiency are also calculated respectively with each corresponding key parameter. As we know, there is a trade-off between the propagation loss and the EO integral factor [Fig.5(d)] [86]. Zhan et al. [87] designed a dual-waveguide stacked graphene EO modulator where the graphene electrode was assumedly positioned between two waveguide arms of the Mach-Zedner interferometer, which is theoretically calculated and discussed. The performance characterized by the half-wave voltage−length product (V_πL) of 0.55 V·cm showing modulation efficiency and the bandwidth of 58.8 GHz describing the operation speed is verified using the finite element method, as shown in Tab.2 [87]. Additionally, we can mutually exploit the large TO effect of the TFLN and the superior thermal conductivity of graphene. Song proposed a novel continuously tunable delay line in x-cut TFLN with the graphene electrodes, and the TFLN TO switch is the critical component of the device. It is noteworthy that the DC bias point in devices utilizing the TO effect demonstrates significantly greater stability compared to those based on the EO effect, thereby enhancing device operational stability. This stability is achieved alongside a reduced V_πL of 1.12 V·cm, with a low switching power requirement of 2.85 mW, as shown in Tab.2. Calculations indicate a trade-off between propagation loss and V_πL, with propagation loss varying according to the distance between the electrodes [Fig.5(e)] [88].

In conclusion, graphene electrodes offer notable advantages, including high optical transparency, mechanical flexibility, and tunable electronic properties, making them well-suited for advanced photonic devices within G-LN integration. As shown in Tab.2, devices of different graphene electrodes based on G-LN integration were achieved before. Surely, these benefits are accompanied by challenges such as environmental sensitivity and stability issues. The choice between graphene and traditional metal electrodes ultimately hinges on the specific application requirements, including operational speed, power efficiency, device integration compatibility, and environmental robustness. It is worth noting that, graphene electrodes have huge technical potential applications in components of PICs, such as EO/TO modulators. As research in graphene fabrication and integration techniques advances, ongoing improvements may address current limitations, further solidifying graphene’s role as a leading electrode material.

5 Surface plasmon polaritons (SPPs)

SPPs represent oscillations of free electrons localized at the interface between materials with contrasting permittivities. These oscillations enable studies of subwavelength optical field confinement, offering a promising approach to surpassing the diffraction limit [89]. The ability to control and manipulate light at subwavelength scales using SPPs has significant implications for advancing photonic devices with applications in environmental sensing, energy harvesting, biological imaging, and medical diagnostics [90]. Graphene, with its strong binding affinity for SPPs, supports relatively long propagation lengths, especially at high Fermi levels where propagation losses become negligible due to reduced electron scattering [91]. Notably, graphene’s optical nonlinearity, high quantum efficiency, and plasmonic excitation properties enhance light-matter interactions, which are essential in advanced SPPs applications. The polar discontinuity at the G-LN interface prompts surface atom reconstruction and facilitates charge carrier transfer to balance the spontaneous polarization of LN. This process induces depolarization and the accumulation of a two-dimensional electron gas at the nanoscale, effectively enabling the excitation of SPPs. Early studies by Wang et al. [92] theoretically explored a ferroelectric domain-controlled SPPs modulator of G-LN integration. They investigated the electronic and optical modulation capabilities of graphene by analyzing both interband and intraband contributions to surface conductivity, as well as SPPs propagation in the transverse magnetic mode. This modulation was achieved by positioning monolayer graphene in either a polarization or depolarization domain with variations in the polarization levels of LN [Fig.6(a)] [92].

Fig.6 Representative demonstration of SPPs. (a) Graphene-based SPPs tuned by ferroelectric domains in LN [92]. (b) THz optics with optically reconfigurable metasurface of G-LN integration [94]. (c) SPPs excitation in n-type doping of monolayer graphene with LN [96]. (d) Graphene-enhanced coupling with LN waveguide [48].

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The coupling and conversion of THz technology based on SPPs of G-LN integration have shown significant promise for subwavelength optical applications, driving considerable research efforts. Qasymeh [93] proposed a novel structure of a graphene parallel plate filled with LN waveguide, for phase-matched coupling and frequency conversion of THz waves based on nonlinear wave interaction. The principle relies on modulating the effective permittivity of the LN waveguide by adjusting the conductivity of graphene via induced nonlinear THz polarization, and then the perturbation theory is employed to model the modulations in conductivity and permittivity. Gorecki et al. [94] demonstrated a reconfigurable THz optic device featuring nonvolatile optical control of THz metasurfaces. These metasurfaces, composed of a metallic split-ring resonator array between monolayer graphene and an LN substrate, exhibit THz frequency-selective transmission tuning performance. The nonvolatile yet reversible photoinduced charge distributions in the LN substrate can locally modify the electrostatic environment of graphene, thereby adjusting its electrical conductivity and altering the resonance spectra of the metasurface [Fig.6(b)] [94]. In subsequent, an iron-doped LN substrate was employed to achieve pattern-free optically reconfigurable graphene metasurfaces instead of the metallic split-ring resonator array in the THz spectral region. Here, LN undergoes charge migration upon structured illumination, with carriers migrating away from illuminated areas to reform spatial charge patterns, enabling electrostatic tuning of the graphene layer [95].

As we know, the photorefractive effect in doped LN enables the formation of phase gratings that facilitate the efficient excitation of SPPs within the evanescent field. Xiong et al. [96] conducted the first experimental investigation of SPPs excitation in G-LN integration by monitoring initial reflections, which were confirmed by observing double-scattering features and a distinct dip in the transmission spectra. Here, the spontaneous polarization of LN induces electron transfer to graphene, resulting in n-type doping of graphene with an electron density of 2.51 × 10¹⁴ cm⁻² [Fig.6(c)] [96]. The subsequent formation of gratings via the LN’s photorefractive effect enabled efficient SPPs excitationally, graphene-covered LN waveguides exhibit strong coupling to near-surface waves under infrared light excitation, even when x-cut LN lacks polarization. Liu et al. [48] identified surface atomic reconfiguration and potential difference at the G-LN interface using Raman spectroscopy, which demonstrated graphene-enhanced coupling of near-surface electromagnetic waves with LN waveguide modes in the infrared range. This enhanced coupling, largely due to SPPs excitation, paves the way for advanced G-LN integration-based devices, including beam splitters and directional couplers [Fig.6(d)] [48]. Sadaghiani et al. [97, 98] modeled two types of waveguides in G-LN integration structure for studying nonlinear effects, such as second harmonic generation and SPPs. Furthermore, SPPs-based sensors have also gained significant attention due to their high sensitivity. Irfan et al. [99] introduced a novel refractive index and temperature sensor leveraging SPPs in G-LN integration, which achieved a numerical sensitivity of 981 nm/RIU in the RI range of 1.33 to 1.40 and a temperature sensitivity of −0.23 nm/°C in the temperature range of 10 °C to 70 °C, with a figure-of-merit of 61.31 RIU⁻¹.

In summary, SPPs propagation in G-LN integration-based devices can be precisely controlled by modulating carrier doping in graphene. By adjusting the intrinsic plasmon resonance, SPPs propagation can be robustly tuned within the polarized LN domain. As interest in nonlinear plasmonic applications grows, a range of innovative G-LN integration-based devices is expected to emerge rapidly. Moreover, the photorefractive effect in doped LN enables the formation of phase gratings that facilitate the efficient excitation of SPPs within the evanescent field. Finally, optic devices based on SPPs enhancing light coupling among LN waveguides and SPPs-based sensors should inspire further advancements.

6 Graphene absorbers

Perfect absorbers, which nearly absorb entire incident light, are of great importance in applications of optic devices such as photodetectors, optical modulators, and sensors. In recent years, graphene-based absorbers have attracted an amount of interest and have been theoretically studied as optical filters. Surely, monolayer graphene alone exhibits a limited absorption of only 2.3% of incident light, despite its broad spectral range from UV to THz [100]. To enhance graphene absorption, two primary G-LN integration structural types have been developed, both demonstrating near-perfect narrow-band absorption. One type involves a monolayer graphene sandwiched between two mirrors of photonic crystals (PhCs) with optimized pair numbers, resulting in reflective filter spectra. The other type employs a resonant grating with optimized parameters adhered to the graphene layer, producing transmission filter spectra.

Rao et al. [101] proposed a composite structure (HL)^MGD(LH)^N of one-dimensional PhCs with a graphene-based defect, integrating LN as the defect layer with an external voltage. They investigated various parameters influencing absorption, such as the angle of incidence, the thickness of the spacer layer, and the period number of PhCs. They achieved and optimized near-perfect absorption at an appropriate incident angle for TE polarization. The resonant wavelength exhibited a linear redshift with increasing external voltage, and multi-resonant peaks were obtained by adjusting the thickness of the LN layer [Fig.7(a)] [101]. Rashidi et al. [102] provided theoretical results showing that nearly perfect absorption can be achieved by adjusting the period numbers of the PhCs, with two typical structures of (LGHG)^MD(GLGH)^N and (LGHG)^MD(GHGL)^N. Similarly, the peak wavelength exhibited a linear relationship with the applied external voltage but a blueshift with increasing voltage, and multi-peak absorption was achieved by varying the thickness of LN [102]. Aly et al. [103] conducted a theoretical analysis using the transfer matrix method, varying several parameters such as defect layer thickness, applied electric field, and incident angle. Their results indicated that graphene enhances the reflectance characteristic by 5% [103]. Chowdhury et al. [104, 105] theoretically explored the enhancement of absorption through the TO effect by temperature change using a proposed MgO-doped LN, which opens up the design of dual-narrowband absorbers in photonics research.

Fig.7 Representative demonstration of graphene absorbers. (a) One-dimensional photonic crystals of period numbers with an x-cut LN layer [101]. (b) A resonant asymmetric grating with G-LN integration [106]. (c) A grating metamaterials with G-LN integration [107].

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Chen et al. [106] constructed a G-LN integration with a resonant grating metasurface adhered to the LN surface, where graphene serves as a thin absorptive layer and a conductive electrode. They achieved 99.99% absorption at 798.42 nm and a 1.14 nm redshift with applied voltages ranging from −150 to 150 V, as shown in Tab.3. Hu et al. [107] proposed a tunable single-/dual-band graphene absorber with a resonant asymmetric grating by varying the incident angle. The resonant wavelengths were adjusted by balancing the applied voltage and the incident angle [Fig.7(b)] [107]. Wang et al. [108] developed an electrically tunable coupling filter utilizing orthogonal gratings together with graphene. The device achieved perfect filtering with an absorption rate of 99% at an 801 nm wavelength, and then the spectral response was explained using coupled-mode theory [Fig.7(c)].

Tab.3 Comparison of different graphene absorbers based on G-LN integration.

Structure type	Spectra type	Wavelength (nm)	Absorption rate (%)	Voltage range (V)	Tuning rate (nm/V)	Ref.
PhCs-(HL)MGD(LH)N	Reflective	776	99.7	−15 to 15	−1.6	[100]
PhCs-(LGHG)MD(GLGH)N&(LGHG)MD(GHGL)N	Reflective	1326&1357	≈100	−105 to 105	−0.9e-4 & −0.6e-4	[101]
PhCs-(HL)MGD(HL)N	Reflective	1222	70	Null	Null	[102]
PhCs-(LGHG)MD(GLGH)N&(LGHG)MD(GHGL)N	Reflective	713.5&798	95.95&92	※a	※a	[103][104]
Grating-symmetric	Transmission	798.42	99.99	−150 to150	−1.14/300	[105]
Grating-asymmetric	Transmission	1550.04	99.9	☆b	☆b	[106]
Grating-symmetric	Transmission	801	99	−150 to 150	−13.4/300	[107]

Note: a※ refers to temperature increasing from 0 to +300 K with a tuning rate of around −15 nm/300 K; b☆ refers to refractive index changing from 2.2 to 2.22 with a tuning rate of −2.2 nm/0.02 RIU.

Tab.3 presents the comparison of various graphene absorbers based on G-LN integration. Two primary G-LN integration structural types of graphene absorbers demonstrate near-perfect absorption, particularly within the visible and near-infrared frequency ranges, as determined by the central wavelengths. The voltage-dependent RI of the LN layer enables linear tuning of the resonant wavelength by varying the applied external voltage. This tuning results in a redshift of the resonant wavelength with increasing voltage, corresponding to the negative value of the tuning rate. However, the two structural types exhibit notable differences in tuning rates across their voltage ranges.

In chief, the tunable perfect graphene absorbers can be achieved by two types of G-LN integration, sandwiching graphene between PhCs with reflective spectra or using resonant gratings with transmission spectra. For reflective spectra, the optimization studies suggest that absorption can be further enhanced by adjusting the period numbers of PhCs, defect layer thickness, and incident angle. For transmission spectra, both structural types can realize dual-band absorption. Optimizing the structural design or a novel structure of the device to achieve a higher tuning rate with a lower applied voltage in performance remains a key goal to pursue.

7 Other devices

The composites of graphene and metal nanoparticles represent a promising category of materials characterized by unique electronic, optical, and chemical properties. Zaniewski et al. [109] experimentally investigated the formation of gold nanoparticles on G-LN integration, revealing that the LN substrate significantly influences the chemical properties of grapheme. From a quantum perspective of particle manipulation by acoustic wave transporting in various media, Yu et al. [110] established an artificial SAWs of G-LN integration platform to study the transport characteristics of dispersed electrons, underscoring the potential of commercially available SAW platforms for fundamental research and practical applications [Fig.8(a)]. Bidmeshkipour et al. [47] conducted microwave characterization of top-gated graphene FETs on LN substrates, demonstrating an intrinsic cutoff frequency of up to 10 GHz. Jiang et al. [111] proposed high-performance G-LN integration-based capacitors with improved kinetics matching between capacitor-type anodes and battery-type cathodes [Fig.8(b)]. Cavallo et al. [112] demonstrated a permanent planar p-n junction in G-LN integration that exhibited photodiode behavior. Jin et al. [113] introduced an ultrasensitive FET biosensor based on G-LN integration, showing great potential for developing a biosensing platform for disease diagnosis [Fig.8(c)].

Fig.8 Representative demonstration of other devices. (a) Surface phononic graphene [110]. (b) High-performance lithium-ion capacitors [111]. (c) Ultrasensitive graphene FET Biosensors [113]. (d) High-performance graphene photodetector [115, 116]. (e) LN waveguide-based photodetectors [117].

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The G-LN integration-based photodetector leverages the unique properties of high carrier mobility and broad light transparency of graphene, combined with the high pyroelectric coefficient of LN. Shimatani et al. [114] proposed a high-responsivity, long-wavelength infrared G-LN integration-based photodetector and analyzed the photogating effect enhanced by the pyroelectric effect of LN due to heat generation. Guan et al. [115, 116] demonstrated a broadband (405−2000 nm) and high-sensitivity (2.92 × 10⁶ A/W) photodetector based on G-LN integration, achieving a very high responsivity (23 ms). The LN substrate absorbs source light, converts it to heat, and induces spontaneous polarization, which generates surface-free electric charges that enter graphene. These changes in the Fermi level of graphene enhance the photoelectric performance of the photodetector [Fig.8(d)] [115, 116]. TFLN has emerged as a promising platform for PICs and optic devices. Zhu et al. [117] proposed an LN waveguide-integrated graphene photodetector that features simultaneous broadband operation at telecom and visible wavelengths, high normalized photocurrent-to-dark current ratios (NPDR) up to 3 × 10⁶ W⁻¹, and large 3dB photoelectric bandwidths of 40 GHz [Fig.8(e)].

8 Outlook and perspective

G-LN integration combines the extraordinary properties of graphene, such as high electron mobility, flexibility, and tunable optical conductivity, with the outstanding electro-optic, acousto-optic, and nonlinear-optic properties of LN. As research on G-LN integration advances, it continues to pave the way for cutting-edge acoustoelectric, photonic, and optic devices, making strides in both fundamental understanding and practical applications. This mini-review has synthesized the theoretical foundations, experimental breakthroughs, and innovative applications of G-LN integration. However, several challenges and opportunities remain, setting the stage for future developments in this field.

The fundamental principles of G-LN integration rest on the synergistic interaction between graphene’s tunable electronic properties and LN’s intrinsic characteristics. A core point is the modulation of graphene’s charge carrier density through the electrostatic fields of LN achieved via polarization effects or induced electric fields, which enables dynamic control of graphene’s electronic, optical, and plasmonic properties. First-principle calculations and experimental studies such as Raman spectroscopy reveal the effects of graphene’s carrier mobility and impurity scattering due to the strong interface interaction of G-LN integration. Continued exploration of interfacial physics reveals new mechanisms may be challenging. The versatility of G-LN integration-based devices can be expanded and unlocked with new functionalities by a comprehensive understanding of fundamental principles. To date, the PMMA transfer technique is commonly used in G-LN integration processes. Despite substantial progress, the fabrication of G-LN integration-based devices is hindered by limitations in scalability, reproducibility, and interfacial quality. The PMMA transfer technique, while effective and low cost, often leads to contaminants, interfacial defects, and strain which can degrade device performance. The automated transfer systems can enable large throughout parallel processing but be limited on the laboratory scale. So, refining the fabrication process for greater uniformity, adhesion, and durability to enhance the interaction control between graphene and LN is needed. Future research could focus on improving the automated transfer technology to a higher quality level and an industrial scale, and advancing alternative methods, such as direct synthesis graphene growth on LN substrate to reduce processing complexity.

For device-level technology, improving the overall performance is crucially important while more novel structure design concepts are always inspired. Because optimization of device performance is essential to unlock the full potential of G-LN integration. SAWs-based sensors, delay lines, filters, and signal-processing devices are available while maximizing SAW coupling efficiency, minimizing insertion loss, and achieving higher operational frequencies will be pivotal. It is noted that a roadmap of SAWs has been given with comprehensively review, and as emphasized in Section 9 “SAWs and 2D materials” [118], integration with 2D materials could lead to highly sensitive sensors and new devices for quantum technology. Graphene electrodes serve as a low-loss, high-transparency, and highly conductive electrode material and exhibit huge potential applications in various electronic devices such as TO/EO modulators. SPPs-based devices enable subwavelength field confinement and efficient light-matter interaction, a promising avenue for applications in spectroscopy and sensing, such as tunable plasmonic sensors, THz waveguides, and directional couplers. The development of near-perfect optical absorbers shows promise in high-performance filters, and the one with asymmetric grating can be used in PICs due to the transmission spectra through TFLN waveguide. At last, G-LN integration also has enabled novel applications such as high-sensitivity biosensors, broadband photodetectors, and photonic crystal-based devices. Moreover, the long-term environmental stability of devices in operational conditions, such as humidity, high temperatures, or exposure to external electromagnetic fields, is another pressing concern. This necessitates the development of robust encapsulation techniques to protect and sustain device integrity over extended periods. Furthermore, the integration of emerging technologies such as artificial intelligence and machine learning in device design and characterization may accelerate the optimization of G-LN integration, enabling adaptive and smart functionalities, and holding the promise of transforming a wide range of technologies.

In a word, the versatility of G-LN integration opens up new frontiers for innovative applications. By addressing current challenges, fostering interdisciplinary collaborations, and exploring novel applications, this field can achieve its full potential, driving breakthroughs in acoustoelectric, photonic, and optic devices. As research progresses, the G-LN integration is poised to play an important role in shaping the future of devices.

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Declarations

The authors declare that they have no competing interests and there are no conflicts.

Acknowledgements

L.J. Zhao was supported by the Youth Teacher Capability Enhancement Plan Overseas Visiting Scholar Program.

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