Plasmonic‐Pyroelectric Materials and Structures

With the growing global energy crisis, research into new energy materials that can potentially transfer heat into electricity has become a worldwide imperative. Pyroelectric materials are polar materials that are able to produce electrical charge in response to temperature change. These materials are of interest for infrared sensing, energy harvesting, and emerging applications in chemistry and biology. However, unlocking their potential requires the temperature changes to be both large and rapid. To achieve this goal, pyroelectric materials can be used in synergy with plasmonic nanomaterials, which provide highly localized and rapid heating upon illumination at the plasmonic resonances. Plasmonic‐pyroelectric combinations are therefore being used for a variety of electrical, thermal, electrochemical, and biological studies and are inspiring new technological applications. In this review, the underlying mechanisms of the pyroelectric and plasmonic effects are introduced and the benefits of combining them are outlined. A range of applications is then overviewed. Critical challenges and future perspectives to further develop the underlying science of these systems and to create highly efficient plasmonic‐pyroelectric materials and structures are discussed.


Introduction
As the 21 st century progresses, it is becoming increasingly apparent that humanity needs to consider new energy sources to sustain its technological development.The advent of the Internet DOI: 10.1002/adfm.202312245   of Things (IoT) also requires advanced sensors.As a result, the creation of new functional materials is a global priority.Within this context, pyroelectric materials are of particular interest for applications related to sensing and thermal harvesting, since they can generate electrical charge and current when subjected to large and rapid changes in temperature.In addition, plasmonic nanomaterials are attractive because, when illuminated by light of a resonant frequency, they can produce large and rapid local temperature fluctuations.Plasmonic materials are typically composed of metal nanoparticles that can rapidly heat up due to their extremely small heat capacities and resonant energy states for surface electrons.Recent efforts to combine plasmonic and pyroelectric materials have emerged as a naturally promising research area.The new combined materials offer large and quick temperature variations (plasmonic behavior) that in turn affect the charge and current generated (pyroelectric behavior).This combination paves the way for a wide range of technological applications, whose practicality and efficiency are governed by the physical principles associated with both pyroelectric and plasmonic effects.To better understand the advantages of combining pyroelectric and plasmonic materials, we need to consider the nature of each material and the physical principles that characterize it.We start with pyroelectric materials.
The word "pyroelectric" is derived from the Greek words "pyr" (meaning "fire") and "electricity". [1]Pyroelectric materials are characterized by a spontaneous dipole moment in the absence of an externally applied electric field.Pyroelectrics are typically crystalline materials and exhibit a net spontaneous polarization that results in a measurable electrical charge (or voltage) due to a change in polarization with temperature. [2]Examples of pyroelectric materials include tourmaline (where the effect was first reported), [3,4] gallium nitride, cesium nitrate, aluminum nitride, and zinc oxide.Their ability to transduce thermal energy into electrical energy makes them highly valuable for applications related to energy harvesting, [2,5] thermal sensing, [6] infrared sensing, [7,8] thermal cameras, [9] detection, [10,11] catalysis [2,12] and others.To date, the development of pyroelectrics has focused on producing a variety of materials, including single crystals, [13] ceramics, [14] polymers, [15] and composites. [16]These materials can be produced in bulk or in thin film form, where the composition and microstructure can be engineered to tailor the dielectric, ferroelectric, and pyroelectric response.The main quantifiers of the pyroelectric response are the pyroelectric charge (Q) and the pyroelectric current (I = dQ/dt, where t stands for time).We will see in this review that the pyroelectric charge and current depend on the temperature change (ΔT) and on the rate of temperature change (dT/dt), respectively. [2]Next, we consider the nature and characteristic physical properties of plasmonic materials.
In plasmonic materials, the illumination of metal nanoparticles by light can lead to coherent oscillations of the surface electrons that are referred to as "plasmons".At resonances, these oscillations are very strong and, as a consequence, the nanoparticles can strongly absorb light with a frequency (or color) that matches the resonances.Given their nanoscale volumes, these nanoparticles can heat up, and at a highly rapid rate, even for low or modest levels of illumination power. [17]As an illustration, Baffou et al. calculated that 100 nm gold nanoparticles produced a temperature change of ΔT ≈ 50 K when illuminated at the surface plasmon resonance (530 nm, green light) at a light intensity of 1 mW μm −2 ; [18] for reference, a commercial (lasers safety class 2) green laser pointer has a power of up to 2 mW.In addition, when 48 nm by 14 nm gold nanorods were illuminated by 760 nm light, Ekici et al. reported a ΔT ≈ 200 K after only a short time period of ≈50-200 ps. [19]This potential to achieve large (high ΔT) and rapid (high dT/dt) changes in temperature finds direct applications when plasmonic nanoparticles are coupled to temperaturesensitive materials.
[25] For information storage, plasmonic nanomaterials heated up by light can help increase the storage capacity of magnetic hard drives by heating domains above the Curie temperature. [26,27]As another example, plasmonic nanoparticles have been successfully combined with temperature-responsive polymers (such as poly-Nisopropylacrylamide, pNIPAM) to create nanorobotic, and mechanical actuators. [28,29,30]Within the context of successfully combining plasmonic and temperature-responsive materials, it is only natural to consider pyroelectric materials.Since plasmonic nanomaterials offer excellent control over both ΔT and dT/dt, they can serve as a natural and tailorable "handle" to control the pyroelectric response, also providing a degree of wavelength selectivity with respect to the incident light.
For further background, there are a several excellent reviews on plasmonic materials, [31] also covering their use for hydrogen generation and carbon dioxide reduction. [32]Pyroelectric materials have also been overviewed for applications related to thermal sensing, catalysis, and energy harvesting. [2,16]There have been in-depth reviews on the development of materials for infrared imaging and photodetectors [33] and achieving a plasmonically enhanced response.Ogawa et al. have reviewed both plasmonic-and metamaterial-based infrared detectors [34] and Stewart et al. recently examined the use of nanophotonic engineering to create thermal photodetectors, which included an overview of pyroelectric-based systems. [35]Finally, Kang et al. [36] have reviewed the potential of combining plasmonic metasurfaces to produce hybrid systems.
Despite these excellent reviews, no review to date has provided a detailed overview of recent developments in combining plasmonic and pyroelectric materials, the range of device structures, and their range of potential applications.Potential tech-noogical implementations include energy harvesting, infrared detectors/photodetectors, catalysis, and bio-applications.In this review, we initially summarize the fundamental physical properties of pyroelectric and plasmonic materials.Understanding these characteristic properties is necessary to clarify how the two materials can be integrated to enhance performance.With a focus on the underlying materials science and device structures, we then overview the key challenges and opportunities for plasmonic and pyroelectric materials and corresponding structures in both fundamental and practical applications.

Fundamental Mechanisms
Since the coupling of plasmonic and pyroelectric materials is a relatively new approach, it is first useful to describe the individual mechanisms to understand the benefits of combining both pyroelectric and plasmonic effects.

Pyroelectric Effect
Pyroelectricity is defined as the property of polar crystals to generate an electrical charge when heated or cooled. [37]This behavior is the result of a change in temperature that leads to a change in the dipole moment and level of polarization of a polar material.This change in polarization leads to the generation of charge and a potential difference across the material under open circuit conditions (V oc ), or a current flow under short circuit conditions (i sc ). Figure 1a shows that the polarization of a ferroelectric typically decreases with an increase in temperature and disappears at the Curie point (T c ), where the material undergoes a change in phase from a polar non-centrosymmetric phase to a non-polar centrosymmetric phase. [2]Simultaneously, the relative permittivity (dielectric constant) increases with temperature and reaches a maximum as it approaches T c and subsequently decreases sharply with an increase of temperature above the T c .
The underlying mechanism of the pyroelectric effect, which leads to charge generation when a material is heated or cooled, is shown in Figure 1b-d.When the polar pyroelectric material is placed between two conductive electrodes and connected to an external circuit at thermal equilibrium (dT/dt = 0), the polarization is unchanged and there is no current flow, as in Figure 1b, since the screening charges present on the surface of the polar material to maintain charge neutrality are balanced. [1,2]When the temperature of the pyroelectric is increased (dT/dt > 0), there is a reduction in polarization, as dipoles lose their orientation.This decrease in polarization leads to screening charges being free to produce a current when the material is connected to an external circuit, as outlined in Figure 1c.Similarly, when the pyroelectric material is cooled (dT/dt < 0), the polarization increases as dipoles regain their orientation, thereby reversing the current flow as screening charges move in the opposite direction to maintain charge neutrality, see Figure 1d.
To exhibit the pyroelectric effect, the material therefore requires a spontaneous polarization, P s , which is defined as the dipole moment per unit volume of the material in the absence of an applied electric field.Small changes in temperature lead  [2,38] the polarization decreases with increasing temperature and reaches zero at the Curie point T c ; simultaneously, the relative permittivity increases with temperature, reaches a maximum at T c and then decreases sharply above T c ,. [2] Copyright 2014, Royal Society of Chemistry A pyroelectric material with a spontaneous polarization, b) at thermal equilibrium the polarization level is constant and there is no current flow since screening charges on the polar surfaces are balanced c) when heated, the polarization reduces as dipoles lose their orientation and screening charges are now free to produce a current and d) when cooled, the polarization increases as dipoles regain their orientation and a current flows in the opposite direction. [39]Copyright 2020, Elsevier.
to proportional changes in dielectric displacement, and the pyroelectric coefficient can be described according to, where p ,E is the pyroelectric coefficient under constant stress and electric field, and  and E correspond to conditions of constant stress and electric field, respectively.The pyroelectric coefficient (C m −2 K −1 ) provides an indication of the level of charge generated per unit area of material and unit temperature increase.The condition of constant stress indicates that the pyroelectric material is not mechanically clamped, and is free to thermally expand or contract in response to a change in temperature. [1]If the material is clamped at a constant strain to prevent thermal expansion or contraction, the change in temperature simply leads to a change in polarization with no change in sample dimensions, and the pyroelectric response is related to the primary pyroelectric coefficient (p primary ).However, where the material is not mechanically clamped (under a condition of constant stress), the overall pyroelectric coefficient is a combination of the change in polarization and its dimensions, as described by Equation (2) [2,40] , where d ij , c ij and  i are piezoelectric coefficient, elastic constant, and thermal-expansion coefficient, respectively.The term is termed the secondary pyroelectric coefficient (p secondary ); it is re-lated to the change in polarization of the material as a result of the piezoelectric charge generated due to thermal expansion.Generally, the primary pyroelectric coefficient is dominant in materials such as lead zirconate titanate (PZT), barium titanate (BaTiO 3 ), and other ferroelectric materials.The secondary pyroelectric effect dominates the pyroelectric response for non-ferroelectric polar materials with lower magnitudes of primary coefficients, such as ZnO, CdS, and other wurtzite-type materials with asymmetric structures. [41]The secondary pyroelectric effect can also be important if the material is subjected to an alternating radiation flux, whose frequency matches its mechanical resonant frequency. [40]nother case is when a pyroelectric material is not uniformly heated or cooled, which generates shear stress and a resulting change in polarization due to the piezoelectric effect; this behavior is termed the tertiary pyroelectric coefficient (p tertiary ).In this case, the pyroelectric coefficient is dependent on the magnitude of the temperature gradient, rather than the magnitude of the temperature change. [40]This effect is generally associated with bulk materials and can be neglected for uniform heating, which is typically the case in thin films. [42]he overall pyroelectric coefficient (p), which can include primary, secondary, or tertiary coefficients, can be used to determine the charge, open circuit voltage and short circuit current generated by a temperature change.If a pyroelectric material is subjected to temperature change (ΔT), the net charge (Q) generated is given by, where A is the electrode area of the pyroelectric material and the pyroelectric coefficient (p) can be determined from Equation (2).The pyroelectric short circuit current (i sc ) generated as a result of a change in temperature is given by, Since pyroelectric materials generate a charge due to a change in polarization, which subsequently leads to a current flow under an electric load, a pyroelectric device can be essentially regarded as a current source.Equation (4) shows that the pyroelectric current is proportional to the area (A), the pyroelectric coefficient (p), and the rate of temperature change (dT/dt).As pyroelectric materials are dielectric in nature, the capacitance (C) of the pyroelectric element is given by the well-known formula for a parallel plate capacitor, where h is the thickness of the pyroelectric material,  T 33 is the relative permittivity at constant stress and  0 is the permittivity of free space.
Based on Equations ( 3) and ( 5), under open circuit conditions, the potential difference (V oc ) across the electrodes as a result of a temperature change can also be expressed as, While the generated charge, current, and voltage are of interest for sensing applications, for energy harvesting applications, it is also of interest to estimate the electrical energy generated from a temperature change.Since the total energy (W) stored in a capacitor is given by W = ½CV 2 , the amount of electrical energy stored in the material due to a temperature change is expressed as [2] Next, we consider the characteristic physical properties of plasmonic nanomaterials with a focus on how they transduce light energy into heat.

Plasmonic Materials and Systems
Surface plasmons are collective oscillations of electrons that occur at the interface between two materials, where the dielectric function experiences a significant change.Typically, surface plasmons can form on the surfaces of metals (such as gold or silver) and dielectrics (such as air or water). [43]When light interacts with a metal surface, its electromagnetic field can couple to the surface plasmons, resulting in propagating waves of electron density oscillations along the surface of the metal, [44] see Figure 2a.The electric field produced by the surface plasmons is strongly localized near the metal-dielectric interface and decays exponentially away from it. [45]Surface plasmons can confine light to subwave- length scales, allowing for the manipulation and control of light at the nanoscale. [46]hen light interacts with metal nanoparticles, such as Au or Ag, [47,48] whose dimensions are significantly smaller than its wavelength, it can excite localized surface plasmon resonances.Plasmonic materials also include Al, Cu, Pt, Li, Na, K, Cs, Rb, and Ni. [49,50]These plasmon resonances are coherent oscillations of the electrons at the surface of the nanoparticles. [51]Classically, the oscillations can be modeled as a simple harmonic oscillator, driven by the force of the electric field of light, with a restoring force originating from the positively charged nuclei within the nanoparticle, see Figure 2b.This oscillator resonates at a specific wavelength that depends on the size of the nanostructure, the material, its geometry, its surroundings, and their distance from each other.Strong electromagnetic scattering and absorption can be observed at this resonance wavelength.
The absorption in plasmonic nanoparticles causes them to heat up, which in turn heats up the surroundings.As a result, a pyroelectric material in the vicinity of a plasmonic nanoparticle can greatly benefit from plasmonic nanoparticle heating, since it can heat up to high temperatures, with rapid heating rates.However, not all the light that is incident on the nanoparticles is transformed into heat and the generation of heat is not instantaneous.There are several physical processes that regulate the flow of energy and they have associate timescales that, taken together, determine the heating response time. [52]It is therefore important to understand the nature and the duration of each process that leads to plasmonic heating.
At a small timescale of up to a hundred femtoseconds, the plasmon resonances having been excited by light can lose their energy by re-emitting it in the form of light [39,53] or by Landau damping.This damping is associated with intra-and inter-band electron transitions in the metal of the nanoparticles.These transitions result from the electric field induced by the surface lattice resonance, and the electrons that are involved in the transitions are often referred to as hot electrons, since their energy can place them high above the Fermi level, see Figure 2c. [54]These hot electrons are non-thermalized, and an effective temperature cannot be assigned to their distribution of energies.However, after electron-electron collisions, the population thermalizes and the energy from these thermalized electrons is then passed to the surrounding lattice, until equilibrium is established.This process can be described by a two-temperature model (TTM).The two temperatures referred to are, first, T e (r,t), the temperature of the electron gas at time t and at coordinate r, and, second, T l (r,t), the temperature of the ion lattice.The twotemperature model consists of two heat equations that describe the corresponding diffusion of heat. [55,56]e T e (r, t) where C and K are the heat capacities and thermal conductivities of the electrons and of the ion lattice, which are indicated with subscripts e and l, respectively; for simplicity, we assume that they are constant.The source terms for these equations are S 1 (r,t), which corresponds to the laser energy deposition per unit area and unit time and S 2 (r,t), which corresponds to external heating of the lattice.Usually, there is no external lattice heating, and therefore S 2 (r,t) can often be neglected, and the electron heat conductivity in metals is much larger than the lattice heat conductivity.A key feature of the TTM is the cross-coupling term, G, that represents the electron-phonon coupling constant and corresponds to the energy transfer between the electrons and the lattice.This energy transfer is important since it clearly regulates the heating of the nanoparticles and G varies for each material.For reference, in bulk gold, G ≈ 3 × 10 16 W m −3 K −1 S −1 and in bulk silver G ≈ 3.5 × 10 16 W m −3 K −1 S −1 . [57]It should be noted that the process has an upper limit, and if too much energy is transferred too quickly, it can lead to distortion and breakdown of the lattice. [58,59]or plasmonic nanoparticles, the source term S 1 (r,t) is proportional to the absorption cross-section of the metal nanoparticle and the photon flux. [60]Importantly, the ultrafast perturbation of the electron distribution leads to a change in the dielectric permittivity () of the nanoparticles.This change affects the surface plasmon resonance, thereby allowing ultrafast manipulation of the spectroscopic properties of nanoparticles. [61]A number of approaches have been used in the literature in order to deepen the TTM; for example, it is possible to include surface and grain boundary scattering, [62] or to discriminate between thermalized and non-thermalized electrons. [63]ypically, the process of energy transfer between electrons illuminated with femtosecond laser pulses and the lattice of the nanoparticles is rapid and of the order of a few ps. [64]A key point of this process is that, as a result of the energy transfer, the lattice is set in motion, thereby forming phonons that can be used to rapidly change the temperature of a nearby pyroelectric phase.
In the simplest case, we can consider spherical nanoparticles with homogeneous and isotropic energy transfer.These conditions result in a breathing phonon mode that corresponds to a radial vibration of the nanoparticle. [65]The radius of the nanoparticle (R) is proportional to the period of oscillation (T' ), following [66,67] : where c l and c t represent the longitudinal and transverse speed of sound, respectively. [68]As an example, the period of oscillation for gold nanoparticles with a diameter > 40 nm is tens of ps, [69] and the oscillation is that of a simple harmonic oscillator with a damping term, where the damping results from a loss of energy to the local environment of the nanoparticles in the form of heat.Changes in the nano-scale particle geometry, such as using nanospheres, nanorods (NRs), nanocages (NCs), and nano nanowires (NWs), as seen in Table 1, can also tune the surface plasmon resonance across the electromagnetic spectrum from the UV, through to the visible and infrared bands.
When considering the combination of plasmonic nanoparticles and a pyroelectric material, it is of particular interest to consider heat transfer from the plasmonic nanoparticle to the surrounding environment, which takes nanoseconds.The model developed by Goldenberg and Tranter provides an intuitive understanding of the temperature T(r, t) at a position r outside the nanoparticle at a time t. [70]This approach assumes a spherical nanoparticle, which is homogeneous and uniformly heated within an infinite, homogeneous medium. [71]The model is based on a heat equation [72] : where c(r) represents the specific heat, (r) is the mass density, K is the thermal conductivity (which we take to be constant for simplicity) and S(r, t) is again a source term.For reference, for gold at 20 °C, K = 314 W m −1 K −1 and for silver, at 20 °C, K = 406 W m −1 K −1 , [73] which are typically much larger than those of pyroelectric materials such as ZnO, at 27 °C, K = 4.5 W m −1 K −1 [74] and polyvinylidene fluoride (PVDF), at 20 °C, K = 0.1848 W m −1 K −1 . [75]Somewhat analogously to Equation (8), the source term is associated with the energy from the laser beam, although here this energy is in the form of heat.The source term is given by S =  abs I, where I is the intensity of light and  abs is the absorption cross-section of the nanoparticle. [76]he latter is obtained from: where  m is the dielectric permittivity of the medium and k is the wavenumber of the electric field.The expression for the source term can be further developed as a function of the timedependent electric field E(r, t) leading to [18] : where  is the frequency of light.
A simplified time-independent heat equation can be obtained for illumination with a continuous wave, or for pulsed light with  a high enough repetition rate compared to the heat transfer rate: where ΔT is the temperature difference with the ambient temperature [77] : with V NP being the volume of the nanoparticle.An important outcome here, for a combined plasmonic-pyroelectric system, is that the temperature at a distance r from the plasmonic nanoparticle increases with the volume of the nanoparticle, with the intensity of illumination, and for a wavelength that matches the plasmon resonance (where  abs is maximal when  + 2 m ≈ 0). [77]The latter dependence is of interest to provide sensors that are wavelength selective, which we will see in Section 3.2 on infrared detectors and photodetectors.It is also of interest to note that the temperature decreases with increasing distance r from the nanoparticle; this dependence indicates the need for a large number of closely packed plasmonic nanoparticles in close proximity to pyroelectrics to increase heat transfer.For simplicity, so far, we have been discussing spherical nanoparticles.In more complex nanostructures surface plasmon resonances, and their heating, depend on the direction of light polarization.

The Benefits of Combining Plasmonic and Pyroelectric Materials
For pyroelectric sensing and harvesting applications, there is a clear need to achieve large changes in local temperature (ΔT) and rapid changes in temperature (dT/dt) to increase the charge (Q, Equation (3)), short circuit current (i sc , Equation ( 4)), open circuit voltage (V oc , Equation ( 6)), and electrical energy (E, Equation ( 7)).For their part, plasmonic nanoparticles can heat up at a ps timescale and, upon excitation at the plasmon wavelength, they can heat a material that is placed in their immediate vicinity to high temperatures.Plasmonic materials have strong and tailorable light absorption characteristics that provide control over ΔT and dT/dt, as well as wavelength and light-polarization selectivity.Therefore, the combined plasmonic-pyroelectric mate-rials offer a step change in performance.We will see in this review that potential applications include i) energy harvesting, ii) infrared detectors with potential for wavelength selectivity and rapid response times, iii) catalysts with combine photo-and pyrocatalytic effects, and iv) new methods for photothermal therapy in bio-applications, as outlined in Figure 3.

Material Combinations Employed in Plasmonic-Pyroelectric Structures
So far, we have overviewed the fundamental principles of pyroelectric and plasmonic materials.We now examine the range of plasmonic-pyroelectric materials that have been explored to date.Table 1 provides an outline of the plasmonic-pyroelectric materials that have been reported in the literature, their typical hybrid structures, and potential applications.It can be seen that a broad range of pyroelectric materials have been explored.They can be classified into ferroelectric polymers, ferroelectric ceramics, and non-ferroelectric ceramics.The ferroelectric polymers include polyvinylidene fluoride (PVDF), and their copolymers such as polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE); these materials are attractive because they can be readily formed into thin films at low processing temperatures, with the potential for embedding plasmonic particles within the polymer (or on its surface) to create composite architectures.While the pyroelectric coefficients of the ferroelectric polymers are relatively low, their low permittivity can lead to high open circuit voltage (Equation 6), and energy harvesting (Equation 7).Ceramic-like ferroelectricbased pyroelectric materials have also been considered; they include lithium niobate (LiNbO 3 ), barium titanate (BaTiO 3 ), lead zirconate titanate (PZT), lead magnesium niobate-lead titanate (PMN-PT), barium strontium titanate (BST) and bismuth ferrite (BiFeO 3 ).These materials are typically produced in bulk or thin film form, where the surface can be decorated with a plasmonic material.The ferroelectric materials also exhibit high pyroelectric coefficients that can lead to a large level of charge generation and high current responsivity (Equation 4).Non-ferroelectric polar pyroelectric materials have also been examined, including zinc oxide (ZnO), and aluminum nitride (AlN), and are readily formed in a thin film or nanorod form.Since these materials are non-ferroelectric their polarization direction cannot be changed and they must therefore be formed in single crystal or highly textured form.Range of applications of plasmonic-pyroelectric materials and structures surveyed in this review: energy harvesting, [78] infrared detector for thermal sensing, [79] photodetector, [80] selective PIR refers to spectrally selective pyroelectric detectors, the Max and Min refer to the simulation absorptivity of the plasmonic-pyroelectric detector (PA-PIR), catalysis to increase of degradation efficiency by generation of reactive oxygen, where hv refers to photon energy.Reproduced with permission from, [81] copyright 2022, Elsevier, and tumor therapy, BTO refers to barium titanate.Reproduced with permission from. [82]Copyright 2021, Elsevier.
The plasmonic materials used to date are less diverse and typically include Au, Ag, and graphene.Both Au and Ag are easy to manufacture and exhibit pronounced plasmon resonance across the visible and near-infrared spectral regions (and ultraviolet for Ag).They can also be readily functionalized with surfactant molecules, which makes them easy to manipulate in liquid media and, as we have seen in Section 2.2, they have substantial cross-coupling G values.The plasmonic materials are used in a variety of nanoscale geometries, such as nanoarrays, nanocubes, nanowires, and nanoparticles that can tailor the surface plasmon resonance across the electromagnetic spectrum, as outlined in Section 2.2.Clearly the range of plasmonic-pyroelectric materials used to date is diverse, with materials that can be selected depending on the requirements of applications based on criteria, such as ease of fabrication, ease of combination, and their pyroelectric, dielectric, and mechanical properties.Next, we overview the range of potential applications.

Applications of Plasmonic-Pyroelectric Materials and Structures
The current applications of plasmonic-pyroelectric materials and device structures can be divided into four categories, where the potential of plasmonic nanoparticles is used to increase the temperature change (ΔT) or the rate of temperature change (dT/dt), for achieving larger or more rapid changes in polarization; these include i) energy harvesting, ii) infrared detectors/photodetectors, iii) catalysis and iv) bio-applications.In this section, each application is addressed.

Energy Harvesting
Energy harvesting can be achieved using hybrid plasmonicpyroelectric structures to scavenge light.For a variety of illumination sources, plasmonic metasurfaces have been produced Reproduced with permissions. [78]Copyright 2018, Wiley.c) Schematic of model (upper image) and plasmonic resonance map (lower image) of BTO-Ag for energy harvesting.Reproduced with permissions. [83]Copyright 2019, Wiley.d) Schematic of pyroelectric nanogenerator.e) Pyroelectric energy harvesting of temperature changes from rain.f) Open circuit voltage of pyroelectric nanogenerator under different temperature of rainwater. [85]Copyright 2022, Elsevier.
to transduce light into thermal energy and subsequently into electrical energy via the pyroelectric effect.We have seen that when a pyroelectric generates charge under open circuit conditions this leads to a potential difference across the material, Equation (6); for example, when operating as a thermal sensor.However, for energy harvesting applications to generate power it is possible to accumulate the pyroelectric charge in a storage capacitor during thermal cycling, or allow the charge to flow through an external electric load. [2]One example is a hybrid energy harvesting device that was created by depositing a gold nanodisk plasmonic array on a transparent indium tin oxide (ITO). [78]On top of the plasmonic nanodisk surface, a pyroelectric P(VDF-TrFE) copolymer layer was placed, which was then coated with a gold film that acted as an electrode, as shown in Figure 4a.When the plasmonic gold nanodisk layer was illuminated, it acted as the light absorber and thermal transducer for the pyroelectric PVDF-TrFE film, resulting in the generation of electrical charge (Equation ( 3)).The device was able to harvest light and generated a voltage of ≈2 mV and an integrated power density of ≈0.4 J m −2 from an unusual fluctuating light source, namely a leaf swinging in the wind to create oscillating shadows; see Figure 4b.The experimental results were consistent with both optical and heat transfer simulations to pro-vide a route to generate power from a fluctuating illumination source.
Based on a similar mechanism, a hybrid device was designed based on combining ferroelectric/pyroelectric BaTiO 3 and a plasmonic Ag nanostructure, which demonstrated an increase in charge density and an enhancement in the effective pyroelectric coefficient by 46% [83] when illuminated with UV light (363 nm).Numerical simulations were undertaken, see Figure 4c, which agreed qualitatively with the experimental results.The authors hypothesized that electronic band bending, coupled with the plasmonic effect, was able to facilitate a pathway for electrons to occupy the surface state of the nanoparticles, leading to an increased the surface potential; further experimental work and simulations to confirm this hypothesis would be of interest.
Another hybrid system was reported by Wu et al., which was based on a pyroelectric electrospun PVDF nanofibrous membrane that contained plasmonic reduced tungsten oxide (WO 2.72 ) nanoparticles to obtain light energy harvesting. [110]When irradiated by infrared radiation for 60 s, the temperature of the PVDF-WO 2.72 composites was 41.5 °C higher than that of the pure PVDF film; the increased change in temperature produced a greater amount of pyroelectric charge (Equation 3), and a higher open circuit potential (Equation 6).The PVDF-WO 2.72 composites were able to produce a maximum open circuit voltage of 1.5 V, which was three times higher than pure PVDF, indicating the capability of harvesting ambient thermal energy and potential applications as a pyroelectric harvesting generator, or as a sensor.The plasmonic nature of the WO 2.72 nanoparticles [110][111][112][113] was thought to originate from the material's semiconducting properties, where WO 2.72 exhibits a more intricate energy band structure compared to conventional plasmonic metals (such as Au), and the bandgap and electronic defects can significantly affect light-matter interaction.Najm et al. undertook simulations of surface plasmonpolaritons (SPPs) in graphene in the THz band, with the potential to be combined with a pyroelectric material. [114]Their work focused on factors affecting the SPPs resonance, including the potential for pyroelectric energy harvesting applications to generate electric energy from waste heat or light.
More recently, a sunlight-triggered pyroelectric nanogenerator (S-PENG) was produced to convert fluctuating solar energy into electrical energy.The generator was incorporated into rotating windmill blades to harvest a fluctuating solar energy source without the need for devices to adjust the intensity of the external light source. [85]As shown in Figure 4d, this novel device structure combined a pyroelectric PVDF polymer layer with a solar-thermal layer that was based on Au plasmonic nanoparticles, which were located on polyethyleneimine (PEI)-modified graphene oxide (GO) (Au@rGO-PEI).This structure enabled solar-thermal conversion by exploiting a localized surface plasmon resonance that enhanced light absorption and subsequent pyroelectric conversion.The structure was able to produce a maximum output power of 940 μW m −2 , which is 35-fold higher than previously reported pyroelectric generators.The device can be seen in Figure 4e where the temperature of the rainwater leads to a fluctuation in temperature between 15 and 35 °C, leading to a maximum open circuit voltage of 10 V, as shown in Figure 4f.
Deepshikha et al. [84] proposed a light harvesting bilayer structure that consisted of a plasmonic absorber that was formed using gold nanoparticles (Au NPs), which were then combined with an organic semiconductor framework, namely polymerized aniline-crystalline Rubrene (PPA-CRB) to provide a pyroelectric response for light harvesting.The experimental results demonstrated that the harvesting device exhibited an enhanced photoinduced pyro-current of 0.02 μA for a hybrid PPA-CRB/Au device and provided a rapid response toward the optical signal at short circuit, with a rise time of 7 ms and a decay time of 10 ms due to the surface dependent polarization and associated pyroelectric effects at the interface of the plasmonic Au nanoparticles and the PPA-CRB.This experimental approach enabled the range of plasmon resonance absorption bands of Au nanoparticles to be extended from the UV to NIR, thereby making it of interest for fast and self-powered broadband light harvesting applications.

Infrared Detectors and Photodetectors
When pyroelectric materials are used as an infrared detector, they produce electrical charge in response to the change in temperature as a result of the absorption of the incoming electromagnetic wave. [39]Plasmonic nanoparticles and pyroelectric materials have therefore been combined in an effort to provide improved infrared sensing.
One example of combining plasmonic and pyroelectric materials is based on the development of gas sensors that operate on the concept of non-dispersive infrared (NDIR) spectroscopy.NDIR spectroscopy does not require dispersive optical elements, such as gratings or prisms, to identify the spectrum of the measured substance.Instead, it relies on a pre-existing knowledge that the target gas emits or absorbs light, within a given spectral window.Therefore, measuring the electromagnetic energy within this spectral window can serve to identify the presence of the target substance. [86]To simultaneously sense multiple target gases and reduce the size of the non-dispersive infrared (NDIR) sensor, a multispectral-array sensor was developed using a metal-insulator-metal structure that exploited a plasmonic metamaterial absorber.To achieve a multi-spectral capability, the plasmonic-pyroelectric absorber combined plasmonic Au nanodisk arrays of different sizes (from 0.68 to 2 μm) with a pyroelectric lithium tantalate thick film, as shown in Figure 5a.Experiments showed that by changing the size of the nanodisk arrays, and their period of oscillation, the system was able to detect eight different wavelengths and target gases; this included H 2 S (2.64 μm), CH 4 (3.27 μm), CO 2 (4.26 μm), CO (4.67 μm), NO (5.26 μm), H 2 CO (5.73 μm), NO 2 (6.20 μm), and SO 2 (7.35 μm).The detector has the potential to be further improved by incorporating dielectric nanoantennae in the upper layer to increase absorption.It could also be adapted to sense more gases, using a narrowband detector array. [86]nother application in near-infrared sensing employed Au nanocages (NCs) as plasmonic nanostructures.In this case, the plasmonic nanoparticles were also used to aid in the fabrication of micropatterned ferroelectric domains in the thermal sensor by inducing a -phase to -phase ferroelectric-paraelectric transition at the Curie point (T c ) of a pyroelectric polymer PVDF thin film, see Figure 5b.The plasmonic Au NCs were mixed with pyroelectric PVDF to produce thin films.Plasmonic heating of the nanocages led to a localized temperature increase in the vicinity of the Au nanocages, inducing a phase transition of the PVDF (to the ferroelectric/pyroelectric -phase) within a few seconds. [87]nce manufactured, the plasmonic Au nanocage/PVDF composite films exhibited a larger increase in temperature compared to the pure PVDF film, with a 570% increase in generated voltage compared to a conventional sensing sandwich structure that had no plasmonic additives; this was due to a combination of plasmonic heating and the presence of the ferroelectric -phase PVDF.Using the same materials, Li et al. [88] created a hybrid mechanical (piezo) and thermal (pyro) sensor.The authors demonstrated that PVDF nanofibers containing plasmonic Au nanocages (Au NC/PVDF) achieved a 12.6-fold increase in tactile force sensing, as a piezoelectric material, and improved NIR sensing, compared to pristine nanocage-free PVDF.This improved behavior was also thought to originate from a combination of a greater -phase content due to the inclusion of Au nanocages in the PVDF nanofibers and the hybrid system effectively converting NIR to heat via the plasmonic nature of the Au nanocages.The experiments demonstrated an enhanced pyroelectric response for the Au NC/PVDF combination compared to pure PVDF, as seen in Figure 5c.In addition, the device exhibited good stability, Figure 5d, indicating a long working life cycle, with the Au nanocages being evenly distributed in the PVDF nanofibers, see Figure 5e.This work demonstrates that the Figure 5. Thermal/infrared sensing applications.a) Schematic of gas sensor, [86] b) structure of device for infrared imaging.Reproduced with permissions. [87]Copyright 2016, Wiley.c,d) Temperature change with time and e) infrared camera images with the 808-nm laser.Reproduced with permissions. [88]Copyright 2013, Royal Society of Chemistry.f) Infrared to thermal conversion model in Au electrodes, g) structure of pyroelectric sensor, [90] h) structure of pyroelectric device, and i) ionic thermoelectric (iTE) device, j) integrated pyro-iTE device. [89]ddition of plasmonic nanocages to a pyroelectric polymer can not only induce local heating to provide a larger pyroelectric response but can also be used for micropatterning during processing by exploiting the localized heating of the plasmonic nanoparticles.
More recently, a thin film poly(vinylidene fluoridetrifluoroethylene) ferroelectric copolymer (PVDF/TrFE) with interdigitated comb-like electrodes (IDT) was prepared by controlling the level of polarization via an electric field that was applied during annealing. [90]The IDT structure and resulting infrared sensor are shown in Figure 5f,g, respectively.The infrared illumination of the IDT structure led to localized heating, aided by plasmonic photothermal heating from the Au electrode that triggered a pyroelectric response from the ferroelectric PVDF/TrFE.This device and electrode structure also avoided dielectric breakdown when subjected to a high electric field; for example, during poling.The measured voltage sensitivity was 242 V/W, which was higher than that of a conventional sandwichtype sensor (72 V/W).Clearly, this improved performance was due to an enhancement in infrared absorption and a conversion of light to heat, resulting from a surface plasmon resonance within the Au-based IDT structure.
Accurate heat sensing capability is essential for electronic skin applications in wearable healthcare and robotics.Recently, Chaharsoughi et al. proposed a new approach that used a gold nanohole film as a plasmonic heating layer, to produce a thermodiffusion-assisted pyroelectric. [89]This approach was aimed at overcoming the lack of a signal (voltage or current) from a pyroelectric at thermal equilibrium (dT/dt = 0), as outlined in Figure 2b.Specifically, the system consisted of a pyroelectric film that was sandwiched between two Au electrodes (Figure 5h).It included a plasmonic Au nanohole metasurface as a lower electrode.The electrode was topped with an ionic thermoelectric layer (see Figure 5i) that provided selective ion thermal diffusion.Ion-electron interactions were formed between the pyroelectric and thermoelectric layers.The thermal diffusion of ions due to heat flow led to a change in the interfacial electrode charge, contributing to a thermoelectric reaction.During the rapid changes in temperature, induced by plasmonic heating of the Au nanohole metasurface, the PVDF-TrFE pyroelectric film was able to produce a rapid transient voltage signal.In addition, a coupling between the ionic thermoelectric layer and the pyroelectric layer provided a stable voltage signal when the system was at thermal equilibrium.As a result of these combined effects, the plasmonic-pyroelectric integrated device (see Figure 5j) demonstrated that it not only produced a rapid response, but also provided a stable and enhanced signal after radiation-induced heating, compared to the pure ionic thermoelectric device.This work holds promise for future "e-skin" applications to detect transient (and static) light signals and temperatures.
Plasmonic-pyroelectric systems have also been explored as photodetectors, including those that require a frequency selective response, where absorption by a plasmonic nanostructure can result in a large and rapid temperature change in a pyroelectric material to produce a high electrical charge (Equation 3), open circuit voltage (Equation 6) and current (Equation 4).Pyroelectric materials utilized to date include PZT, PVDF, PVDF-TrFE, PMN-PT, AlN, LiNO 3 , LiTO 3 , and ZnO, as summarized in Table 1.
By combining a pyroelectric PZT layer and a gold plasmonic crystal structure, a pyroelectric infrared detector based on a multilayer structure composed of Pt/PZT/Pt/Ta/SiNx/Si/SiNx was fabricated, where the manufacturing process is shown in Figure 6a. [92]It is worth noting that the gold plasmonic layer not only functioned as an absorber, but also functioned as an optical filter to provide a degree of wavelength selectivity.The experimental results demonstrated that the responsivity of this detector at wavelengths of 7.14 to 8.33 μm was enhanced, compared to a system without a gold plasmonic crystal structure.By integrating this detector with non-dispersive infrared spectroscopy, the limit of detection (LOD) for methane gas sensing achieved a 13-fold enhancement, compared to the device without a gold plasmonic structure.
In the visible and near infrared region with a wavelength of 0.66-2 μm, a plasmonic metasurface was coupled to a pyroelectric aluminum nitride (AlN) thin film to act as both an absorber and as a spectral filter; see Figure 6b.Combining these two materials enabled the detector to achieve a rapid 700 ps response time, which is close to six orders of magnitude faster than other thermal detectors reported for spectra selection. [97]Another approach by Kuznetsov et al. formed a detector based on a commercial pyroelectric film that was combined with a plasmonic metamaterial absorber structure to detect millimeter-wavelengths, as shown in Figure 6c. [115]However, the specific pyroelectric and plasmonic absorber materials utilized in the detector were not stated.
Figure 6d illustrates a photodetector that used an Au array as a metamaterial absorber that was combined with ferroelectric LiNbO 3 in thin film form, for incident longwave detection (8-12 μm); the hybrid combination resulted in an 86% efficient absorption and 28.9 ms thermal time constant. [94]Similarly, for longwave selection (1.6-16.7 μm), Pan et al. produced a detector based on a pyroelectric polymeric PVDF-TrFE thin film and a planar periodic gold array as a plasmonic metasurface.The detector provided a plasmonic metasurface-based absorber for long wavelength infrared (LWIR) spectral range. [91]For operation in the longwave infrared region, with a 332 nm wide spectra band, John et al. combined a pyroelectric AlN film with a plasmonic Au array of fine-scale holes to achieve 95% absorption and 18.9 dB of spectral selectivity. [96]For far-infrared detectors at THz wavelengths beyond 20 μm, Arose et al. investigated the factors that affected the design of a photodetector using simulations based on a plasmonic metal film with a periodic array of subwavelengths that was deposited on the surface of a range of pyroelectric materials, which included bulk LiTaO 3 and AlN films. [95]n addition, a number of detectors have been reported that are based on pyroelectric ZnO, which is attractive in this context since it can be readily formed in thin film, nanoparticle, or nanowire form.To achieve a compact, high-performance and low-cost detector with multispectral selectivity for modern spectroscopic applications, Doan et al. [99] designed a quad-wavelength detector, as seen in Figure 6e.This design integrated four different plasmonic absorbers based on an Al-disk-array that was placed on an Al 2 O 3 -Al bilayer; this array was combined with a ZnO thin film as the pyroelectric material.The device exhibited spectral selective infrared detection at wavelengths of 3.3, 3.7, 4.1, and 4.5 μm with a corresponding responsivity of 125, 150, 126, and 128 mV/W, respectively.These results demonstrate the Figure 6.Plasmonic-pyroelectric for spectral selective infrared detection.a) Microfabrication process for pyroelectric infrared detector based on Au coupled to PZT. [92] b) Schematic of plasmonic Ag absorbers coupled to AlN film.Scale bar: 400 nm.Reproduced with permission from. [97]Copyright 2019, Springer Nature.c) Metallic metamaterial coupled to commercial pyroelectric film. [115]d) Schematic of Au absorber coupled to LiNbO 3 film and corresponding simulation of optical power absorption temperature and DC electric field. [94]e) Schematic of structure of quad-wavelength detector (E and H refers to electric field and magnetic field, respectively). [99]otential of multispectral selective plasmonic-pyroelectric detectors for infrared spectroscopic device applications.
Figure 7a shows a hybrid ZnO-based plasmonic-pyroelectric detector that acted as an uncooled mid-wavelength infrared device.This device exhibited narrowband spectral selectivity by integration of a pyroelectric ZnO film with a plasmonic Au microhole array.The latter served as the upper electrode and a lower electrode was made from Pt. [80] Figure 7b shows a numerical simulation whereby the geometrical parameters of the holes in the upper Au electrode determine the absorption wavelength of the detector.When illuminated by 3.66 μm light, the plasmonically enhanced electromagnetic near-fields located at the surface of the Au hole array led to localized heating of the pyroelectric ZnO layer, thereby enhancing the responsivity of the detector.Another form of ZnO-based detector is shown in Figure 7c, which represents a multispectral array for light detection in the 3-5 μm range.In this case, Au metasurface absorbers were coupled to a pyroelectric ZnO film to enable the detection of four narrowband spectral responses at given plasmonic resonances. [100]o further enhance the responsivity of the photodetector, Basumatary et al. fabricated a structure as seen in Figure 7d, which integrated plasmonic Au nanoparticles as a substructure that was sandwiched between graphene oxide (GO) and ZnO.The latter helped with additional photo-induced charge carrier generation. [103]Due to the interband transition of Au nanoparticles, the plasmonic effect of Au was coupled with the pyrophototronic effect to enhance the photocurrent and responsivity of the photodetector.
A ZnO-based band-selective photodetector was presented by Zhu et al., which was based on pyroelectric ZnO nanowires and plasmonic Au and Ag nanoparticles.Again, this combination of materials aimed to enhance the spectral selectivity, based on the localized surface plasmon resonances (LSPR). [101]Figure 7e shows that upon illumination by 405 nm light, the photoresponsive current of the detector was both rapid and large in magnitude, as indicated by the I pyro+photo+LSPR that was produced by p-Si/metal plasmonic nanoparticle/ZnO nanowire photodetector.This high current was obtained due to interfacial plasmainduced hot electron injection and scattered electrons-phonons coupling that produced heat, as outlined in Section 2.2.This optical-thermal coupling effectively enhanced the intensity of the pyroelectric charge, indicating a potential application in multispectral sensing.
More recently, to detect UV and visible light, Li et al. demonstrated a photodetector based on integrating pyroelectric ZnO/CuO core-shell nanorods and plasmonic nanoparticles; the fabrication process can be seen in Figure 7f. [102]Specifically, the ZnO seed layer was initially deposited on the substrate and grown to form pyroelectric ZnO nanorods via a hydrothermal method.The ZnO nanorods were subsequently coated with CuO through a reaction in a solution.Each CuO-coated ZnO nanorod was finally decorated with Au nanoparticles, and silver paste, where a fluorine-doped tin oxide (FTO) substrate was employed as the upper and lower electrode.The experimental result demonstrated that the optical responsivity and detectivity upon irradiation with 325 nm light were 1.4 × 10 −4 AW −1 and 3.3 × 10 11 Jones, respectively.When illuminated by 532 nm light, at a variety of power densities, the current delivered by the CuO/ZnO/Au nanoparticle photodetectors and the corresponding pyroelectric current in-duced by a pulse light are shown in Figure 7g.Finally, Wang et al. fabricated a photodetector based on pyroelectric ZnO nanowires coated with Au nanoparticles, [79] where the corresponding scanning electron microscopy images can be seen in Figure 7h.Compared to ZnO-based photodetectors, the response time was reduced to 50 μs, and the corresponding responsivity and detectivity of the Au-coated ZnO-based detector increased by 212% and 266%, respectively.
The examples highlighted above demonstrate the significant potential of combining plasmonic and pyroelectric materials to enhance photodetectors.Important parameters that can be improved include large (ΔT) and rapid rate of temperature change (dT/dt) in the pyroelectric material, high voltage and current responsivities, rapid response times, as well as wavelengthspecificity by tailoring the dimensions of the plasmonic component.ZnO has been studied extensively due to its ease of fabrication, although materials with higher pyroelectric coefficients, such as ferroelectric materials, could also be examined to further increase performance.

Catalysis
Plasmonic noble metals, which include Au, Ag, Pt and Ni, have significant photocatalytic effects; they can strongly and selectively absorb light due to localized surface plasmon resonances. [117] particularly promising application to exploit plasmonicpyroelectric hybrids is in the field of catalysis to enhance pyrocatalytic effects, photo-catalytic effects, or combined effects.The applications include exploiting the oxygen reduction reaction (ORR), [104] CO 2 reduction reaction (CO 2 -RR), [116] degradation of a dye in solutions, [81,105] and the detection of biomolecules, [106,107] as summarized in Figure 8. Pyro-catalytic systems have been widely reported in applications related to the degradation of organic contaminants and the disinfection of bacteria in an aqueous environment, primarily due to their potential non-toxic nature and high efficiency. [118]The process of pyro-catalysis can be initiated by temperature fluctuations, leading to the generation of pyro-generated positive and negative charges that can facilitate reduction-oxidation (redox) reactions, to generate strong oxidant radical species on the surface of pyroelectric materials, thereby leading to the degradation of dyes in contaminated water or the destruction of microorganisms. [105]Liu et al. reported on particles, where a combination of pyroelectric and plasmonic materials showed an enhanced efficiency of Rhodamine B (RhB) degradation compared to either a random mixture of Au and BaTiO 3 nanoparticles, or pure BaTiO 3 nanoparticles. [105]A schematic of the reaction mechanism of pyro-catalytic dye degradation is shown in Figure 8a and the corresponding chemical equations are shown below: q − + Au → Au(q − ) ( 18) Reproduced with permission from. [80]Copyright 2016, American Chemical Society.c) Schematic of Au absorber coupled to pyroelectric ZnO film. [100]d) Structure of the device with GO/Au_np/ZnO film. [103]e) Current response of p-Si/Ag NPs/n-ZnO under 405 nm light illumination.Reproduced with permission from. [101]Copyright 2021, Elsevier.f) Schematic of preparation of vertically aligned ZnO/CuO/Au nanoparticle core/shell photodetector.g) Photocurrent of CuO/ZnO/Au nanoparticle photodetector under 532 nm irradiation.Reproduced with permission from. [102]opyright 2021, Wiley.h) SEM image of Au-coated ZnO NWs.Scale bar: 100 nm. [79]Reprinted with permissions from. [105]Copyright 2019, American Chemical Society.c) Pyroelectric nanogenerator with plasmonic layer (graphene@Ag nanowires) on a PVDF film.d) Current of pyroelectric generator with four plasmonic layers under different light levels (0.75, 1.125, 1.5, 1.725, and 2.4 W), inset refers to temperature distribution.Reprinted with permissions from. [104]Copyright 2021, Elsevier.e) Mechanism of pyro-catalysis and photo-catalysis synergy.Reprinted with permissions from. [81]Copyright 2022, Elsevier.f) Schematic of system g) DNA-based biomolecules uracil detection through SERS by an externally electric field applied to the template surface. [106]h) Photocatalytic mechanism of BP/WO heterostructures.ET and TR refer to electron transfer and thermal radiation, respectively.Reprinted with permissions from. [116]Copyright 2022, Elsevier.
When subjected to a temperature change, the generated electrons and holes from the pyroelectric BaTiO 3 accumulate on the surface of the BaTiO 3 particles (Equation ( 17)).The electrons are subsequently transferred from the conduction band of BaTiO 3 into the Au nanoparticles since the conduction band of BaTiO 3 is higher than the Au potential, [105] thereby forming a Schottky barrier at the metal-semiconductor contact region for improved charge separation (Equation ( 18)). [119]The electrons accumulated on the surface of the Au nanoparticles can be rapidly transferred to surface-absorbed oxygen to form activated superoxide radicals for pyro-catalysis (Equation ( 19)).Similarly, the holes in the valence band of BaTiO 3 combine with surface-adsorbed hydroxyl ions (OH − ), thereby forming hydroxyl radicals (•OH); see Equation (20).These strongly oxidizing superoxide radicals ( • O − 2 ) and hydroxyl radicals (─OH) can initiate the decomposition of dyes in a solution through electrochemical oxidation reactions, as a result of pyro-catalytic effects (Equation ( 21)).The experimental results showed that the pyro-catalytic degradation efficiency of the Au-BTO hybrid NPs could reach ≈95% after 4 h of thermal cycling; see Figure 8b.
In addition to pyro-catalysis, applications being considered include photo-catalysis using ferroelectric materials or plasmonicsemiconductors to act as a highly efficient, clean, and environmentally friendly catalytic technology for wastewater treatment.During photo-catalysis, electron-hole pairs are generated on the surface of the catalyst by illumination, which subsequently combine with oxygen species to attack dye molecules. [120]Li et al. prepared a pyroelectric nanogenerator (NG) based on a surfaceenhanced Raman scattering (SERS) substrate by combining a pyroelectric PVDF film with a plasmonic layer of graphene@Ag nanowires (NWs), [104] as shown in Figure 8c.The plasmonic Ag nanowires acted as the absorption layer, which transferred heat to the pyroelectric PVDF.By virtue of their localized surface plasmon resonance, the Ag nanowires also contributed to surfaceenhanced Raman scattering.The pyroelectric PVDF film was able to convert transient heat from the Ag nanowires into electrical energy via the pyroelectric effect.This dual effect resulted in an increase of the local charge density on the surface of the plasmonic layer, which in turn increased the SERS signals to drive the oxidation reaction and to form oxygen reduction reaction intermediates.The preserved time of the electric potentials was increased to approximately 100 s, providing sufficient time for SERS detection.With an increase in light intensity, both the output current and the corresponding temperature distribution of the nanogenerator increased, as shown in Figure 8d.
Based on the dual effects mentioned above, direct Z-type heterojunctions which consisted of pyroelectric Ba 0.8 Sr 0.2 TiO 3 (BST) nanotubes and plasmonic nanoparticles of Ag 2 O to form an Ag/Ag 2 O semiconductor structure were prepared by an electrospinning and co-precipitation method. [81]Electrons and holes, induced by the pyroelectric BST, and the photo-induced electrons and holes, formed by the Ag nanoparticles, were thought to be separated efficiently.The degradation efficiency of an organic dye, reactive brilliant red (Rhr-xb), reached 100% after only 20 min under visible light due to the combined effects of pyrocatalysis and photo-catalysis; see Figure 8e.
The combined plasmonic and pyroelectric systems described above are based on dielectric pyroelectric materials, rather than pyroelectric semiconductors.However, by using highly aligned diphenylalanine peptide nanotubes (FF-PNTs) as a wide bandgap semiconductor and Ag nanoparticles (NP), [106] researchers have prepared a plasmonic-pyroelectric semiconductor system, as shown in Figure 8f.They investigated the impact of applying an external electric field on the catalytic oxidation of paminothiophenol (PATP) to pnitrothiophenol (PNTP) by SERS detection, when illuminated with 532 nm light.Figure 8f shows the SERS detection spectra of PATP on an FF-PNT/Ag nanoparticle template at a variety of electric potentials, from 0 to 10 V, which indicates that the application of a sufficient potential (and an associated electric field) can enhance the catalytic process of the template.Specifically, when illuminated by a laser, the heat generated from the plasmonic Ag nanoparticles was able to enhance the photo-catalytic reaction, where electron-hole pairs are generated on the surface of the catalyst.In addition, upon application of a bias electric field, the pyroelectric FF-PNTs operate in a dielectric mode, in which the pyroelectric effect is proportional to the applied electric field strength.The temperature of FF-PNTs therefore increased linearly with electric field strength, facilitating charge transfer to the Ag nanoparticles.Using this material, it was possible to detect a series of biomolecules, such as uracil (see Figure 8g), and achieve an eight-fold increase in SERS intensity.
Plasmonic semiconductor heterostructures composed of pyroelectric black phosphorus (BP) and plasmonic tungsten oxides (WO) have also been fabricated for CO 2 reduction, [116] where the operating mechanism of the heterostructures is outlined in Figure 8h and the plasmonic properties of WO have been reported in Refs.[110,112,116] Upon irradiation by visible and near-infrared (NIR) light, the temperature was increased by the plasmonic WO, thereby triggering the pyroelectric response of black phosphorus and the generation of pyroelectric carriers.This process promoted electron transfer from the pyroelectric black phosphorus to WO and increased the electron concentration for strong SPRs.Reducing CO 2 , the experimental results showed that the heterostructures produced 26.1 μmol h −1 g −1 CO with a selectivity of 98%, which is 7 and 17-fold higher than that of pure WO and black phosphorus, respectively.
More recently, to increase the pyro-catalytic production rate, You et al. [108] synthesized 3D hierarchically structured coral-like BaTiO 3 nanoparticles via a hydrothermal method.The BaTiO 3 nanoparticles acted as the pyro-catalytic material and were decorated with Au nanoparticles to act as plasmonic heat sources, thereby forming Au/BaTiO 3 plasmonic/semiconductor nanoreactors to operate over multiple thermal cycles.The experimental results showed that a high hydrogen production rate of 133.1 ± 4.4 μmol g −1 h −1 under pulsed laser irradiation of 532 nm was achieved.Plasmonic-pyroelectric hybrid systems have therefore attracted interest in the quest to enhance pyro-catalytic effects, photo-catalytic effects, or combined effects, although more research on the specific catalytic mechanism is required.Next, we consider the production of reactive oxygen species for bioapplications.

Bio-Applications
Photothermal therapy involves the use of photo-absorbing agents to produce heat from light, for medical treatment.The generated heat can lead to thermal destruction of cancer cells. [121]o apply a photodynamic therapy (PDT) for hypoxic tumor treatment, a plasmonic-pyroelectric heterostructure was designed to generate O 2 -independent reactive oxygen species (ROS). [82]The system was based on a localized surface plasmon resonance effect that used gold nanorods embedded in pyroelectric barium titanate (BaTiO 3 ) by forming Au@BTO core-shell nanostructures (CSNS); see Figure 9a.Upon irradiation with an 808 nm laser, the plasmonic Au nanorods were able to transfer heat to the pyroelectric BaTiO 3 shell, to raise its temperature, which resulted in a decrease in the polarization of BaTiO 3 and increased the number of holes released from the BaTiO 3 shell surface.Thereby, O 2 -independent HO• reactive oxygen species were formed.In addition, computed tomography (CT) images demonstrated that the pAu@BTO CSNS accumulated in the tumor, inside injected mice; see Figure 9b.It was further shown that, under 808 nm laser irradiation, the pAu@BTO CSNS were able to effectively kill tumor cells and inhibit tumor cell proliferation (Figure 9c).
Another photothermal application used a BaTiO 3 @Au coreshell nanostructure for breast cancer therapy. [109]The manufacturing process for the nanostructure is shown in Figure 9d.At equilibrium, the spontaneous polarization by the BaTiO 3 core and Au shell is presented Figure 9e.When illuminated by an 808 nm laser, the plasmonic Au shell absorbs NIR light and generates heat.The polarization of the BaTO 3 core is then reduced, producing charge that can react with H 2 O. Electrons react with O 2 to generate O 2 − reactive oxygen species, as seen in Figure 9f.A polyethylene glycol coating can also be used to enhance the dispersion in a physiological solution and to generate a large amount of reactive oxygen species.The 808 nm laser irradiation thus destroys the mitochondrial electron transport chain, inhibiting mitochondrial oxidative phosphorylation (OXPHOS), cutting off the supply of lipids, and reducing the production of adenosine triphosphate (ATP) in triple-negative breast cancer (TNBC) cells (Figure 9g).

Conclusion and Outlook
We have seen that combining plasmonic and pyroelectric materials is attracting significant interest.For plasmonic-pyroelectric materials, the key enabling factor is that illumination at the plasmon resonances leads to large and rapid temperature variations that enhance the pyroelectric response.The plasmonic heating can also induce a phase transition in the pyroelectric, with potential benefits for micro-patterning of structures or enhanced charge generation.We have reviewed the underlying pyroelectric and plasmonic mechanisms, the range of material combinations employed, the variety of structures formed, and the potential applications of plasmonic-pyroelectric hybrids.We have seen in this review that potential technological implementations include energy harvesting, infrared detectors with possible wavelength selectivity and rapid response times, catalysts with combined photo-and pyro-catalytic effects, and novel photothermal therapy applications, as outlined in Figure 3. Based on this review, a number of challenges and future perspectives for developing highly efficient plasmonic-pyroelectric systems can be identified.
The overarching challenge for plasmonic-pyroelectric materials is to develop low-cost, large-scale devices that, upon illumina-tion, can rapidly heat up and achieve large changes in temperature to produce the desired electrical response.Fundamentally, these material combinations can transduce light into heat and then heat into electrical charge.They can therefore offer improvements to energy conversion efficiency.By improving our understanding of the energy conversion mechanisms, it is possible to optimize the performance of these materials, with the most obvious benefits for applications in energy harvesting, and sensing.To gain a deeper understanding and design tailored hybrid systems, it would be of interest to develop multi-physics modeling tools that can simulate plasmonic-pyroelectric combinations and device structures.Such simulations would provide valuable insights and aid in the optimization of these systems for each specific application.
An important challenge is to successfully combine the plasmonic-pyroelectric materials.Such combinations include forming plasmonic arrays, particles, and coatings on pyroelectric layers or particles, as well as creating composite architectures.Efficient control of the interface is also key, as the material response needs to be homogeneous.A large inhomogeneity can result in some parts of the material overheating, thereby generating heat in excess of what is required to produce the desired electric response and so wasting energy.Conversely, other parts of the material could be underheated, thereby failing to heat up to the minimum temperature required to produce the desired electric response.It is therefore necessary to be able to produce plasmonic particles with narrow distributions of size and aspect-ratio, that are positioned at regular distances from each-other throughout the pyroelectric.In addition, precise control of dimensions at the sub-nanometer scale provides a route to reliably tune the frequency-response and the absorption properties of the plasmonic particles.Such control in dimensions is also important in the research field of metasurfaces and metamaterials.Plasmonicpyroelectric materials can therefore greatly benefit from adopting fabrication techniques developed for this field.
There are also a number of opportunities that arise upon considering the selection of the pyroelectric material.For example, while the majority of plasmonic-pyroelectric materials combine nanoscale plasmonic and bulk pyroelectric constituents, there is potential to nano-structure the pyroelectric.Moreover, while the majority of pyroelectrics used to date have been based on noncentrosymmetric crystals, there is potential to engineer the interface, where the number of atoms can be too small to form a sufficiently thick polar layer for the accumulation of neutralization and depolarization screening charges.In this context, recent studies have shown that centrosymmetric crystals can also exhibit pyroelectric effects at their surfaces/interfaces, [84,[122][123][124][125] revealing a new route to combine plasmonic and pyroelectric effects.It is worth considering the electrochemical interactions between the plasmonic and pyroelectric constituents, where these interactions provide intriguing opportunities for photo-catalysis, [108] bio-applications, water splitting, and water treatment.
In many cases, plasmonic-pyroelectric materials need to be homogeneous, which requires fine control of the material dimensions over large sample regions.However, fine control over processing can also provide potential to purposefully-induce a degree of inhomogeneity.For example, non-symmetrical plasmonic layers could be used to produce temperature gradients, broadband operation, and generate shear stresses.Inside a Figure 9. Plasmonic-pyroelectric system for bio-applications.a) Schematic of plasmon resonance-induced pyroelectric effect and O 2 independent HO• formation in Au@BaTiO 3 (BTO) with therapeutic effect for hypoxic tumor.b) CT images of mice at 0 and 24 h injection c) representative photos of mice at the end of the treatment.Reprinted with permission from. [82]Copyright 2021, Elsevier.d) Preparation of BaTO 3 @Au core-shell nanostructure.e) Schematic of spontaneous polarization by Au shell at equilibrium state.f) Schematic of plasmonic-induced pyroelectric effect and reactive oxygen species (ROS) generation.g) Mitochondrial oxidative phosphorylation (OXPHOS)-inhibiting effect of pBaTO 3 @Au on triple-negative breast cancer TNBC cells.Copyright 2023, Springer Nature. [109]yroelectric, these inhomogeneities can create large piezoelectric shear coefficients, and this mechanism would lead to a new kind of plasmonic-pyroelectric architecture, one that would depend on the magnitude of the temperature gradient, rather than on the uniform temperature change.Similarly, if only a small region of a pyroelectric is heated by a plasmonic component, the active area may be clamped by the remaining pyroelectric element.There is therefore potential to exploit this clamping effect to reduce secondary pyroelectric contributions.[40] In terms of applications, plasmonic-pyroelectric material combinations are of particular interest because they offer multifunctionality.In addition to the solar energy harvesting, infrared sensing, catalysis, and bio-applications, that are discussed within this review, new multifunctional designs could be imagined that mix functionalities, such as combining both sensing and actuation functionalities.Plasmonic structures provide the potential to generate localized heat upon optical excitation, which induces thermal expansion in the pyroelectric material, leading to mechanical actuation.This mechanism could be useful in future Nanomechanics and Microelectromechanical Systems (MEMS) for nanorobotic applications.Moreover, since plasmonic materials tend to be metals, they can also serve for thermal management; nanostructured (plasmonic) metal structures can efficiently dissipate waste heat, while the pyroelectric material can convert it into usable electrical energy.Such novel metalpyroelectric combinations could enable thermal management and energy harvesting in a single material system.Furthermore, in nanoelectronics, plasmonic-pyroelectric nanomaterials could be used as light-sensitive parts of an electronic circuit, which enables the development of hybrid devices with enhanced functionality.By combining plasmonic-pyroelectric structures with transistors, diodes, or other electronic elements, it becomes possible to achieve active control within the circuit by illumination at the surface plasmonic resonance wavelength to produce a voltage.
Addressing the above-mentioned challenges and creating new opportunities through the development of new fabrication techniques, improved characterization methods, and computational modeling will contribute greatly to the advancement of plasmonic-pyroelectric materials and revealing their future applications.

Figure 1 .
Figure 1.Schematic of the pyroelectric effect.a) Polarization and dielectric constant versus temperature,[2,38] the polarization decreases with increasing temperature and reaches zero at the Curie point T c ; simultaneously, the relative permittivity increases with temperature, reaches a maximum at T c and then decreases sharply above T c ,.[2] Copyright 2014, Royal Society of Chemistry A pyroelectric material with a spontaneous polarization, b) at thermal equilibrium the polarization level is constant and there is no current flow since screening charges on the polar surfaces are balanced c) when heated, the polarization reduces as dipoles lose their orientation and screening charges are now free to produce a current and d) when cooled, the polarization increases as dipoles regain their orientation and a current flows in the opposite direction.[39]Copyright 2020, Elsevier.

Figure 2 .
Figure 2. Plasmonic heating.a) Schematic of a surface plasmon resonance.b) Diagram of a localized surface plasmon resonance.c) Timescales of the processes that lead to plasmonic heating.

Figure 3 .
Figure3.Range of applications of plasmonic-pyroelectric materials and structures surveyed in this review: energy harvesting,[78] infrared detector for thermal sensing,[79] photodetector,[80] selective PIR refers to spectrally selective pyroelectric detectors, the Max and Min refer to the simulation absorptivity of the plasmonic-pyroelectric detector (PA-PIR), catalysis to increase of degradation efficiency by generation of reactive oxygen, where hv refers to photon energy.Reproduced with permission from,[81] copyright 2022, Elsevier, and tumor therapy, BTO refers to barium titanate.Reproduced with permission from.[82]Copyright 2021, Elsevier.

Figure 4 .
Figure 4. a) Structure of device: the upper electrode is transparent indium tin oxide (ITO), and the lower electrode is an Au film.Plasmonic nanodisks were formed on the upper surface of the pyroelectric P(VDF-TrFE) copolymer.b) Experimental configuration for energy harvesting light fluctuations.Reproduced with permissions.[78]Copyright 2018, Wiley.c) Schematic of model (upper image) and plasmonic resonance map (lower image) of BTO-Ag for energy harvesting.Reproduced with permissions.[83]Copyright 2019, Wiley.d) Schematic of pyroelectric nanogenerator.e) Pyroelectric energy harvesting of temperature changes from rain.f) Open circuit voltage of pyroelectric nanogenerator under different temperature of rainwater.[85]Copyright 2022, Elsevier.

Figure 8 .
Figure 8. Hybrid systems in catalysis applications.a) Schematic of mechanism of pyro-catalytic degradation.b) Pyro-catalytic degradation efficiency for a dye solution.Reprinted with permissions from.[105]Copyright 2019, American Chemical Society.c) Pyroelectric nanogenerator with plasmonic layer (graphene@Ag nanowires) on a PVDF film.d) Current of pyroelectric generator with four plasmonic layers under different light levels (0.75, 1.125, 1.5, 1.725, and 2.4 W), inset refers to temperature distribution.Reprinted with permissions from.[104]Copyright 2021, Elsevier.e) Mechanism of pyro-catalysis and photo-catalysis synergy.Reprinted with permissions from.[81]Copyright 2022, Elsevier.f) Schematic of system g) DNA-based biomolecules uracil detection through SERS by an externally electric field applied to the template surface.[106]h) Photocatalytic mechanism of BP/WO heterostructures.ET and TR refer to electron transfer and thermal radiation, respectively.Reprinted with permissions from.[116]Copyright 2022, Elsevier.

Table 1 .
Plasmonic-pyroelectric material combinations examined in the literature, and the range of illumination sources and applications.