Semiconductor nanomaterial-based polarized light emission: From materials to light emitting diodes

The overall optical efficiency of backlight-based liquid crystal displays (LCDs) is less than 5% due to the loss of backlight source by polarizers, color filter, liquid crystal layer and so on. Self-emissive light emitting diodes (LEDs) have been undergoing huge development due to their substantial market potentials to meet the demand of future display. More importantly, the polarized LEDs could enhance the energy utilization efficiency by avoiding light loss caused by polarizers. Therefore, it is desirable to look for effective methods to assemble high-quality anisotropic nanomaterial films so as to fabricate polarized LEDs with high degree of polarization and external quantum efficiency. Here, the photoelectrical properties of some semiconductor nanomaterials and their potential applications for polarized LEDs are introduced. The research progress in the field of polarized light emission from materials to films and then to LEDs is reviewed. Mechanisms of polarized emission, and different assembly strategies for polarized light emitting films and LEDs are also summarized and compared. Finally, several current challenges are discussed, and perspectives on future potential commercial application of polarized LEDs are offered. We hope this review will provide a valuable summary on current status and stimulate some new insightful ideas for future development of polarized LEDs.


INTRODUCTION
It has been estimated that the display market may grow to over USD $200 billion by 2031 [1][2][3][4]. The display screens used for TV, laptop, mobile phone, tablet and smart watch take advantage of numerous technologies such as liquid crystal display (LCD), light emitting diode (LED), and organic LED. Since 1980s, LCDs have been utilized as the main display device [5,6]. Traditional LCDs rely on backlight unit display technology, which requires complex multi-layer structures, and among them vertical and horizontal polarizers are especially essential in LCD module (Scheme 1a) [6][7][8][9]. Unfortunately, the overall optical efficiency of the LCD system from the backlight source is less than 5% [7,8,10]. The polarizers and color filter are the two main LCD components responsible for the low utilization of backlight source [7,[11][12][13]. In order to achieve more smart and precise brightness adjustment, the backlight unit of LCDs has been developed from white lamp to white LED, and then to more and smaller mini-LED backlight partitions [14][15][16]. Nonetheless, the color gamut and color resolution of LCDs are still far from satisfactory [17,18]. Developing new displays with high color purity, high efficiency, high resolution and wide color gamut range is an urgent issue to be addressed in the future display field.
Semiconductor nanomaterials, such as II-VI colloidal and perovskite nanomaterials are emerging as excellent luminescent materials in color display devices, due to their unique optical properties such as easily tunable emission color, narrow and symmetric photoluminescence (PL) peaks, high color purity, high PL quantum yield (PLQY), as well as their competitive lowcost solution synthesis methods [1, 5,7,8,[18][19][20][21][22]. Currently, the quantum dot enhancement film (QDEF) consisting of green and red emitting semiconductor nanomaterials dispersed in a polymer film has been widely used in backlight unit for LCD [5,7,8,18,[23][24][25]. The QDEF plays a very important role as a photoconversion layer that can absorb blue backlight and convert the blue light into green and red light with high color purity, improving the color gamut of LCD. Sony, Samsung, TCL and LG released several flagship devices equipped with QDs to their display markets [1]. However, the QDEF still caused a quite high emitting-light power loss (>10%) due to the light absorption loss from the polymer of QDEF [5].
In addition, the process of generating polarized light in the LCD system will cause energy loss of backlight, because only the electric field parallel to the polarizer filter can pass through [7,9,11]. Therefore, the use of polarized light source can reduce light power loss when passing through the vertical polarizer filter [1]. Compared with QDs, the anisotropic nanomaterials, such as one-dimensional (1D) nanorods (NRs), 1D nanowires (NWs) and 2D nanosheets (NSs), have the advantages of low non-radiative recombination rate, high electrical conductivity, good stability, and especially good polarized light emission [13,[26][27][28][29][30][31]. The anisotropic nanomaterials being randomly arranged in macro-scale have no polarized light emission properties, but the film of orientation-aligned anisotropic nanomaterials can exhibit polarization characteristics [32]. Instead of QDEF, if we integrate aligned anisotropic nanomaterials EF (ANEF) in the LCD backlight unit, the polarized light can be obtained (Scheme 1b) [9,33]. The degree of polarization (DOP) is a very important parameter for nanomaterials and ANEF in practical application [34]. The direct cause of polarized light emission for nanomaterials is that their radiative transition dipole moments tend to be distributed along a certain direction [1, 32,[35][36][37]. Nanomaterials with anisotropic shapes usually exhibit higher DOP. This is because anisotropic nanomaterial can break the spherical symmetry, enabling polarized light emission along the long axis direction [7,26,[38][39][40]. Based on above arguments, in order to obtain a high DOP in film emission, we need to use anisotropic nanomaterial emitters with high DOP, and align the anisotropic nanomaterials in a highly ordered and compact manner in a large area.
Different from backlight-based display technology in LCDs, the self-emissive QD LEDs are targeted to meet the demand of future display for high efficiency, high color purity, high resolution and wide color gamut (Scheme 1c) [41][42][43][44][45][46]. The external quantum efficiency (EQE) of red and green QD LEDs progressed significantly and achieved over 20% rapidly [43,44,47]. However, the metal electrodes in LEDs will reflect ambient light, which severely affects the display effect. Therefore, the circular polarizers are required to suppress ambient light reflection from the surroundings for high image contrast and outdoor visibility. Nevertheless, the use of circular polarizers loses more than 50% of the light energy from self-emissive QD LED. Fortunately, the construction of polarized LED could reduce energy loss, showing great promise in flexible displays and large-scale integration (Scheme 1d). Because the polarized LED based on orientation-aligned anisotropic nanomaterials can directly emit polarized light [9,[48][49][50][51][52], when the polarized light is incident at a suitable angle relative to the main axis of the circular polarizer (0°or 90°), only a slight energy loss will be caused after passing through the circular polarizer. Though various approaches have been developed to arrange anisotropic nanomaterials into ordered patterns, the DOP and EQE of polarized LEDs are still limited due to the low DOP of films, the limitation of preparation methods, the poor quality of films and so on. Therefore, it is more desirable to look for effective aligning methods compatible with the standard LED fabrication process so as to fabricate polarized LEDs with high DOP and EQE.
Taking into account the potential importance of polarized light emission in future display market, a timely and thorough overview with the current literature related to polarized light emission is highly desirable. Therefore, in this review, we give a comprehensive summary on the state-of-the-art progress in the field of polarized light emission from materials to films, with a particular emphasis on polarized LEDs (Fig. 1). We begin with the theories of polarized light emission and analyze the factors that influence DOP. Sequentially, the polarized light emission properties from representative materials of QDs, NRs, NWs and NSs are described. Then, the assembly strategies for constructing polarized light emitting films and LEDs are summarized. Finally, we discuss the current challenges such as effective aligning methods compatible with the standard LED fabrication process, polarized light emission and charge carrier transport mechanisms for polarized LEDs, and give a brief conclusion and outlook Scheme 1 The structure schematics of (a) traditional LCD system, (b) LCD system with ANEF, (c) QD LED, and (d) polarized QD LED.

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on the potential commercial application prospect of the polarized LEDs. We hope this review would provide a valuable summary for current status of this research direction and stimulate some new insightful ideas for future development of this promising field.

POLARIZED LIGHT EMITTING MATERIALS
Polarization is a special physical property of light, which has significant application value not only in display, but also in imaging and information storage [53][54][55][56]. Some nanomaterials especially the anisotropic nanomaterials exhibit excitation and emission polarization [57,58]. To evaluate the effectiveness of polarized light emitting materials, some characterization definitions and mechanisms need to be quantified and well understood. In this section, the key parameters and influencing factors of polarized luminescence will be briefly introduced, and polarized light emission properties from QDs, NRs, NWs and NSs will also be summarized and compared.

Mechanism for linearly polarized light emission
The DOP (P) and anisotropy (R) value are important parameters used to measure the polarization ratio, which are defined as [34] (2) where I ∥ and I ⊥ are the light emission intensities parallel and perpendicular to the chosen alignment orientation, respectively. Normally, I ∥ is also the maximum intensity of the polarized light, and I ⊥ is the minimum. The value of P must be between 0 and 1, where P = 0 means that the material shows no polarized light emission. We pursue higher values of P for practical applications. When measuring the polarized light intensity, a rotatable linear polarizer needs to be placed between the material and the spectrometer in order to allow the polarized luminescence to pass through the polarizer. Then, the polarization angledependent luminescence signals can be obtained by the rotation of the polarizer periodically. And the DOP can be calculated via Equation (1). The size and morphology, in particular aspect ratio (AR) of the materials, are of vital importance to polarized light emission. It was reported that the nanomaterials with AR values more than 1.2 can realize partially polarized light emission [1, 35,57,[59][60][61].
In general, the polarized light emission is determined by anisotropic effective transition dipole moments, dielectric confinement of optical electric field, the splitting of the exciton fine structure and so on [32,35,[62][63][64][65][66][67][68]. The polarized light emission mechanism for colloidal NRs is similar with that of core/shell colloidal NRs. Current polarized light emission mechanisms mainly focus on anisotropic core/shell colloidal nanomaterials [35].
For anisotropic 1D colloidal nanocrystals (NCs), their radiative transition dipole moments tend to be distributed along a certain axis, which is the direct cause of their polarized luminescence. Talapin et al. [64] demonstrated that efficient shape control may be achieved in the shell of colloidally grown semiconductor NCs, exhibiting high linearly polarized emission. The lowest excited state of wurtzite spherical NCs is generally an Atype exciton, which is optically forbidden along the c axis, whereas emission occurs from a higher B-type state with a transition dipole moment orthogonal to the c axis of the wurtzite structure. A non-spherical shape can lead to a swapping of the two states, with a strongly allowed dipole resulting in the direction of growth (c axis) due to the perturbation induced by the crystal field [64].
Another reason for linearly polarized luminescence of 1D colloidal NCs is the dielectric confinement effect. It originates from the difference in the dielectric constant of 1D colloidal NCs and the environment in which they are located. According to the theory of Wang et al. [65], when the lateral size of 1D nanomaterials is larger than 10 nm, since the size has exceeded the exciton Bohr radius of the same component bulk material, the quantum confinement effect will become weak. The classical electrodynamic theory can give a good explanation of the observed polarization anisotropy. The electric field strength in the 1D nanomaterial is determined by the dielectric constant of the 1D nanomaterial and the external environment, which can be expressed as where E i is the electric field in the 1D nanomaterial, E e is the excitation field, ε and ε 0 are the dielectric constants of the 1D nanomaterial and the external environment, respectively. According to the above expression, when the nanomaterial with a high dielectric constant is in an external environment with a low dielectric constant, if the incident field is polarized parallel to the nanomaterial, the electric field inside the nanomaterial will not reduce. But when polarized perpendicular to the nanomaterial, the electric field amplitude will attenuate [65].
The splitting of the exciton fine structure is another important reason for linearly polarized luminescence of 1D colloidal NCs. The spherical symmetry of the dot-like core should lead to a band-edge exciton structure with the same symmetry as for the spherical NCs, which show a low DOP for the emission. Vezzoli et al. [35] gave a detailed discussion on polarized emission and the fine structure of CdSe/CdS dot-in-rods made of a spherical core of CdSe surrounded by a rod-like shell of CdS. They presented a model of the polarized emission of CdSe/CdS dot-inrods including the band-edge exciton fine structure, the shell anisotropy and the measurement configuration. They showed that the DOP at room temperature was closely related to the fine structure. As can be seen in Fig. 2a, the valence band of CdSe consists of three sub-bands: the heavy hole band, light hole band and split-off band. The net-splitting ∆ is the energy splitting at k = 0 of the heavy hole and light hole bands. The originally eightfold degenerate band-edge exciton ground state (1S 3/2 1S e ) of CdSe NCs splits into eight fine structures of states |±2>, |±1 L >, |±1 U >, |0 L >, and |0 U > [36]. The superscripts L and U are used to distinguish the sublevels with same projection but different total angular momenta. As shown in the middle of Fig. 2a, the U states have higher energies than the L states. It has been shown that a level swapping of the fine structure appears for a certain AR of the CdSe NRs. It can be explained by the band structure of CdSe, where an inversion of the heavy hole and light hole subbands energy ordering at k = 0 implies a change in the fine structure level ordering.
The optically forbidden |±2>states have no contribution to room-temperature emission. The |0 L > state is also optically inactive. Therefore, the room-temperature emission is mixed with recombination from the |0 U > state and from the degenerate |±1 L >, |±1 U >states [36]. The |0 U > state is related to a linear 1D dipole that oscillates along the c-axis of the crystal and emits linearly polarized photons. The |±1 L > and |±1 U > can be seen as 2D dipoles and they oscillate inside a plane. Because of the level degeneracy, the emission is an incoherent superposition of σ + and σ − components, and the corresponding dipole is called a degenerate 2D dipole [69][70][71]. The two types of dipoles and the polarization of their far field emission are shown in Fig. 2b. These dipoles are contained in the plane perpendicular to the caxis of the crystal, so the polarization azimuths for the two oscillators are mutually perpendicular. Therefore, the DOP for the total emission strongly depends on the relative oscillator strengths of two oscillators mutually oscillating in an orthogonal plane [1]. Fig. 2c presents the fine structure levels calculated for the DOP values of 0.38, 0.50 and 0.62.
Bai et al. [37] revealed the role of AR in the PL emission of single CdSe/CdS dot-in-rods using single-dot PL spectroscopy. They modeled the interrelation between DOP and AR by calculating the band-edge exciton fine structure. Fig. 2d shows the fine structure energy levels of CdSe/CdS dot-in-rods with different ARs. The |±1 L >state contributes little to DOP, while the |0 U > and |±1 U > states contribute more significantly in determining the DOP. They found that when the AR reached 7, the contribution of the dielectric effect to the DOP became limited. The strain effect between the CdSe core and the CdS shell can change the ordering of the energy levels and the populations of emission states, thus increasing the DOP. Therefore, the DOP increased from 0.23 to 0.58 as the AR increased from 7.1 to 40.9.
With regard to perovskites, Yin et al. [72] studied bright exciton fine structure splitting in single perovskite CsPbI 3 NCs. Ramade et al. [73] investigated the fine structure of excitons and electron hole exchange energy in CsPbBr 3 single NCs. Folie et al. [74] studied the dynamics of band edge excitons in inorganic perovskite NWs. These studies proved that the polarized light emission from perovskites is also determined by the splitting of the exciton fine structure, anisotropic effective transition dipole moments, dielectric confinement of optical electric field and so on.

QDs
In general, compared with QDs, the anisotropic nanomaterials with larger AR values such as 1D NRs, 1D NWs and 2D NSs tend to have higher DOP [39,75]. However, the QDs can also emit polarized light in some cases. Wang et al. [39] synthesized CsPb(Br x I 1−x ) 3 full inorganic perovskite QDs by changing the halide ratio and systematically studied polarized PL properties from these QDs for the first time. When x = 0, the DOP of CsPbI 3 was as high as 0.36 in solution and 0.40 as a film. Fig. 3a shows the experimental setup for polarized PL measurement. Fig. 3b, c show the polarization properties for CsPbI 3 and all the CsPb(Br x I 1−x ) 3 QDs in hexane solution. The change of polarized emission intensity for CsPbI 3 QDs is consistent with the sine function. The DOP of CsPb(Br x I 1−x ) 3 QDs improves with the increase of iodine amount. This may be due to the following two reasons. On one hand, the CsPbBr 3 perovskite belongs to the cubic crystal structure, while the CsPbI 3 perovskite is distorted cubic structure. The distorted cubic structure can break the space inversion symmetry and result in an asymmetrical structure, so as to enhance the polarization properties [40]. On the other hand, the CsPb(Br x I 1−x ) 3 QDs are highly ionized, which facilitates self-organization forming of ordered packing structures in hexane. Shinde et al. [75] reported the structural distortions and polarized light emission from CsPbBr 3 nanocubes with different sizes, and indicated that smaller sized CsPbBr 3 nanocubes have higher polarized emission efficiency.
In addition to the full inorganic perovskite QDs, some organic-inorganic hybrid perovskite NCs also show polarized emission effects. The MAPbI 3 NCs exhibit a DOP of 0.28, which may be caused by the uniform alignment and distorted crystal structure of NCs [76]. Liu et al. [77] observed the linearly polarized emission in single FAPbBr 3 NCs at room and cryogenic temperatures. They speculated that the polarization phenomenon is attributed to the large energy-level splitting of the bright-exciton states, which leads to efficient exciton recombination from the lowest state with a 1D dipole moment.
The crystal structure of perovskite QDs can change in some solvents. Sun et al. [78] discovered that the polar solvent molecules can induce the phase transition of CsPbI 3 nanocubes from cubic to orthorhombic phase. Such lattice distortion also contributes to the hierarchical self-assembly of CsPbI 3 nanocubes into single-crystalline NWs. Besides, they revealed that more rapid self-assembly and phase transition processes can be induced by more amount or stronger polarity of solvents. Fig. 4a shows the schematic diagram of self-assembly process of CsPbI 3 from nanocubes to NWs. Although they did not investigate the polarized light emission properties, it was thought that the DOP of NWs was improved because of the asymmetrical structure.
The polarization anisotropy can also be achieved in dimers of two laterally coupled QDs, because the separate excitons in the two QDs can be coupled through the dipole-dipole interaction. Kim et al. [79] studied the polarization anisotropy in a dimer of laterally coupled GaAs QDs and found that the emission from laterally coupled QDs is strongly polarized along the coupled direction. As can be seen in Fig. 4b, c, a DOP of 74% can be observed when the excitation polarization is parallel to the coupled direction, while it is only 10% when the excitation polarization is perpendicular to the coupled direction. At last, they proved that the dipole-dipole interaction across two separate QDs is mediated. The anisotropic wavefunctions are determined by the excitation polarization.

NRs
NRs exhibit stronger polarized light emission property over QDs because of their higher anisotropy [37,[80][81][82]. Colloidal semiconductor NRs are important polarized light emitting materials researched most early, including NRs, dot-in-rod and rod-in-rod core/shell nanostructures [26,[83][84][85]. In 2001, Hu et al. [57] studied linearly polarized emission of CdSe colloidal NCs. Through empirical pseudopotential calculations, they predicted that different from plane-polarized light for spherical dots, the slightly elongated CdSe NCs can emit polarized light along the long axis. They also confirmed that there is a sharp transition from nonpolarized to purely linearly polarized emission at the AR of 2 by single-molecule luminescence spectroscopy mea- surements. Since then, polarized light emission optimization from length-diameter ratio AR adjustment, structure optimization, surface modification and doping regulation has developed rapidly in the following years.
Carbone et al. [86] reported a seeded-growth method to synthesize CdSe/CdS dot-in-rod core-shell NRs with asymmetric structure, regular morphology, narrow distribution of size and linearly polarized emission. More importantly, the AR could be adjusted easily through simply changing the dosage of the precursor and reaction temperature. They also achieved the lateral and vertical alignments of CdSe/CdS NRs on the substrates up to micrometer-size. Fig. 5a shows the schematic diagram of the seeded growth method. Fig. 5b-f show the transmission electron microscopy (TEM) images of CdSe/CdS NRs with different sizes. Sitt et al. [87] synthesized CdSe/CdS rod-in-rod core/shell NRs by the seeded-growth method. The NRs exhibited uniform size, high emission quantum efficiencies and highly polarized emission. They found the DOP was controlled by the inner core rod dimensions. They also measured the polarization property at different excitation wavelengths and studied the interaction between electronic contribution and dielectric effect in determining the absorption and emission polarization. Fig. 5g, h show the parallel and perpendicular polarized emissions of core/shell and rod-in-rod CdSe/CdS nanostructures, respectively. As can be seen, the rod-in-rod CdSe/CdS nanostructure shows much higher difference between the polarization components, the DOP is calculated to be 0.37. Instead of CdSe/CdS heterostructure, Hadar et al. [88] synthesized CdSe/Cd 1−x Zn x S NRs with high green emission quantum yield and high polarization through controlled addition of Zn atoms (Fig. 5i).
Metal halide perovskite NRs are emerging as materials with prominent optical and electronic properties, which can also offer the remarkable property of linearly polarized light emission [89].
Li et al. [89] realized the direct synthesis of two kinds of CsPbBr 3 NRs in polar alcohols with the average lengths of 10.8 and 23.2 nm. The PLQY of 76% is realized in the short CsPbBr 3 NRs. As can be seen in Fig. 6a, the CsPbCl 3 and CsPbI 3 NRs can be easily synthesized by anion-exchange reactions, and the emission can vary across the full visible wavelength range. TEM images provided in Fig. 6b confirm the NR shape of the pristine CsPbCl 3 , CsPbBr 3 and CsPbI 3 NRs with similar size. Fig. 6c shows the different PL intensities of long CsPbBr 3 NRs measured in two different polarizer directions. Fig. 6d compares the R values of CsPbBr 3 NCs, short NRs and long NRs. Among them, the CsPbBr 3 NCs presented a featureless emission polarization spectrum; and the long CsPbBr 3 NRs showed the highest anisotropy because of their highest length-diameter ratio. Dou et al. [30] realized an enhanced R value in the CsPbBr 1.2 I 1. 8 NRs compared with pristine CsPbBr 3 NRs. because the substitution of smaller bromine anions with larger iodine anions can induce structural distortion of PbX 6 octahedra units and so as to break the lattice symmetry. As can be seen from the PL anisotropy curves shown in Fig. 6e, the CsPbBr 1.2 I 1.8 NRs show the PL R value around 0.4, which is four-fold higher than that of pristine CsPbBr 3 NRs. Zhao et al. [90] found the PL intensity and radiative lifetime are sensitive to the polarization direction. As can be seen in Fig. 6f, the PL of CsPbBr 3 NRs shows clear changes in intensity, with the strongest intensity in parallel and the weakest in vertical directions to the NRs, while the PL from CsPbBr 3 NCs remains unchanged with the different polarization angles. Moreover, they found that the radiative lifetimes of CsPbBr 3 NRs are affected by different excitation polarization angles (Fig. 6g). Except the inorganic halide perovskite NRs, the rod-like aggregates MAPbBr 3 QDs also show different emission intensities in the parallel and perpendicular to the rod-like packing directions (Fig. 6h) [91].

NWs
Compared with QDs and NRs, the NWs possess inherently larger AR, giving them a competitive advantage in anisotropy and polarized light emission [65,[92][93][94][95][96][97][98]. Colloidal semiconductor NWs are therefore attractive materials with strong polarized light absorption and emission. Yang et al. [93] synthesized ultralong colloidal CdSe x S 1−x NWs through a solutionliquid-solid method. The optical band and PL spectra of CdSe x S 1−x NWs can be finely tuned from 508 to 628 nm by varying the ratio of Se/S. More importantly, the CdSe x S 1−x networks show strong polarized emission with DOP up to 0.6, which is much higher than that of the CdSe/CdS dot-in-rod or rod-in-rod samples. It is also feasible to detect polarized light by colloidal NWs. Yu et al. [99] synthesized randomly oriented CdSe NW networks for photocurrent measurements. They proved that the photocurrent is sensitive to the orientation between the incident polarized light and the external applied electric field, and the maximum photocurrent occurs when the light polarization is parallel to the electric field direction.
There are also many studies exploring the synthesis methods of lead halide perovskite NWs. Yang's group [94,95] made a lot of efforts in this regard. They reported a catalyst-free, solutionphase method to synthesize CsPbX 3 single-crystalline NWs with uniform diameter that crystallize in the orthorhombic phase [94]. Afterwards, they used highly monodisperse CsPbBr 3 NWs as the templates to synthesize CsPb(Br/X) 3 (X = Cl, I) NWs [95]. The prepared alloy NWs have well-preserved morphology and crystal structure. As shown in Fig. 7a, the NWs with a wide range of alloy compositions can emit bright PL over almost the entire visible spectrum. Then, they successfully synthesized and purified highly uniform single crystal ultrathin CsPbBr 3 NWs through a catalyst-free colloidal synthesis followed by a stepwise purification method [100]. After surface treatment, the ultrathin CsPbBr 3 NWs with a diameter of 2.2 nm and length up to several microns have bright PL, and the PLQY can be up to about 30%. Gao et al. [101] synthesized the well-aligned CsPbBr 3 and CsPbCl 3 NWs with thicknesses of 15 and 7 nm using a vapor phase van der Waals epitaxial method. They demonstrated the NWs with thicknesses less than 40 nm have strong emission anisotropy with DOP up to 0.78 due to the electrostatic dielectric confinement. Fig. 7b compares the polarization dependence of integrated PL intensity as a function of polarization angle. The NWs of 15 nm thick have obviously higher anisotropy than that of 250-nm-thick NWs. As shown in Fig. 7c, the PL images of the NWs under different excitation polarization angles are also in accord with the PL intensity. Zhou et al. [102] developed flexible linearly polarized photodetectors based on CsPbI 3 NWs. Because of the excellent crystal structure and morphology anisotropy of CsPbI 3 NWs, the polarized photodetector is highly sensitive to the polarization direction of light. The flexible device shown in Fig. 7d has high flexibility, transparency and polarizationdependent photocurrent response.

NSs
2D NSs exhibit exceptional photophysical properties, for example, increased exciton binding energy, low threshold sti-
Cassette et al. [108] reported the synthesis and properties of novel 2D CdSe/CdS dot-in-plate NCs with polarized emission. The NCs were synthesized by growing the disciform CdS shell on spherical CdSe cores. Fig. 8a, b show the CdSe/CdS dot-inplate NCs lying flat or standing on their edges. The CdSe/CdS dot-in-plate NCs exhibit strong polarized emission in the plane perpendicular to the wurtzite crystal c axis (Fig. 8c). Fig. 8d shows the schematic of energy levels and the orientation of the corresponding transition polarizations with respect to the crystallographic structure in CdSe/CdS dot-in-plate NCs. The energy levels and oscillator strengths of the optically active ones, the |0 U >, |±1 U >, and |±1 L >states, control the emission properties, especially the polarization. The |0 U >transition is linearly polarized along the c axis (π transition) [108]. Controlling the orientation of a single fluorescent nanoparticle in a desired orientation and measuring their emission spectra are still a big challenge. Feng et al. [110] proved the single cubic CdSe/CdS NSs have different fluorescence dipoles in vertical and horizontal orientations. As can be seen in Fig. 8e, the cubic-shaped CdSe/ CdS NSs are structured with a thin CdSe core layer sandwiched in a thick CdS shell. Fig. 8f compares the polarization analysis curves for two typical NSs, where the DOP for the horizontal NSs is very close to 0, while the vertical NSs exhibit a much higher DOP of 0.79. Besides, the measurements and calculations show a good agreement in 2D dipole orientation (Fig. 8g). Ma et al. [111] reported anisotropic PL from isotropic optical transition dipoles in semiconductor NSs. Feng et al. [112] demonstrated that the emission is polarized and the emission patterns are anisotropic for rectangular NSs; while they showed nonpolarized and isotropic properties for square NSs.
Recently, perovskite crystals also play an important role in 2D NSs. Bekenstein et al. [104] reported highly luminescent perovskite cesium lead halide colloidal NSs. They observed that the reaction temperature is vitally important for the shape and thickness of CsPbBr 3 NCs. As can be seen in Fig. 9a-c, reactions conducted at 150, 130, and 90°C produce NCs, NSs and thin NSs, respectively. They also have different-color PL emission. Sheng et al. [113] synthesized all-inorganic CsPbX 3 perovskite 2D NSs. They have uniform thickness and tunable size, and they can be easily controlled by changing the component and amount of metal halides, temperature, time, and ligands. Fig. 9d shows that differently shaped perovskite NCs can be obtained by changing the dosage of CuCl 2 . Zeng et al. [114] synthesized CsPbBr 3 NSs with a PLQY of 97.4% using the aged Pb-oleate precursor. Fig. 9e shows the diagrams and TEM images of freshprecursor sample (top) and aged-precursor sample (bottom).
Liu et al. [115] synthesized the CH 3 NH 3 PbBr 3 NSs through a self-organization method. The spherical nanodots will transform to square or rectangular NSs when the colloidal NCs are kept at a high concentration for three days, or the synthesis of nanodots is combined with self-organization. Additionally, after being stretched for 2-3 times, the polymer composite film embedded with CH 3 NH 3 PbBr 3 NSs exhibits expected polarized emissions, and the DOP is 0.11. As can be seen in Fig. 9f, the PL emission of stretched film shows the cos/sin intensity dependence on the polarization angles. Yang et al. [116] proposed a two-step nucleation strategy to synthesize CsPbBr 3 NSs. Subsequently, they prepared the CsPbBr 3 NS films by a dip coating method for the polarization measurement. Fig. 9g-i show the integrated PL intensities as a function of polarization angles for the samples of CsPbBr 3 NCs, stacked shaped NSs and tongue-shaped NSs, respectively. Fig. 9j compares the polarization effect of different perovskite nanomaterials, where the DOP of 0.58 for CsPbBr 3 NS film is relatively high.
In addition to the above-mentioned polarized light emission materials, the tetrapod-shaped, bone-shaped and some other anisotropic heterostructures may also have the potential for polarized light emission with high DOP [117,118].

FABRICATION OF LINEARLY POLARIZED LIGHT EMITTING FILMS
It is expected that the aligned ANEF with large size, good uniformity and densification would emit polarized light with high DOP, so as to further enhance the efficiency of LCD and benefit the fabrication of polarized LEDs [9,18,32,119]. Since the alignment of anisotropic nanomaterials is vitally important, many techniques have been explored to obtain high-quality ANEFs recently, including ink-jet printing, mechanical stretching, electrospinning, photoalignment, mechanical rubbing, selfassembly at the liquid/air interface, electric field-assisted assembly, template-assisted assembly, and off-center spin-coat-ing. Several other less significant methods have also been investigated. Most of these methods need the assistance of external force, therefore limiting the flexibility and local alignment orientation to a certain extent. And most importantly, some methods are inapplicable to the fabrication of LEDs. In this section, we would summarize and compare the different methods to construct polarized light emitting films.

Ink-jet printing
The non-contact ink-jet printing technique offers a possibility to achieve uniform, well-aligned and large-scaled polarized light emitting films. This method would have a broad application prospect in polarized optoelectronic devices, LCD backlights and some other fields where polarized light is needed. The year of 2019 was a boom year to align semiconductor and perovskite nanomaterials by ink-jet printing. Gupta et al. [120] demonstrated that the ink composition and printing conditions have a great influence on the alignment quality of CdSe/CdS rods. Their ANEFs show a DOP of 0.76. When combining ANEFs with blue LED as the backlight display unit for LCD, the system provides an unprecedented increase of optical efficiency for 77% and simultaneously offers better color space. Fig. 10a shows the schematic of the ANEF printing process followed by photopolymerization under UV light. Fig. 10b gives the schematics of different flow processes in the ink-jet printing films. The color triangle of the LCD shown in Fig. 10c covers 109% of NTSC on the CIE1976 color triangle.
Zhou et al. [38] made a great contribution for the practical application of ink-jet printing. Using this method, they produced many devices by 3D-printed nanocomposite inks com-

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posed of perovskite NWs and block copolymer matrix, such as optical storage, encryption, sensing, and full-color displays. These devices have high polarized absorption and emission properties. Fig. 10d, e show the applications of ink-jet printing for security encryption and optical strain sensors. Fig. 10f presents the spectral emission profiles of the pixel array based on hexagonal tiles of red, green, and blue perovskite nanomaterials printed along three directions. Zhou et al. [121] aligned the CdSe/CdS core/shell quantum rods by the contact ink-jet printing method. When printing the ink, the quantum rods will align along the fluid ink flow direction, resulting in aligned quantum rods and forming a polarized light emission film with a DOP of 0.42.

Mechanical stretching
Mechanical stretching is a simple and common method for arranging nanomaterials, which is suitable for producing largearea-polarized light source [9,32,119,[122][123][124]. During the stretching process, the nanomaterials are rotated and aligned along the stretching direction by the shear stress provided by the polymer segment. This method requires that the nanomaterials have good compatibility with the selected polymer, and the polymer can also protect the nanomaterials from the environment. Besides, the polymer needs to have a large stretching ratio so that the nanomaterial film could have a large degree of orientation.
Cunningham et al. [9] obtained semiconductor-polymer polarized light emission composite films by this method. The films with high DOP of 0.7 are viable candidates for practical application in LCD devices. Compared with the emitter layers with randomly oriented CdSe/CdS nanomaterials, the composite films with aligned CdSe/CdS nanomaterials exhibit more than two times enhancement of brightness. Fig. 11a presents the composite films under UV light before and after stretching. Fig. 11b shows the polarized PL dependence from the composite films before (squares) and after (circles) stretching. The DOP of stretched films is 0.6, while it is only about 0.05 for the nonstretched films. Raja et al. [123] demonstrated enhanced water and light stability of colloidal perovskite nanomaterials by encapsulating them into polymeric matrices. Fig. 11c, d show the images of aligned perovskite NWs and polymer composite film observed by optical microscope and confocal microscope, respectively. As can be seen in Fig. 11e, the DOP of the aligned NWs polymer composite is about 0.44, which is more than five

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April 2023 | Vol. 66 No. 4 times higher than that of NCs.
Wang et al. [124] synthesized CsPbBr 3 NWs and polymethyl methacrylate (PMMA) polymer composite films with strong polarized PL. The optimized stretched composite film can achieve a DOP of 0.42. Then, they used the film as a fluorescence conversion layer to enhance the UV polarization light detection. Their results demonstrated that the response is greatly enhanced after spectral conversion, and different responsivities correspond to different polarization states. Fig. 11f shows the images of unstretched film and other seven samples with different stretched lengths. Fig. 11g shows the polarization curve of different samples. After stretching, the DOP of the film is improved significantly. Fig. 11h explains the reason of improved polarization through mechanical stretching. Under the stretching force, the CsPbBr 3 NWs in the PMMA polymer film would deflect in the stretching direction.
Zhong's group [32,119] carried out a series of work by this method. Lu et al. [32] reported strong polarized PL from stretched perovskite nanomaterials and polymer composite films. The perovskite QDs in the composite films would be aligned into wires and emit polarized light with a DOP up to 0.33. Fig. 11i shows the photograph of films under UV irradiation. As can be seen from the polarization measurements of composite films in Fig. 11j, the composite films exhibit non-polarized emission before stretching. The polarization measurement results of stretched films fitted well with trigonometric function. Ge et al. [119] reported that the aligned CdSe@CdS NRs in polyvinylidene fluoride (PVDF) films show a DOP of 0.52. Then they used the composite film as the downshifting material for polarization-sensitive UV detection. Fig. 11k gives the schematic diagram of the construction of the CdSe@CdS/PVDF composite films. Fig. 11l shows the angle-dependent emissions of the composite films before and after stretching. The DOP increased four and five times for the films stretched to lengths two and

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four times that of the original, respectively.

Electrospinning
Electrospinning is a versatile, easy and viable technique to fabricate composite nanofibers, with advantages of both the continuous production and wide application [125][126][127][128][129][130][131]. More importantly, the aligned composite nanofibers could be used for polarized light emission. Jiang et al. [132] adopted the electrospinning method to realize in situ fabrication of Cs 3 Cu 2 I 5 nanostructures embedded in PVDF films for polarized light emission. The polymer PVDF films can also protect the Cs 3 Cu 2 I 5 nanomaterials. Due to the directional transition dipole moment induced by the anisotropic crystal structure of aligned Cs 3 Cu 2 I 5 nanomaterials and dielectric confinement effect of the PVDF polymer film, the Cs 3 Cu 2 I 5 / PVDF composite nanofibers exhibit a high DOP of 0.4. Fig. 12a shows the schematic diagram of the fabrication of Cs 3 Cu 2 I 5 / PVDF composite nanofibers by the electrospinning method. Using the polarization measurement setup in Fig. 12b, they demonstrated the maximum DOP of the nanofiber film can reach 0.40, while it is only 0.18 for the microfiber film. Their polarization properties are shown in Fig. 12c, d.
Meng et al. [133] reported an in-situ fabrication of MAPbX 3 @polyvinyl alcohol (PVA) nanofibers through electrospinning. The strongly polarized emission of optimized MAPbX 3 @PVA nanofibers can be attributed to the quantum confinement and dielectric confinement effects. Moreover, a DOP of 0.42 was achieved for the films of well-aligned MAPbX 3 @PVA nanofibers with a macroscale size. Fig. 12e shows the schematic diagram of the fabrication of MAPbX 3 @ PVA nanofibers by the electrospinning method. From the scanning electron microscopy (SEM) image in Fig. 12f, we can see the MAPbX 3 @PVA nanofibers are uniform and smooth. The emission anisotropy of the MAPbX 3 @PVA nanofiber film with different precursor concentrations is shown in Fig. 12g. Fig. 12h-j show the fluorescent optical microscopy images of MAPbX 3 @PVA nanofiber films, demonstrating that the electrospinning method can be used to fabricate perovskite nanofibers with macroscale alignment. Meng's group [134] fabricated inch-size aligned MAPbX 3 @PVA nanofiber films by adapting an electrospinning technique. The aligned electrospun polyurethane fibers containing CsPbBr 3 NWs also show a high DOP of 0.30 [135]. Hasegawa et al. [136] fabricated the nanofiber sheets consisting of aligned electrospun polymer nanofibers embedded with CdSe/CdS NRs. The DOP of the nanofiber sheets can be up to 0.6. Aubert et al. [137] embedded silicacoated CdSe/CdS NRs in polymeric nanofibers by electrospinning, and achieved large-scale and electrically switchable polarized emission.

Photoalignment
The photoalignment method can achieve precise alignment of

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April 2023 | Vol. 66 No. 4 nanomaterials in the desired position at the nanoscale, because the photo-reorientation of chromophores offers large anchoring energy with a low pretilt angle [7,138,139]. Besides, the linear arrays of smectic liquid crystal defects can play a role of smart matrix to manipulate the position and direction of NRs [140]. Fig. 13a shows the straightforward but multistep process of the photoalignment of the NRs in the liquid crystal polymer films [7]. This process includes spin coating and subsequent photoalignment, after spin coating a mixture of nanomaterials and liquid crystal polymer monomers in toluene, the polymerization under UV light would ensure the stability of the composite films. The photoinduced surface energy of the alignment layer gives a torque to the liquid crystal polymer molecules and aligns them in the direction parallel to the easy axis of the chromophore. Meanwhile, the repulsive intermolecular forces between the nanomaterial ligands and liquid crystal polymer molecules would exert counter-torque, and align the nanomaterials perpendicular to the easy axis of the chromophore. The polarization angle dependences of the emission intensity for red and green films are presented in Fig. 13b, c, respectively, which show good agreement with the Malus's law.
The photoalignment approach also offers the possibility to align the light emitting nanomaterials on patterned surfaces. Schneider et al. [138] used photoalignment to align CdSe/CdS core-shell NRs dispersed in a liquid crystal polymer into microscale patterns. After unidirectional alignment, the patterns were fabricated by a second irradiation with different polariza-tion azimuths and the employment of a photomask. Fig. 13d presents the concept of the angular dependence of the emission intensity for 2D fluorescent grating patterns. Depending on the directions of the NRs and excitation polarization, the patterns exhibit the variation of dark and bright states. When the excitation polarization azimuth is 45°, the absorption and emission intensities are equal. The fluorescence micrographs show the minimum (dark) and maximum (bright) states when the polarization azimuth is 0°and 90°, respectively.

Mechanical rubbing
The mechanical rubbing is a very simple and convenient method to align NRs on large scales [50,141,142]. During rubbing process, the dragging force produced by the rubbing fibers leads to deflection and reorientation of the NRs along the direction of rubbing. However, the relatively rough films constructed by this method are to some extent difficult to achieve high EQE for LEDs.
Amit et al. [141] reported the alignment of CdSe/CdS semiconductor NR film for macroscopic scale through this method. Fig. 14a shows the schematic of the mechanical rubbing process. Fig. 14b gives the SEM image of CdSe/CdS semiconductor NR film characterized by the random NR orientation. After mechanical rubbing, the NRs are aligned along the rubbing direction (Fig. 14c). They studied the effects of substrate treatments on the DOP and showed that partially hydrophobic surface is beneficial for the alignment of CdSe/CdS NRs. In

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addition, the optimal amount of organic ligands added to the deposited solution was found to yield high-quality alignment. The prepared CdSe/CdS film exhibited high linearly polarized emission with a DOP of 0.56. While the contrast experiments of rubbed spherical QDs and spin-coated films of NRs exhibited no polarized light emission. Wei et al. [50] aligned CsPbBr 3 anisotropic NWs using the mechanical rubbing method. Fig. 14d shows the schematic diagram of the preparation process for aligned CsPbBr 3 films by mechanical dry rubbing and mechanical wet rubbing methods. Fig. 14e, f show the SEM images of CsPbBr 3 NW films after mechanical wet rubbing and mechanical dry rubbing, respectively. The directions of NWs are in accordance with the rubbing directions for both wet rubbing and dry rubbing methods. The dry-rubbed film shows a little lower polarization PL behavior with a DOP of 0.50. At last, they also used the dry-rubbed film for polarized LED device fabrication.

Self-assembly at the liquid/air interface
Usually, the surface tension of liquid is used in this method to achieve self-assembly. Rizzo et al. [49] developed a simple and effective method to assemble colloidal CdSe/CdS NRs on the surface of water (Fig. 15a). This technique includes two steps. Firstly, the CdSe/CdS NR solution is floated onto a water surface. During the solvent evaporation process, the NRs in the concentrated organic solution would self-assemble into an ordered NR film at the liquid/air interface. Secondly, a polydimethylsiloxane (PDMS) stamp is used to fish and transfer the NR film by contact printing from the water surface. By exploiting this contact printing method, they fabricated an LED with strongly polarized emission. Kim et al. [143] developed a simple method to make optically anisotropic films by shearoriented assembly of LaPO 4 NRs. Fig. 15b illustrates a schematic of the customized coating machine. In this method, shearing a thixotropic rod gel may induce such a gel-to-sol transition and simultaneously the orientation of rods as they recover the individual mobility. Firstly, a certain amount of NR solution was dropped onto the substrate. Secondly, the substrate was dragged under the coating blade at a constant speed and a fixed distance from the blade plane. The film thickness can be well controlled by the gap thickness. Then, the substrate with sheared suspension was heated to 140°C to evaporate the solvent. At last, the solidified film was annealed at 500°C for 2 h. Deng et al. [144] aligned perovskite NWs by a modified evaporation-induced self- assembly method. The growth mechanism is schematically described in Fig. 15c. The possible growth mechanisms are as follows. At the beginning, the NWs grow from the edge of the substrate due to the heterogeneous nucleation growth. As evaporation continues, the liquid/substrate interface line driven by evaporation and gravity moves along the evaporation direction. As a result, the NWs can be aligned very well quasi-vertically to the substrate edge.
Langmuir-Blodgett assembly is a simple and feasible method to assemble monolayer films in a large-area at the water/air interface [145]. The constructed films can be transferred to various substrates. Rhee et al. [48] carefully optimized the Langmuir-Blodgett technique deposition condition and achieved highly dense and smooth NR thin film. Fig. 15d shows the deposition process of NR film by the Langmuir-Blodgett method. Firstly, amphiphilic NRs dissolved in hexane were dropped in the Langmuir-Blodgett trough filled with deionized water. The NRs will rapidly spread on the water surface without sinking or mixing. Secondly, after evaporation of the hexane, the two barriers were moved toward the center so as to compress the floating NR film. The increased surface pressure is helpful to form highly dense film on the surface of water. Thirdly, a pre-immersed substrate was pulled up slowly to transfer the aligned NR film to the substrate. The DOP of thin film with high order of alignment of NRs is 0.21.
However, because water is used during the preparation process, this method cannot be used to assemble perovskite nanomaterials directly because of their poor stability. The chemical surface modification by a protection layer with strong affinity should be a reasonable method not only to obtain individually dispersed NWs with good water stability but also to retain the optoelectronic properties of NWs. Liu et al. [145] introduced an amphiphilic block copolymer to modify the surface of CsPbBr 3 NWs. The surface-modified core shell perovskite NWs revealed enhanced photoluminescent emission and stability against water. Afterwards, they further applied the Langmuir-Blodgett method to assemble highly aligned NW monolayer. At last, they achieved a DOP of 0.36 in the aligned monolayer.

Electric field-assisted assembly
Electric field-assisted assembly can align anisotropic nanomaterials with high uniformity under the action of local electric fields [86,[146][147][148][149][150]. However, the disadvantages for this method could be the smaller size of the film and the requirement of

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patterned substrate [146][147][148]. Hu et al. [146] reported a method to control the position and orientation of colloidal CdTe and CdSe NRs by the local electric fields generated from an interdigitated electrode. Fig. 16a shows the schematic of the device. Different electric fields could be achieved by varying the width (W) and length (L) of the device. Fig. 16b shows the schematic of the experiment. A drop of hexane/octane solution containing NRs dries on the surface while voltage is applied to the electrodes. They reveal strong alignment of NRs to the direction of the applied field and dense accumulation around and onto the voltage-biased electrodes (Fig. 16c). The degree of alignment under the applied electric field is quantified by a nematic order parameter S ≈ 0.8 in contrast to the zero-field case where S ≈ 0.1. Carbone et al. [86] also assembled their synthesized CdSe/ CdS dot-in-rod core-shell NRs in the lateral and vertical directions on the substrates up to micrometer-size. Fig. 16d, e show the lateral alignment of NRs with electric fields. The red arrow represents the direction of the applied electric field. At last, a DOP of 0.45 is observed on micrometer scale.
Kaur et al. [147] fabricated a functional film on the interdigitated electrode substrate to align CdSe/CdS NRs. Fig. 16f illustrates the steps for the formation of the NR film. Without the applied electric field, the reactive mesogen molecules are randomly oriented. As the voltage increases, the reactive mesogen molecules are driven by the external bias to minimize the electrostatic energy of the system. The applied electric field can control the position and orientation of NRs. Then, the NRs are fixed in position by the polymerization under the exposure of UV light. The DOP of aligned NRs was measured to be 0.48. Persano et al. [148] fabricated aligned CdSe/CdS core/shell NR film by drop casting the NRs from a solution on substrates with prepatterned electrodes. Fig. 16g, h show the SEM images of aligned CdSe/CdS NRs at different magnifications. Gryn et al. [149] showed anisotropic NRs can be aligned and organized in a large area by anisotropic interactions with the bulk molecular director and linear topological defects. Mohammadimasoudi et al. [150] produced a hybrid luminescent layer based on CdSe/ CdS NRs dispersed in a liquid crystal. Then, the NRs are aligned by applied electric field and polymerized by UV illumination. The DOP of the CdSe/CdS NR film can be up to 0.6.

Template-assisted assembly
The template-assisted assembly method can be used to fabricate polarized light emitting films, but it is not compatible with the standard LED fabrication process. Wang et al. [151] aligned a needle-like superparticle structure into unidirectional line patterns on Si 3 N 4 substrates through capillary forces (Fig. 17a, b). Furthermore, the patterns can be readily transferred into uniform and removable thin films of PDMS with large size. The resulting thin films exhibit strong polarized light emission with a high DOP of 0.88, which is much higher than that of individual single CdSe/CdS NRs (Fig. 17c). This enhanced polarized light emission mechanism can be attributed to the combination of the dielectric effect and collective electric dipole coupling effects among the CdSe/CdS NRs inside the elongated needle-like superparticle structure embedded in PDMS films. The template-assisted assembly method can also be used to produce highly aligned single-crystalline MAPb(I 1−x Br x ) 3 (x = 0, 0.1, 0.2, 0.3, 0.4) NW arrays with a continuously adjustable absorption range from 680 to 780 nm [152]. Deng et al. [152] used the periodically aligned SU-8 photoresist stripes on the SiO 2 /Si substrate as the template to fabricate the MAPb(I 1−x Br x ) 3 perovskite NWs by a facile fluid-guided antisolvent vaporassisted crystallization method. The schematic illustration of the preparation method is shown in Fig. 17d. The SU-8 template is dipped into MAPbI 3 /DMF solution and then placed in the CH 2 Cl 2 solvent. The diffusion of antisolvent leads to the precipitation of MAPbI 3 NCs along the sides of SU-8 template. Fig. 17e shows the large-area continuous and uniform NW arrays. From the SEM image in Fig. 17f, we can see the aligned MAPbI 3 NWs are preferentially deposited on the two sides of SU-8 template rather than the channels. The magnified SEM image shows that the MAPbI 3 NW has smooth surface (Fig. 17g). Lutich et al. [153] fabricated highly ordered CdSe/ CdS core-shell NRs by infiltration of the NRs into the pores of transparent porous anodic alumina membrane. The CdSe/CdS NR films have highly polarized PL and absorption properties.

Off-center spin-coating
Off-center spin-coating is a very simple strategy to align aniso-tropic nanomaterials. It has large potential in the polarized LED, because it is compatible with the standard LED fabrication process. Yuan et al. [154] developed the method of off-center spin-coating. As shown in Fig. 18a, in this method, the substrate is placed far away from the center of the spin coater. When the spin coater begins to rotate, the nanomaterials in the solution could be aligned by the outward centrifugal force. Spin-coating is widely used in the LED fabrication due to its easy process, high quality film and free of the complicated film transfer process. Off-center spin-coating combines the advantages of spincoating and oriented assembly. Even though there is only one study using this method to fabricate LED so far, we believe it is a very promising approach [155]. Kim et al. [156] prepared highly aligned organic polymer semiconductor films by off-center spincoating of the pre-aggregated conjugated polymer solution. Fig. 18b shows the schematic illustration of aligned polymer films under the action of centrifugal force. Fig. 18c shows the SEM image of the aligned CsPbBr 3 NW films prepared by offcenter spin-coating [155]. However, the quality of CsPbBr 3 NW film is still far from satisfactory. Because the organic ligand on the surface of NWs would cause strong van der Waals force between adjacent NWs. At last, the resulted CsPbBr 3 NW bundles will inevitably reduce the film quality. Furthermore, this method can also be used to align silver, silicon NWs and poly- mer chains [157][158][159]. Fig. 18d compares the silicon NW alignment effects made by the conventional spin-coating and off-center spin-coating methods. From the optical microscopy images, we can see the ordered silicon NWs outward the radial direction made by off-center spin-coating.

POLARIZED LIGHT EMISSION LEDs
The practical use of polarized light emission LEDs in the display industry can eliminate the need of a front polarizing film, which reduces the manufacturing cost and time. As mentioned earlier, many methods have been used to achieve polarized light emission in aligned anisotropic nanomaterials, but some of the aligning methods are not compatible with the standard LED fabrication process, leading to poor device performance. Therefore, it is urgently needed to identify effective methods to align anisotropic nanomaterials without destroying the LED structure, thereby achieving high-performing polarized LEDs. In this section, we would summarize and compare several different methods to construct polarized light emission LEDs in two parts: the II-VI colloidal nanomaterials-based polarized LEDs and the perovskite nanomaterials-based polarized LEDs.

II-VI colloidal nanomaterials-based polarized LEDs
Hikmet et al. [40] constructed the first polarized light emission LED in 2005. They used the core/shell CdSe/CdS NRs with a PLQY about 50% as the emitting layer. The CdSe/CdS NRs are aligned by rubbing. Fig. 19a shows the electroluminescence (EL) spectrum of the polarized LED in the parallel and perpendicular directions to the rubbing. Obviously, the parallel direction has stronger emission intensity. However, the blue emission from polyvinylcarbazole (PVK) can also be seen in the spectrum, indicating some electron-hole pair recombination around the PVK interface. Rizzo et al. [49] also constructed a polarized LED based on highly ordered arrays of colloidal CdSe/CdS core/shell NRs as the active species. The alignment method for CdSe/CdS film is discussed before. Then this technique was used to transfer floating NR film onto an organic layer to fabricate polarized LED. The inset of Fig. 19b shows the device structure. Fig. 19b shows the current voltage and luminance voltage properties of NR LED. The maximum brightness of 170 cd m −2 achieves at 19 V and 140 mA cm −2 . The EL spectrum reveals that the light emission comes from the CdSe/CdS NRs. The inset of Fig. 19c compares the EL spectra for parallel and perpendicular directions.
Rhee et al. [48] reported polarized EL emission based on highperformance CdSe/CdS NR LED. The Langmuir-Blodgett technique they used to obtain highly dense and smooth NRs thin film is discussed in section "Self-assembly at the liquid/air interface" in detail. Fig. 19d shows the experimental setup for measuring polarized LED under electrical excitation. A linear polarizer is placed between the LED device and detector, and the EL intensities of different angles can be measured by rotating the polarizer from 0°to 180°. A constant current should be applied to maintain the EL of LED during the measuring process. As can be seen in Fig. 19e, the EL spectrum exhibits 1.25-fold higher intensity when the polarizer is parallel to the alignment direction of NRs. Fig. 19f shows the peak intensity with respect to the rotational angle. At last, the polarized LED fabricated by the Langmuir-Blodgett technique shows a low turn-on voltage of 1.85 V, a decent peak EQE of 10.33%, and a high maximum luminance of 56,287 cd m −2 . This is the highest EQE of polarized LED ever reported. Compared with spin coating, this method just causes a slight deterioration of the device performance. It can be attributed to the degradation of CdSe/CdS NRs during the assembly process in water surface. Therefore, this method is not applicable to perovskite materials.

Perovskite nanostructure-based polarized LEDs
Due to their excellent photoelectric properties and promising applications, anisotropic perovskite nanomaterials have also drawn significant attention in polarized LEDs. However, some methods of constructing polarized light emitting films discussed above are inapplicable to perovskite nanomaterials because of their instability. The mechanical rubbing and off-center spincoating are the only two methods used to fabricate CsPbBr 3 NWs polarized LED for now. Nevertheless, because of the strict requirements on the film quality of LED devices, the perovskite nanostructure-based polarized LEDs prepared by these two methods have not achieved desired results. Wei et al. [50] successfully fabricated CsPbBr 3 NWs polarized LED using a mechanical rubbing method in 2020. Fig. 20a, b show the luminance-voltage and current density-voltage characteristics of the CsPbBr 3 NW LEDs. The inset of Fig. 20a shows the device structure of polarized LED. Combined with an optimal device structure, the turn-on voltage has been as low as 6.5 V. They studied the device performances with different hole transport materials and found the single-layer poly-TPD-based device showed the best device performance. However, the highest EQE was only 0.08%. Then, they used the off-center spin-coating method to align CsPbBr 3 NW films and fabricated the polarized LED [155]. Fig. 20c shows the EL spectrum dependence of the voltage. The emission peak is located at 523 nm. The film quality and device performance in this work need to be further enhanced.

CURRENT CHALLENGES IN THE POLARIZED LIGHT EMISSION LEDs
Effective aligning methods compatible with the standard LED fabrication process Table 1 summarizes the polarized LEDs' performance based on different materials and assembly methods. As can be seen, the fabricated polarized light emission LEDs still have some problems until now: the low efficiency and DOP, which are most significantly affected by the aligning method of anisotropic nanomaterials. In this review, we have discussed many methods of aligning anisotropic nanomaterials to construct polarized light emitting films. However, some of the aligning methods produce limited anisotropic nanomaterials alignment, irregular and low-quality films, introduction of other assisting materials and, most importantly, are not compatible with the standard LED fabrication process, leading to poor device performance. Overall, we put great expectation on off-center spin-coating and electric field-assisted assembly to align anisotropic nanomaterials without destroying the LED structure, thereby achieving high-efficiency polarized LEDs. Spin-coating is a mature method and widely used in the LED fabrication. Off-center spin-coating has a large potential in the polarized LED, because it combines the advantages of easy process, high-quality film and free of complicated film transfer process from spin-coating and oriented assembly from other methods. The only one study using this method did not achieve high LED performance, because the surface organic ligand of NWs would cause the CsPbBr 3 NWs to form big bundles, so as to reduce the film quality, whereas a proper surface ligand treatment may solve this problem and then induce uniform films.
Electric field-assisted assembly has been proved to align anisotropic nanomaterials with high uniformity under the action of local electric fields. The problems for this method could be the smaller size and patterned substrate. The strategy of electric field-assisted assembly combined with dip-coating should be a feasible method. Firstly, the anisotropic nanomaterials can be aligned orderly in solution under applied electric fields. Then, the pre-immersed substrate is pulled up slowly. After drying, the well-ordered NR film can be achieved on the substrate. It should be noted that a slight deterioration of LED performance might occur due to the degradation of transport layer during immersing in NR solution. Highly tolerant transport layers deposited by atomic layer deposition and magnetron sputtering are expected to avoid this damage.

Polarized light emission and charge carrier transport mechanisms for polarized LEDs
The direct cause of polarized light emission for anisotropic nanomaterials is that their radiative transition dipole moments tend to be distributed along their long axis. For PL, the polarized luminescence properties of anisotropic nanomaterials are determined by the quantum confinement effect and dielectric confinement effect, which are related to their lateral size and the dielectric constant of the external environment. With regard to EL, the electron distribution of the anisotropic nanomaterial surface and the environmental permittivity will affect the DOP [160]. Because the anisotropic nanomaterials are perpendicular to the direction of the applied electric field, the applied electric field will inevitably affect the electron distribution and permittivity [161]. The characteristics of the carrier migration in the anisotropic nanomaterials under the action of the electric field force need to be further studied. Therefore, it is vitally important to reveal the mechanisms of polarized light emission and charge carrier transport for polarized LEDs, so as to construct the LEDs with high DOP and EQE [160][161][162][163][164].

Key challenges for the commercial development
Although the self-emissive polarized LEDs have huge market development potentials to meet the demand of future display, the research on polarized LED devices is still in its infancy, and some key challenges for the commercial development still remain in this field. Firstly, high EQEs are necessary for commercial applications of polarized LEDs (especially blue polarized LED). There is still a need to further improve the efficiency by optimizing the device structure, designing suitable transport layer materials, preparing highly ordered light-emitting layers and so on. Secondly, the long-term stability of both nanomaterials and devices for LEDs is a well-recognized challenge, hindering their practical application severely. As we all know,   the perovskites are sensitive to water and oxygen, which will degrade rapidly when exposed to the air. The long-term stability of nanomaterials needs to be further improved by ion doping, surface coating, polymer encapsulation and so on. With regard to polarized LED devices, introducing ion-blocking interlayers and all-inorganic charge transport layers is a feasible strategy to improve the device stability and performance. However, since the source of the instability of polarized LED devices is mainly material-related issues, the development of luminescent materials with good stability is the top priority to ensure high stability of polarized LEDs. Thirdly, methods that enable large-scale production are very popular and necessary for factories, such as spin-coating and spray coating. Besides, the development of nontoxic or low-toxic polarized light emitting materials is also an important challenge.

CONCLUSIONS AND OUTLOOKS
Different from the backlight-based display technology in LCDs, the energy-saving self-emissive polarized LEDs without using polarizers have huge development and display market potentials, which can also meet the demand of future display for high color purity, high resolution, wide color gamut, and so on. Various approaches have been carried out to align anisotropic nanomaterials into films, while the DOP and EQE of polarized LEDs are still limited. The high-quality anisotropic nanomaterial films and effective aligning methods compatible with standard LED fabrication processes are required to fabricate polarized LED with high DOP and EQE. Overall, we have great hopes for methods of modified off-center spin-coating and electric fieldassisted assembly mentioned in this review to achieve highefficiency polarized LEDs.
In this review, we give a comprehensive summary on the stateof-the-art progress in the field of polarized light emission from materials to films and then to LEDs. We start with the brief introduction on the theories of polarized light emission and analyze the factors that influence the DOP. After that, the polarized light emission properties from representative materials of QDs, NRs, NWs and NSs are described. Then, the alignment strategies for constructing polarized light emitting films and LEDs are summarized. Finally, we discuss the current challenges such as effective aligning methods compatible with the standard LED fabrication process, polarized light emission and charge carrier transport mechanisms for polarized LEDs. In summary, we strongly believe the future of polarized LEDs is bright and board, which can bring huge economic benefits. We hope this review will provide a valuable summary for current status of this research direction and stimulate some new insightful ideas for future development of this promising field.