To achieve simultaneous protection against both pulsed and continuous wave (CW) or quasi-CW Lasers Protection, significant research effort has been devoted to the state-of-the-art optical limiting (OL) materials and processes in an attempt to achieve some measures of protection against such laser beams in the past decades. Two-dimensional (2D) nanomaterials with a lot of unique properties, including graphene, transition metal dichalcogenides, black phosphorus and others, have aroused the extensive research interest of many researchers. In this review paper, we describe systematically the OL mechanisms and the recent achievements in the 2D nanomaterials and their organic/polymeric derivatives for laser protection. In an effort to sustain the advantage of 2D nanomaterials, one can not only introduce the functional molecules or polymers to blend with them to form a complex multi-phase material system, but also embed the soluble 2D nanosheets covalently functionalized with organic/polymeric materials in a polymer host to form host-guest composite materials that are expected to improve the OL performance of the whole system. All in all, an optimized complex multi-component nanomaterial system enormously enhances the performance and applicability of OL devices. In addition, the fundamental studies of the photophysical and photonic properties of 2D nanomaterials and their derivatives in various solid hosts are of significance for modifying the nanomaterials at a molecular level.
In addition to being widely used in civilian fields, lasers have also been developed into various types of laser weapons. With their characteristics of high speed, repeated strikes, accurate target killing, controllable degree of damage, resistance to electromagnetic interference, and economical operation costs, they will play an important role in future wars, counter-terrorism, and It has unique and important strategic and tactical value in security and rescue. Western developed countries led by the United States, while attaching great importance to the research and development of advanced laser weapons, are also vigorously promoting laser protection research, looking forward to all high-value military/civilian platform optoelectronic loads, military Equipment and personnel for effective laser protection. Over the past two decades, people have made unremitting efforts to obtain functional materials that can effectively protect lasers, such as fullerenes, carbon nanotubes (CNTs), graphene , porphyrin, phthalocyanine, naphthalocyanine, mixed metal complexes, carbon black suspension, metal/metal oxide nanoparticles/nanowires, semiconductor nanoparticles/nanowires, polymers and their composites, organic/inorganic Composite materials and other nonlinear optical materials are gradually being prepared.
In recent years, two-dimensional functional materials (graphene, hexagonal boron nitride, transition metal halides, graphitized carbon nitride, layered metal oxides, etc.), two-dimensional polymers, metal-organic frameworks, perovskites, black Phosphorus (black phosphorus, BP), etc. (Figure 1) and their derivatives are used in field effect transistors, optical modulators, mode-locked and Q-switched lasers, optical limiting, information and energy storage, Fields such as radio frequency devices and chemical sensors have shown increasingly important potential application value. In 2014, sixty-four materials scientists from all over the world jointly wrote the "Development Roadmap for Graphene and Other Two-Dimensional Materials", which provided The future development of two-dimensional materials points the way. However, these two-dimensional materials are not soluble in any organic solvents, severely limiting the solution processing and application capabilities of the materials. Using "pre-synthesized organic or polymer grafting to (grafting to ) or "grafting organic groups or polymer chains directly from the surface of two-dimensional materials" can design and prepare a large number of organic/polymer based on two-dimensional nanomaterials. Molecular optoelectronic functional materials. This article reviews the research progress in the field of optical limiting in recent years based on the most representative two-dimensional materials and their organic/polymer derivatives such as graphene, BP, transition metal sulfides, and perovskites. , existing key scientific issues and future development trends.
Several typical two-dimensional materials and their application schematics
In terms of working principles, laser protection technology can be divided into two categories: laser protection technology based on linear optics principles and laser protection technology based on nonlinear optics (NLO) principles. In addition, there is also thermally induced phase change protection technology and mechanical microstructure protection technology, etc. Relatively speaking, laser protection materials based on nonlinear optical principles have broad-spectrum resistance to variable wavelength lasers, fast response times, and activation of the protector does not affect the detection or image processing and transmission capabilities of the instrument. , can effectively reduce the laser intensity to a level acceptable to optical instruments, military equipment and the human eye. It has extremely high practical application value and is also a key research topic in this field internationally. As shown in Figure 2, the most important The laser protection (optical limiting, OL) mechanism mainly includes excited state reverse saturable absorption (RSA), two photon absorption/multi-photon absorption (TPA/MPA), free Carrier absorption (free-carrier absorption, FCA), nonlinear refraction (NLR) and nonlinear scattering (NLS). In the visible light region, the protection range of RSA materials in solutions and solid films is between 400 -600 nm, while TPA materials produce optical limiting effects due to excited state absorption in the 600-800 nm region. The optical limiting effect area of NLS materials can extend to the near-infrared region. RSA, FCA and thermal effects induce Nonlinear refraction involves cumulative nonlinear effects, while nonlinear refraction caused by MPA and free electron effects are instantaneous nonlinear effects. The former depends on the energy flux deposited in the sample, while the latter depends only on the incident laser. The instantaneous intensity. RSA is usually produced from a molecular system in which the excited state absorption cross section is larger than the ground state absorption cross section. As the energy of the incident light increases, the absorption of light by the anti-saturable absorption material further increases, and the degree of light transmittance decreases. MPA (especially TPA) ) is an important instantaneous nonlinear effect that is easily observed in many semiconductor materials. Electrons in the valence band absorb multiple photons through a virtual intermediate state to excite the transition to the conduction band of the material. For FCA, Carriers generated through photon absorption or thermal effects in the conduction band (electrons) and valence band (holes) can continuously absorb photons and transition from low energy levels to high energy levels. When the number of generated free carriers is large, this The process can play some role. NLR can come from the real part of χ(3) (electron Kerr nonlinearity), which is instantaneous or transient nonlinearity, or it can come from cumulative carrier generation effects induced by photon absorption or thermal effects. From The self-focusing or self-defocusing of NLR can be applied to optical limiting. NLS plays an important role in optical processes based on nanomaterials. Scattering usually includes Rayleigh scattering, Tyndall scattering and Raman scattering. When the particle size is smaller than Or when it is much smaller than the wavelength of the incident light (less than one-tenth of the wavelength), the intensity of the scattered light in each direction is different, which is inversely proportional to the fourth power of the wavelength of the incident light. This phenomenon is called Rayleigh scattering. At this time, Rayleigh scattering theory can be used for analysis. However, when the size of the scattering center is equal to or larger than the wavelength of the incident light, the scattering intensity is proportional to the square of the frequency, and the scattering is greater in the forward direction of the light than in the backward direction. Strong, the directionality is relatively obvious, and Mie scattering theory can be used for analysis at this time. Like MPA, NLS is not sensitive to the narrow resonance wavelength range of the incident light, so it may contribute to the broadband optical limiting response. It has been proposed in the literature There are many ways to induce scattering centers. This scattering center can come from the generation of solvent bubbles or from the refractive index discontinuity caused by the plasma formed on the surface of the nanomaterial and the thermal effect of the solvent surrounding the nanoparticles. From From a practical application perspective, it is ideal to design nonlinear optical materials with multiple optical limiting mechanisms (such as anti-saturation absorption, two-photon, light scattering, etc.) to achieve broad-spectrum laser protection, but it is quite challenging.
Optical limiting mechanisms: (a) Nonlinear scattering; (b) multi-photon absorption;
(c) reverse saturable absorption; (d) free-carrier absorption
2D Nanomaterials in Laser Protection Field Application--Graphene And Its Derivatives
To achieve simultaneous protection against both pulsed and continuous wave (CW) or quasi-CW Laser Protection, significant research effort has been devoted to the state-of-the-art optical limiting (OL) materials and processes in an attempt to achieve some measures of protection against such laser beams in the past decades. Two-dimensional (2D) nanomaterials with a lot of unique properties, including graphene, transition metal dichalcogenides, black phosphorus and others, have aroused the extensive research interest of many researchers. In this review paper, we describe systematically the OL mechanisms and the recent achievements in the 2D nanomaterials and their organic/polymeric derivatives for laser protection. In an effort to sustain the advantage of 2D nanomaterials, one can not only introduce the functional molecules or polymers to blend with them to form a complex multi-phase material system, but also embed the soluble 2D nanosheets covalently functionalized with organic/polymeric materials in a polymer host to form host-guest composite materials that are expected to improve the OL performance of the whole system. All in all, an optimized complex multi-component nanomaterial system enormously enhances the performance and applicability of OL devices. In addition, the fundamental studies of the photophysical and photonic properties of 2D nanomaterials and their derivatives in various solid hosts are of significance for modifying the nanomaterials at a molecular level.
1. Graphene and its derivatives
The most representative products of nanotechnology are based on carbon nanomaterials: zero-dimensional (0D) fullerene, one-dimensional (1D) CNTs, two-dimensional (2D) graphene and three-dimensional (3D) carbon nanoparticles and graphite. All of them Allotropes of carbon exhibit various NLO responses. For example, carbon black suspension has a strong thermally induced NLS effect and is suitable for protecting against strong nanosecond pulse lasers; fullerene has a large third-order optical response in a specific wavelength band. Nonlinear and RSA properties; CNTs exhibit ultrafast second- and third-order nonlinearities and saturable absorption (SA) properties in the near infrared region (NIR). The optical limiting response of CNTs suspension mainly comes from Thermally induced solvent bubble NLS, while soluble CNTs exhibit a strong solution concentration-dependent optical limiting effect, which belongs to the NLA mechanism. Even though the linear transmittance of CNTs suspension is the same as that of solvated CNTs At the same rate, the optical limiting performance of CNTs suspension is still independent of the concentration of CNTs. A better optical limiting effect will be obtained when CNTs are dispersed in a solvent with low boiling point, low viscosity and low surface tension. At high incident energy intensity or density, the higher the concentration of CNTs contained in the polymer/CNTs, the more effectively it can block the incident laser. However, the optical limiting mechanism in these composites is quite complex. Graphene has highly conjugated π electron conjugation The linear dispersion relationship between the system and the electronic band structure exhibits ultrafast carrier relaxation dynamics and ultra-broadband resonance nonlinear optical response properties. Similar to CNTs, no matter what kind of graphene nanostructure they are, they exhibit ultrafast carrier relaxation dynamics and ultra-broadband resonance nonlinear optical response properties. Both exhibit strong broadband optical limiting characteristics at 1064 nm. The OL mechanism of this type of material in suspension is mainly NLS, while the optical limiting mechanism of soluble graphene and its derivatives is NLA (especially TPA mechanism) and the NLS mechanism.
There is a linear dispersion relationship between the energy and momentum of Dirac electrons in graphene near the Dirac point, resulting in a continuously resonant optical response in the broadband spectral range from the ultraviolet to the near-infrared region (> 2.5 μm). Single-layer graphite The linear light absorption of graphene has nothing to do with wavelength. For any low-intensity beam, the absorbance of each layer of graphene strictly follows π·α ≈2.3%. α is the fine structure constant, so the absorbance of multi-layer graphene is proportional to the number of layers. . Depending on different experimental conditions, graphene and GO will exhibit NLS, ESA, TPA or SA effects. Four-wave mixing technology confirmed the effective nonlinear magnetic susceptibility of graphene sheets |χ(3)| up to 10–7 esu. GO has a two-dimensional network structure formed by a mixture of sp2 and sp3 carbon bonds. The isolated nanoscale sp2 regions appearing in the sp3 region give GO an energy band gap. Therefore, the energy band can be controlled by size, shape and the ratio of sp2 clusters. Width, thereby obtaining adjustable photoluminescence and electroluminescence properties. Due to the existence of ultrafast carrier-carrier scattering and carrier-phonon scattering, graphene has ultrafast carrier dynamics. Under femtosecond (fs) pulse laser excitation, the in-band equilibrium time is as short as about 100 fs, and the in-band relaxation time is in the picosecond (ps) level.
A few examples will be given to briefly introduce the application research progress and future development directions of graphene and its derivatives in the field of laser protection. Girisun et al. [35] reported different proportions of Au-Fe2O3 (15 wt%, 25 wt%, 40 wt% )-doped reduced graphene oxide (RGO) nanocomposite Au-Fe2O3-RGO under 700-900 nm femtosecond pulse laser irradiation, as shown in Figure 3. Using 700, 800 When irradiated by 700 nm and 900 nm lasers, the material exhibits typical RSA signals. By comparison, it can be found that Au-Fe2O3-RGO doped with a ratio of 15 wt% has the best optical limiting performance under 700 and 800 nm laser conditions. , and under 900 nm laser conditions, 15 wt% and 25 wt% have similar optical limiting performance. This study verified that the surface plasmon resonance effect of inorganic metal nanoparticles promotes the optical limiting performance, through different loads The proportion of inorganic nanoparticles to the surface of graphene can realize the adjustment of the nonlinear optical properties of the material in the near-infrared region.
(a) Synthesis of Au-Fe2O3-RGO composites; open aperture patterns of the samples at (b) 700, (c) 800, and (d) 900 nm
Compared with graphene loaded with inorganic nanoparticles, graphene derivative materials modified by organic small molecules and polymers usually have better solubility, stability and processability. Liu et al. respectively used Pt complexes through cyclization Reaction and electrostatic adsorption to modify the GO surface (Figure 4(a)), and the impact of the synergistic effect between different optical limiting mechanisms on material properties was systematically studied. With GO, Pt-1, Pt-2 and C60 Compared with the Pt complex-modified materials GO-Pt-1 and GO-Pt-2, the optical limiting performance is greatly improved (Figure 4(b) and Figure 4(c)). This is due to the fact that in the modified material system , Pt complexes containing acetylene groups and GO play the roles of electron acceptors and electron donors respectively, which not only reduces the ground state absorption of the material, but also greatly enhances the excited triplet state absorption of the material. The electrons and energy between the donor and the acceptor The synergistic effect of transfer with NLS and TPA of graphene dispersion allows the Pt complex-modified material to exhibit excellent laser protection effect. Interestingly, in traditional understanding, non-covalently modified graphene materials are Due to poor solubility and dispersion stability, its nonlinear optical properties are usually slightly inferior to covalently modified graphene materials. However, in this report, the non-covalently modified GO material GO-Pt-2 using electrostatic adsorption has It has better optical limiting performance than covalently modified GO-Pt-1. As shown in Figure 4(b), under the test conditions of 532 nm laser wavelength, the normalization of GO-Pt-1 and GO-Pt-2 The normalized transmittances are 45% and 28% respectively. At the same time, the GO material GO-Pt-2, which is non-covalently modified by the Pt complex, also has the lowest limiting threshold (0.85 J/cm2, Figure 4(c) ). On the same unit area of GO surface, GO-Pt-2 has more Pt complex molecules than GO-Pt-1, so it has better light limiting response. In addition to Pt complexes, porphyrin phthalocyanines Because of its unique 18π electron macrocyclic structure, the material has also been widely used to covalently modify graphene to improve its processability and nonlinear optical properties. Du et al. combined porphyrin molecules and porphyrin group-containing The copolymers were covalently modified on the graphene surface (Figure 4(d)), and the optical limiting performance of the material under 532 and 1064 nm laser irradiation was studied (Figure 4(e) and Figure 4(f)). By Comparison found that the performance of porphyrin-functionalized RGO (PF-RGO, ZnP-RGO) composite materials is better than that of porphyrin-functionalized GO (PF-GO, ZnP-GO) composite materials. At the same time, in the same kind of graphene Among the materials, the performance of materials modified with porphyrin copolymers is better than that of graphene materials modified with porphyrin small molecules. Among this series of materials, the copolymer-modified graphene material PF-RGO has the lowest normalized transmittance rate and the largest nonlinear extinction coefficient. This is because the system has the largest π conjugated system, which is conducive to the rapid transmission of electrons and energy. The enhancement of optical limiting performance is also attributed to the electron and energy transfer effect between systems. The synergistic effect of nonlinear scattering of graphene dispersions and TPA.
(a) Synthesis of GO-Pt-1 and GO-Pt-2; (b) typical open-aperture Z-scan data and (c) optical limiting performance of the samples at 532 nm; (d) schematic illustration of the structure of PF-GO and ZnP-GO (insert shows the photographs of dispersions in DMF: (I) ZnTNP-PAES; (II) GO; (III) ZnP-GO; (IV) PF-GO; (V) PF-RGO; (VI) ZnP-RGO.); open-aperture Z-scan curves with normalized transmittance (open symbols) and scattering signal (solid symbols) for the samples at (e) 532 and (f) 1064 nm
Generally, aggregation-induced emission (AIE) organic/polymer materials are not suitable for nonlinear optical materials because strong intermolecular interactions often lead to an increase in relaxation channels in the system and a shortened excited state lifetime. Thereby reducing the nonlinear absorption capacity of the material. In order to make full use of this type of material, Liu et al. [37] designed and prepared a polymer PAHFTP with aggregation-induced emission characteristics, and then covalently grafted it onto RGO to obtain a New material PFTP-RGO (Figure 5). The charge transfer effect between PAHFTP and RGO caused the fluorescence intensity of the original PAHFTP in the solid film to decrease by 91.89%. PFTP-RGO was embedded in non-optically active polymethacrylate. The PFTP-RGO/PMMA film prepared in polymethyl methacrylate (PMMA) exhibits broad-spectrum laser protection capabilities, while the PMMA film based on pure PAHFTP does not exhibit any nonlinear optical properties. Compared with PFTP-RGO/PMMA and RGO/ Compared with PMMA films, PFTP-RGO/PMMA treated by thermal annealing at 100°C in a nitrogen atmosphere shows better nonlinear optical properties, with optical limiting thresholds of 0.24 and 0.18 GW/cm2 obtained at 532 and 1064 nm, respectively. Thermal damage thresholds are approximately 33.88 J·cm–2 (value measured under 532 nm, 900 μJ laser irradiation) and 37.32 J·cm–2 (value measured under 1064 nm, 1000 μJ laser irradiation). ).
(a) Synthesis of PFTP-RGO. (b) Variation of the normalized transmittance as a function of input laser intensity for the films: (b1) at 532 nm; (b3) at 1064 nm; the corresponding βeff coefficients as a function of the excitation pulse energy (b2), (b4)
2D Nanomaterials in Laser Protection Field Application--Black Phosphorus And Its Derivatives
To achieve simultaneous protection against both pulsed and continuous wave (CW) or quasi-CW Laser Protection, significant research effort has been devoted to the state-of-the-art optical limiting (OL) materials and processes in an attempt to achieve some measures of protection against such laser beams in the past decades. Two-dimensional (2D) nanomaterials with a lot of unique properties, including graphene, transition metal dichalcogenides, black phosphorus and others, have aroused the extensive research interest of many researchers. In this review paper, we describe systematically the OL mechanisms and the recent achievements in the 2D nanomaterials and their organic/polymeric derivatives for laser protection. In an effort to sustain the advantage of 2D nanomaterials, one can not only introduce the functional molecules or polymers to blend with them to form a complex multi-phase material system, but also embed the soluble 2D nanosheets covalently functionalized with organic/polymeric materials in a polymer host to form host-guest composite materials that are expected to improve the OL performance of the whole system. All in all, an optimized complex multi-component nanomaterial system enormously enhances the performance and applicability of OL devices. In addition, the fundamental studies of the photophysical and photonic properties of 2D nanomaterials and their derivatives in various solid hosts are of significance for modifying the nanomaterials at a molecular level.
1. Black phosphorus and its derivatives
As the most stable allotrope among phosphorus materials, BP was successfully synthesized through high-temperature and high-pressure calcination of red phosphorus as early as 1914. With the development of physical and chemical exfoliation technology, it was not until 2014 that scientists successfully used exfoliation technology for the first time. A two-dimensional layered few-layer BP nanosheet material (also known as phosphorene) was prepared, and the first two-dimensional BP-based field effect transistor device and other corresponding optoelectronic devices were successfully prepared using this material. Figure 6 gives the top view and side view of the BP lattice structure. Since then, research results on two-dimensional BP materials have experienced explosive growth. Two-dimensional BP materials have many excellent properties:
1) Different from the zero-bandgap two-dimensional graphene material, the BP two-dimensional material has a directly adjustable band gap from 0.3 eV (bulk BP) to 2.0 eV (single-layer BP), making the two-dimensional BP material possess semiconductor properties , has huge application potential in various types of optoelectronic materials;
2) Due to the wrinkle structure on the surface of the two-dimensional BP material, BP has strong in-plane anisotropy that is different from other two-dimensional materials;
3) Through physical doping and chemical means, the band gap of BP two-dimensional materials can be effectively adjusted, allowing BP to be used in the fields of communications and thermal imaging, and its absorption can cover almost the entire electromagnetic spectrum, which is unique to other two-dimensional materials. incomparable;
4) High carrier mobility, up to 6500 cm2·V–1·s–1.
However, while the BP two-dimensional nanomaterial has many advantages, its shortcomings are also more prominent: when stripped to a two-dimensional nanometer size, the two-dimensional BP material will undergo rapid chemical degradation in the atmosphere in the presence of oxygen, causing its A sharp decline in semiconductor performance. Like most two-dimensional nanomaterials, two-dimensional BP nanomaterials have poor solubility in commonly used organic solvents, resulting in severe phase separation, which greatly limits their use in preparing film-forming devices. Applications in fields. Despite these outstanding shortcomings, the emergence of two-dimensional BP nanomaterials still provides significant opportunities for the research and development of new organic optoelectronic materials, but also brings challenges.
(a) Top view of the puckered honeycomb lattice of black phosphorus; (b) lateral view on the lattice in armchair direction. Insets: BP lattice with a six-membered ring in chair configuration highlighted in red; scanning tunneling electron microscopyimage of the BP lattice.
Lu et al. and Guo et al. reported the nonlinear optical properties of multi-layer BP nanosheets and few-layer BP nanosheets in the solution state and film state respectively. The multi-layer and few-layer nanosheets prepared by liquid phase exfoliation technology, whether at 400 Under laser excitation of 800 nm or 800 nm, BP nanosheets showed a typical saturated absorption response. Shi et al. prepared a D-A type BP/C60 blend using few-layer BP as the electron donor and C60 as the electron acceptor, and doped it with The first BP/fullerene/PMMA composite laser protection functional material was constructed by mixing it into optically inactive PMMA (Figure 7), and the nonlinear optics and optical limiting of the material at 532 nm were studied using Z-scanning technology. Ability. Compared to C60, BP and unannealed BP/C60/PMMA composites, the annealed composites showed significantly enhanced nonlinear optical and laser protection responses. The observed nonlinear coefficients were at 400 μJ pulse The energy increases from 39.39 cm/GW before annealing to 241.73 cm/GW, which is much larger than C60/PMMA (162.79 cm/GW), BP (–5.33 cm/GW, negative values indicate saturated absorption), which is at 400 μJ and 700 The limiting threshold and laser damage threshold obtained at μJ are 4.5 J/cm2 and 19.54 J/cm2 respectively.
(a)−(e) Typical open-aperture Z-scan data with normalized transmittance as a function of the sample position Z for the samples embedded in PMMA matrix under the excitation of 6 ns pulses at λ = 532 with different energies. The solid lines are the theoretical fitting results. (f) Structure of BP:C60 blends.
Nonlinear optical properties of dispersions of two BP nanosheets (BP-Small and BP-Big) with different sizes and thicknesses. The lateral size of BP-Small is about 114 nm and the thickness is 8-9 nm, corresponding to the number of nanosheet layers of about is 4 layers; the lateral size of BP-Big is about 338 nm, the thickness is 22 nm, and the corresponding number of layers is about 11. When irradiated by 532 nm, 6 ns pulse laser, these two BPs of different sizes showed typical SA signal (Figure 8(a) and Figure 8(b)). Through comparison, it is found that BP-Big has better NLO responsiveness than BP-Small and has stronger SA capability (Figure 8(c)).
Nonlinear optical properties of few-layer BP nanosheets under laser conditions of different wavelengths and pulse widths. As shown in Figure 8(d), Figure 8(e), Figure 8(g), and Figure 8(h), BP nanosheets Under either 340 fs or 6 ns low-energy pulse laser excitation, typical SA signals were exhibited in the visible and near-infrared regions. It was further found that BP nanosheets have better SA response in the visible range than in the near-infrared range. . Then they also studied the relationship between the optical properties of BP nanosheets and the incident laser energy under 532 nm, 6 ns pulse laser. As shown in Figure 8(f) and Figure 8(i), under low-energy pulse laser Under excitation, BP nanosheets still show typical SA signals and no scattering signals. As the incident laser energy further increases, saturation absorption and anti-saturation absorption appear at the same time and show a competing relationship. At the same time, scattering The signal is greatly enhanced, so the optical limiting response of the BP nanosheet dispersion can be attributed to NLS.
Open-aperture Z-scan fitted data of (a) BP-Big and (b) BP-Small; (c) NLO response of BP nanosheets with variable sizes BP-Big and BP-Small as a function of pulse fluence; open-aperture Z-scan results of the BP dispersion for nanosecond pulse excitation at (d) 532 nm and (e) 1064 nm and femtosecond pulse excitation at (g) 515 nm and (h) 1030 nm; (f) open-aperture Z-scan result and (i) corresponding scattering signal of BP dispersions at a 532 nm ns laser.
Although non-covalent modification (physical doping) is simple and easy to implement, it cannot effectively solve the problems of poor stability of BP and easy phase separation of the system under high load. In comparison, organic/polymer covalent modification of BP It is a good method that can not only improve the environmental stability of BP, improve the solubility of the material in organic solvents, but also greatly promote the photoelectric and nonlinear optical properties of the material. Liu et al. used newly synthesized zinc based on dodecafluoride lateral substitution Phthalocyanine diazonium salt F12PcZn-N+2BF−4
Successfully covalently grafted fluorine-containing zinc phthalocyanine onto the surface of two-dimensional BP, obtaining a new D-A type nanohybrid material F12PcZn-BP (Figure 9). This material has very good performance in common organic solvents Solubility and excellent environmental and thermal stability. Compared with BP/PMMA and F12NH2PcZn/PMMA films, F12PcZn-BP/PMMA films exhibit better nonlinear optical and optical limiting properties. The F12PcZn-BP/PMMA films were After thermal annealing at 200°C for 30 minutes in the atmosphere, the nonlinear absorption coefficient at 532 nm increased from 177.10 cm/GW before annealing to 263.47 cm/GW, and the optical limiting threshold increased from 0.72 GW/cm2 before annealing. reduced to 0.61 GW/cm2.
(a) Schematic illustration of the fabrication F12PcZn-BP; (b) (I)−(III) typical open-aperture Z-scan data of the samples and (IV) variation in the normalized transmittance as a function of input laser intensity for the PMMA-based films at 532 nm.
2D Nanomaterials in Laser Protection Field Application--Molybdenum Disulfide And Derivatives
To achieve simultaneous protection against both pulsed and continuous wave (CW) or quasi-CW Laser Protection, significant research effort has been devoted to the state-of-the-art optical limiting (OL) materials and processes in an attempt to achieve some measures of protection against such laser beams in the past decades. Two-dimensional (2D) nanomaterials with a lot of unique properties, including graphene, transition metal dichalcogenides, black phosphorus and others, have aroused the extensive research interest of many researchers. In this review paper, we describe systematically the OL mechanisms and the recent achievements in the 2D nanomaterials and their organic/polymeric derivatives for laser protection. In an effort to sustain the advantage of 2D nanomaterials, one can not only introduce the functional molecules or polymers to blend with them to form a complex multi-phase material system, but also embed the soluble 2D nanosheets covalently functionalized with organic/polymeric materials in a polymer host to form host-guest composite materials that are expected to improve the OL performance of the whole system. All in all, an optimized complex multi-component nanomaterial system enormously enhances the performance and applicability of OL devices. In addition, the fundamental studies of the photophysical and photonic properties of 2D nanomaterials and their derivatives in various solid hosts are of significance for modifying the nanomaterials at a molecular level.
1. Molybdenum disulfide and its derivatives
Transition metal sulfides have a layered structure similar to graphene, and its general structural formula is MX2 (M is Mo, W, Nb, Ta; X is S, Se). A single-layer transition metal sulfide nanosheet contains three atoms The middle layer is a transition metal layer, and the single-layer nanosheets are stacked on each other through van der Waals forces. The biggest structural feature of transition metal sulfides is that they form different crystal structures depending on the stacking method. The most typical transition Metal sulfide represents molybdenum disulfide (MoS2) as an example. MoS2 nanosheets have large in-plane carrier mobility (200-500 cm2·V-1·s-1) and small band gap (1.2-1.9 eV), high fluorescence quantum yield, large surface area and good photoelectrochemical stability, etc., showing great potential application value in many high-tech fields. MoS2 has 1H, 1T, 1T′, 2H and 3R 5 crystal configurations. Different crystal structures also have different effects on the carrier dynamics and nonlinear optical response properties of the material. 2H phase MoS2 with a triangular prism structure and semiconductor properties is widely used in transistor devices. The less stable 1T′ crystal configuration exhibits dipolar ferroelectricity, semiconducting properties, and quantum spin Hall effect. The 1T phase MoS2 has an octahedral structure and usually exhibits metastable metallic properties. By By inserting alkali metal hydrides into the MoS2 lamellar structure as electron donors, Tan et al. [58] successfully achieved the transformation of the MoS2 crystal configuration from 2H to 1T′ configuration. Under 900 nm femtosecond laser irradiation, 1T The ′ phase MoS2 nanosheets exhibit a typical saturable absorption effect (Figure 10), while the 2H phase MoS2 nanosheets are saturated absorption at low energy and TPA at high energy. This transformation is due to the 1T′ phase MoS2 Nanosheets have higher conduction band electron occupancy, optical transmittance and optical Kerr nonlinear effect.
Open (a) and closed (b) aperture Z-scan measurements of h-LiMoS2 and MoS2 at different input laser power, indicated at the top left of each curve, showing saturable absorption and self-focusing behavior of h-LiMoS2 at a lower pumping power.
MoS2 nanosheet dispersion has ultrafast SA properties under 800 nm femtosecond laser pulse. Thin-layer MoS2 nanosheets of different sizes were obtained by gradient centrifugation method. These materials showed size-dependent properties under 532 nm picosecond pulse laser excitation. NLO phenomenon. It was found that the OL performance of MoS2 nanosheets is better than that of graphene under high pulse energy. Similarly, the solid-state optical limiter prepared by embedding MoS2 nanosheets in a PMMA film has femtosecond and picosecond performance at 800 nm. It has a large nonlinear absorption coefficient and a very small limiting threshold under laser pulse. TiS2 nanosheets have optical limiting performance comparable to C60 under 532 nm high-energy nanosecond pulse laser excitation. This is due to the TiS2 nanosheet dispersion. Excellent TPA and ESA properties.
The biggest advantages of using chemical synthesis technology to covalently modify MoS2 nanosheets are:
1) It can effectively prevent the agglomeration of MoS2 nanomaterials, and the consequence of agglomeration is to reduce or significantly reduce the nonlinear optical properties of the material;
2) Utilizing the intramolecular charge transfer effect between polymers with electron-donating properties and MoS2 will greatly improve or promote the nonlinear optical and optical limiting properties of the material;
3) Since the material is soluble in common organic solvents, it brings great technical convenience to low-cost materials and device processing. Based on these ideas, pre-synthesized MoS2-DDAT (S-1-dodecyl-S′ -(α, α′-dimethyl-α′′-acetate)trithiocarbonate, DDAT) reversible addition-fragmentation chain transfer agent (RAFT) was synthesized in situ for the first time. Vinylcarbazole (Poly[N-vinylecarbazole], PVK) covalently grafted MoS2 laser protection functional material (MoS2-PVK) (Figure 11). Compared with MoS2 and MoS2/polymer co-mixture, polymer covalently grafted The branched MoS2 derivatives exhibit strong broad-spectrum laser protection properties both in N, N-dimethylformamide (DMF) solution and in PMMA solid films. When the excitation wavelength is 532 and 1064 nm, the maximum nonlinear coefficients of MoS2-PVK/PMMA are 917.00 cm/GW (MoS2/PMMA is 100.69 cm/GW, MoS2/PVK/PMMA is 125.12 cm/GW) and 461 cm/GW (MoS2/ PMMA is –48.92 cm/GW, MoS2/PVK/PMMA is 147.56 cm/GW). The optical limiting thresholds of MoS2-PVK/PMMA are 0.3 and 0.5 GW·cm–2 respectively. Interestingly, at the 532 nm excitation wavelength When the MoS2-PVK/PMMA film was irradiated by laser energy of 250 μJ, and then the laser energy was reduced to 50 μJ, the material showed an RSA response, instead of the SA response shown before at 50 μJ. . This phenomenon is most likely related to the thermal effect produced by laser action on the film. In order to verify this inference, the author annealed MoS2-PVK at 200 ℃ in an N2 atmosphere, and tested the sample film under 532 nm, 50 μJ laser radiation. NLO performance when irradiated. It was found that the NLO response mechanism of annealed MoS2-PVK/PMMA is RSA response. After the material is annealed at high temperature, on the one hand, the degree of crystallization of PVK increases, on the other hand, the intramolecular carbazole unit of PVK The π-π stacking effect is enhanced, which increases the intramolecular charge transfer efficiency. In addition, heat treatment also causes changes in the surface morphology of the material. After high-temperature annealing treatment, MoS2-PVK becomes a coral reef shape. These results show that the annealing treatment induces The molecular and electronic structure of MoS2-PVK changes, which ultimately causes the NLO response mechanism of the material to change at 50 μJ.
Fig. 11. (a) Synthesis and (b), (c) NLO (OL) performance of MoS2-PVK
Considering that in MoS2-PVK, MoS2 and PVK play the roles of electron acceptor and electron donor respectively, polyacrylonitrile (PAN) covalently modified MoS2 nanomaterials (MoS2- PAN, Figure 12(a)). When excited by 532 and 1064 nm lasers, the nonlinear absorption coefficients of MoS2-PAN/PMMA films reached 1015.75 cm/GW (504.15 cm/GW for MoS2/PAN/PMMA) and 534.00 cm/ respectively. GW (245.07 cm/GW for MoS2/PAN/PMMA). The optical limiting thresholds of MoS2-PAN/PMMA are 1.98 J·cm–1 (532 nm) and 3.60 J·cm–1 (1064 nm) respectively. High temperature annealing The MoS2-PAN/PMMA material (pyro-MoS2-PAN/PMMA) after treatment (processed at 200°C for 4 h in a nitrogen atmosphere) has better nonlinear optical and optical limiting responses. The nonlinear absorption coefficient reaches 1151 cm/GW respectively. (532 nm) and 699 cm/GW (1064 nm), the optical limiting threshold is further reduced to 1.56 J·cm–1 (532 nm) and 2.67 J·cm–1 (1064 nm). From Figure 12(b) ) It can be seen that when polyacrylonitrile is thermally cracked at 200-300°C, adjacent cyano groups will each open a bond and couple with each other to form a -N=C-N=C-conjugated structure system. The electrons and Holes can migrate along this conjugated chain. In MoS2-PAN before annealing (Figure 12(c)), the nuclear electron energy spectrum of Mo 3d shows four peaks, respectively located at 228.1 (Mo 3d5/2), 231.3 (Mo 3d3/2), 229.1 (Mo 3d5/2) and 232.2 (Mo 3d3/2) eV. The two peaks of 228.1 and 231.3 eV belong to 1T-MoS2, while the two peaks of 229.1 and 232.2 eV belong to 2H-MoS2. The peak at 225.9 eV is attributed to S 2s. After annealing MoS2-PAN at 220 °C for 4 h, the intensity of the peak in pyro-MoS2-PAN attributed to the MoS2 1T phase decreases significantly (Figure 12(d) ), and the intensity of the peak attributed to the 2H phase is greatly enhanced, which means that a phase transition from 1T phase to 2H has occurred, indicating that the MoS2 nanosheets in pyro-MoS2-PAN are better than the MoS2 nanosheets in MoS2-PAN. With higher semiconductor properties. An asymmetric porphyrin covalently modified molybdenum disulfide quantum dots (MQDs) composite material MQDs-TPP. Nonlinear absorption coefficient and limiting threshold of MQDs-TPP under 532 nm laser radiation Reaching 1059 cm/GW and 1.62 J/cm2 respectively; the corresponding parameters under the 1064 nm wavelength laser are 831 cm/GW and 1.97 J/cm2 respectively.
Fig. 12. (a) Synthesis of MoS2-PAN and pyro-MoS2-PAN; (b) pyrolytic process of PAN; the Mo 3 d core level XPS spectra of (c) the non-annealed MoS2-PAN and (d) the pyro-MoS2-PAN. The 2 H and 1 T contributions are represented by red and green plots, respectively.
2D Nanomaterials in Laser Protection Field Application--Metal Halide Perovskites
To achieve simultaneous protection against both pulsed and continuous wave (CW) or quasi-CW Laser Protection, significant research effort has been devoted to the state-of-the-art optical limiting (OL) materials and processes in an attempt to achieve some measures of protection against such laser beams in the past decades. Two-dimensional (2D) nanomaterials with a lot of unique properties, including graphene, transition metal dichalcogenides, black phosphorus and others, have aroused the extensive research interest of many researchers. In this review paper, we describe systematically the OL mechanisms and the recent achievements in the 2D nanomaterials and their organic/polymeric derivatives for laser protection. In an effort to sustain the advantage of 2D nanomaterials, one can not only introduce the functional molecules or polymers to blend with them to form a complex multi-phase material system, but also embed the soluble 2D nanosheets covalently functionalized with organic/polymeric materials in a polymer host to form host-guest composite materials that are expected to improve the OL performance of the whole system. All in all, an optimized complex multi-component nanomaterial system enormously enhances the performance and applicability of OL devices. In addition, the fundamental studies of the photophysical and photonic properties of 2D nanomaterials and their derivatives in various solid hosts are of significance for modifying the nanomaterials at a molecular level.
1. Metal halide perovskites
Organometal halide perovskite (CH3NH3PbI3) and its derivatives [(C4H9NH3)2(CH3NH3)x–1SnxI3x+1, CH3NH3PbBrxCl3–x, CH3NH3PbBrxI3–x, CH3NH3MX3 (M = Pb, Sn; X = Cl, Br, I), CH3NH3PbClxI3–x and CH3NH3SnBrxI3–x] have high linear absorption coefficient (1.5 × 104 cm–1 at 550 nm), tunable energy band gap (1.17–2.3 eV), and long exciton diffusion length (100- 1000 nm), high mobility (the mobility of CH3NH3PbI3 is 66 cm2·V–1·s–1, the mobility of CH3NH3SnI3 reaches 2320 cm2·V–1·s–1), high crystallinity and solution processability, etc. The excellent performance has aroused widespread research interest in the fields of organic solar cells, photodetectors, transistors, light-emitting diodes, etc. The general chemical formula of inorganic perovskite materials is ABX3, in which the size of cation A is larger than that of cation B, and X is an anion that can combine with these two cations. Similar to transition metal sulfides, perovskites also have three different crystal types (100, 110 and 111, Figure 13).
Fig. 13. (a) Key structural factors that influence the properties of halide perovskites; (b) (I) representative crystal structures of halide perovskites in different dimensions; (II) nanoscale morphologies of halide perovskites; (III) schematic representation of the 2D organic-inorganic perovskites from different cuts of the 3D halide perovskite structure.
CsPb(Br/I)3 perovskite quantum dots with high crystallinity generally have better TPA response than traditional metal chalcogenide quantum dots. CH3NH3PbI3 films show strong nonlinear refractive effects and SA caused by Pauli blocking effect. Effect. The nonlinear refractive index of CH3NH3PbI3 and CH3NH3PbI3–xClx perovskites is three orders of magnitude greater than that of silicon. The nonlinear refractive index of CH3NH3PbI3–xClx is twice greater than the refractive index of CH3NH3PbI3. Colloidal halide perovskites were prepared Quantum dots. Research has found that the TPA cross-section of CH3NH3PbBr quantum dots is 5.23 × 106 GM (1 GM = 10–50 cm4·s·photon–1·molecule–1), which is one order of magnitude higher than CsPbBr3 quantum dots (1.2 × 105 GM). Under 800 nm laser excitation, the nonlinear absorption coefficient of CH3NH3PbBr3 is 8.2 cm·GW–1, which has a similar band gap to epitaxial single crystal semiconductors. The third-order nonlinear coefficient and decay time are related to the halide composition (CH3NH3PbBr3, CH3NH3PbBr2I), The relationship between temperature and excitation wavelength. The maximum third-order nonlinear magnetic susceptibility χ(3) is about 1.6 × 10–6 esu, which is similar to or larger than many ordinary third-order materials. Crystalline semiconductors and heavy atoms can Produces a large χ(3) value, about 10–5 esu. As the temperature increases, non-resonant excitation significantly reduces the nonlinear optical properties. Using an I instead of a Br in Br3 makes the nonlinear absorption The coefficient is reduced by at least one order of magnitude, and it is speculated that the reason may be due to the lack of strong exciton resonance in CH3NH3PbBr2I. The rapid formation of free electrons and holes in CH3NH3PbBr2I leads to a strong Coulomb force effect. Mixing of triiodide perovskite CH3NH3PbI3 and iodine chloride Nonlinear optical properties of perovskite CH3NH3PbI3–xClx (Figure 14). Both materials exhibit typical SA responses at 532 and 1064 nm and maintain very good absorption effects even at high laser energies. The nonlinear absorption coefficient and saturation strength of CH3NH3PbI3 are –2.25 cm·MW–1 and 12.71 GW·cm–2 respectively, while the corresponding parameters of CH3NH3PbI3–xClx are –2.03 cm·MW–1 and 12.61 GW·cm–2 respectively.
Fig. 14. (a) Illustration of halide perovskites based NLO materials; (b) typical open-aperture Z-scan curves of CH3NH3PbI3 and CH3NH3PbI3–xClx at 1064 nm; (c) typical open-aperture Z-scan curves of CH3NH3PbI3 and CH3NH3PbI3–xClx at 532 nm.
Polyvinyl carbazole (PVK)/organic perovskite (CH3NH3PbI3) composite functional material PC, the nonlinear optical and optical limiting behavior of the material in DMF solution and PMMA polymer matrix was systematically studied using Z-scanning method (Figure 15) . When irradiated by 532 and 1064 nm lasers, neither PVK, CH3NH3PbI3 nor their blended composite PC has any nonlinear optical properties in DMF solution. After annealing at 200 ℃ for half an hour, PC has 532 nm in the same solution. It exhibits saturable absorption properties at 1064 nm, exhibits saturable absorption properties at low pulse energy, and exhibits anti-saturation absorption properties when irradiated with higher pulse energy. When PC is embedded in non-optically active PMMA The center and rear end show RSA performance and excellent optical limiting performance whether it is irradiated by 532 nm or 1064 nm laser.
Fig. 15. Typical open-aperture Z-scan data of the CH3NH3PbI3:PVK/PMMA films with different CH3NH3PbI3:PVK concentrations. The annealing condition: 200 ℃ for 30 min in N2.
Conclusion and Outlook
Laser destroys the target through thermal effects (causing holes or dents), mechanical damage (causing deformation and cracking of the target object), and radiation damage (the laser attacks the object, causing the vaporized material to produce a plasma cloud that can radiate X-rays and ultraviolet rays) , thereby causing further damage to the target), it can effectively destroy aircraft, missiles, tanks, ships and other military targets in a very short time, and damage human eyes. Ideal laser protection materials have high linearity in response to weak radiation The transmittance can effectively prevent the damage of any intensity of incident laser to photoelectric load devices, military equipment and human eyes in a wide spectral range. It has fast response time, high damage threshold, small limiting threshold, and reaches nonlinearity. It maintains good nonlinear optical response over a wide range of light flux before saturation, but it is quite difficult to meet these requirements. For two-dimensional nanomaterials, the biggest problem currently faced is:
1) These materials have a strong aggregation effect in the solid state, are difficult to dissolve in any common organic solvents, and have poor dispersion stability, which brings serious difficulties to the processing of materials;
2) It is difficult to accurately prepare two-dimensional materials with controllable number of layers and controllable size on a large scale and at low cost. Therefore, future work will mainly focus on the efficient and low-cost controllable preparation of two-dimensional materials and the covalent chemistry of two-dimensional materials. Innovative design of modified derivatives, structural design and optimization of high-performance solid laser protection devices, etc. In addition, how to accurately understand and master nonlinear photophysical processes based on two-dimensional nanomaterials, reveal the excited state properties of materials, and light scattering and the impact of the charge/energy transfer process in the material system under the action of light on the nonlinear optical properties of the material, and further optimize the material structure design and material composition control, carry out device structure optimization design and packaging process research, and break through the low and uniform damage threshold of composite materials Engineering technical difficulties such as poor properties and easy agglomeration are crucial to the realization of efficient wide-spectrum laser protection, and they are also problems that must be effectively solved during the research and development process.
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