“Matryoshka Doll” Heterostructures Induce Electromagnetic Parameters Fluctuation to Tailor Electromagnetic Wave Absorption

Components manipulation and structure engineering have shown powerful approaches for tailoring electromagnetic (EM) parameters. However, the integration of controllable architectural and compositional complexity into one multiple‐layer heterostructure seems significantly effective but remains a considerable challenge, and correlative quantized modulation of EM parameters is scarce. Herein, three types of metal–organic frameworks (MOFs) hybrids and derived sulfides are ingeniously fabricated by MOF architecture engineering and solvothermal sulfuration. Compared to the random assembly of MOFs in the “Chaotic” structure (Structure 1), the regular arrangement of MOF‐on‐MOF heterostructures in the “Matryoshka doll” structure (Structure 2) guarantees improved lattice strain and defect arising from abundant heterointerfaces in core–shell heterostructure. Impressively, such aforementioned advantages together with sufficiently exposed defect sites and conductivity display an interesting “quantized state” between “0 state” and “1 state” in the subsequent ion exchange process for “Matryoshka doll” structured Zn–Co sulfides, benefiting EM parameters “quantization” and boost electromagnetic wave (EMW) absorption. Further, introducing metal ions in a cation‐doped “Matryoshka doll” structure (Structure 3) subtly optimizes composition and defect, leading to enhanced impedance matching and effective absorption bandwidth of 7.8 GHz at 2.6 mm. This study highlights the significant eﬀect of multiple‐layer heterostructures on EM parameters fluctuation, which is in synergy with lattice defect, sulfur vacancy, and conductivity to tailor desirable EMW absorption capacity.


Introduction
Pursuing high-performance electromagnetic wave absorbing materials (EMWAMs) is of far-reaching significance, and extensive efforts have focused on designing multicomponents manipulation and ingenious structure engineering to simultaneously unlock the bottleneck of mismatched impedance and narrow absorption bandwidth issues for conventional absorbers. [1][2][3][4] On account of elegant structures and elaborate functionalities, hybrid materials particularly nanostructures with high complexity are emerging as an important research hotspot. The nanostructural complexity can be classified according to composition, architecture, and the integration of both compositional and architectural diversity into one nanoarchitectonics. [5] As an emerging class of crystalline porous materials, metal-organic frameworks (MOFs) are promising candidates owing to their flexible tunability in composition and architecture. [6][7][8] The size, structure, and morphology of MOFs are feasible to be controlled to possess well-defined architecture. [9] More importantly, such well-defined architecture with controlled components can provide an ideal model to unveil Components manipulation and structure engineering have shown powerful approaches for tailoring electromagnetic (EM) parameters. However, the integration of controllable architectural and compositional complexity into one multiple-layer heterostructure seems significantly effective but remains a considerable challenge, and correlative quantized modulation of EM parameters is scarce. Herein, three types of metal-organic frameworks (MOFs) hybrids and derived sulfides are ingeniously fabricated by MOF architecture engineering and solvothermal sulfuration. Compared to the random assembly of MOFs in the "Chaotic" structure (Structure 1), the regular arrangement of MOF-on-MOF heterostructures in the "Matryoshka doll" structure (Structure 2) guarantees improved lattice strain and defect arising from abundant heterointerfaces in core-shell heterostructure. Impressively, such aforementioned advantages together with sufficiently exposed defect sites and conductivity display an interesting "quantized state" between "0 state" and "1 state" in the subsequent ion exchange process for "Matryoshka doll" structured Zn-Co sulfides, benefiting EM parameters "quantization" and boost electromagnetic wave (EMW) absorption. Further, introducing metal ions in a cation-doped "Matryoshka doll" structure (Structure 3) subtly optimizes composition and defect, leading to enhanced impedance matching and effective absorption bandwidth of 7.8 GHz at 2.6 mm. This study highlights the significant effect of multiple-layer heterostructures on EM parameters fluctuation, which is in synergy with lattice defect, sulfur vacancy, and conductivity to tailor desirable EMW absorption capacity.
multiple-layer heterostructures on the quantized modulation of EM parameters for EMWAMs.
As is known, MOFs have been used as templates/precursors for the fabrication of carbonaceous materials, metal oxides/ sulfides for EMWAMs. [10][11][12] However, single MOFs and their derivatives always suffer from inferior EMW dissipation ability, and inevitably lead to indigent EMW absorption bandwidth. [13] In spite of the achievements made, their EM response abilities and impedance matching are still unsatisfactory due to the limited heterointerfaces, insufficiently exposed defect sites as well as inhibited electron formation/transfer. [14,15] Intuitively, these issues may be simultaneously addressed by modulating compositions and architectures via the integration of two or more different types of MOFs, that is, MOF-on-MOF nanoarchitectonics. This MOF-on-MOF nanoarchitectonics is anticipated to have several advantages: 1) Abundant heterogeneous interfaces included by the core-shell MOF@MOF heterostructures construction that cannot be directly obtained with pristine MOFs; 2) The presence of a lattice strain in the crystal arising from the core-shell structure, [16,17] and the enriched lattice defects as well as exposed vacancy sites generated by the post-treatment [18,19] (such as anion exchange between S 2À and 2-MeIM during solvothermal sulfuration; and 3) The controllable electron formation/transfer/ aggregation behavior [20][21][22] and the resulting EM parameters by flexibly but effectively modulating the components and layer numbers of such well-designed MOF-on-MOF nanoarchitectonics together with post-treatment. Therefore, to maximize heterostructure merits, well-defined multiple-layer heterointerfaces and various components within an integral MOF-on-MOF framework remain highly desirable but experimental challenge.
Although MOFs adopted to build MOF-on-MOF hybrids are intensively developed for various applications, [23][24][25] traditionally reported MOF-on-MOF nanoarchitectonics is mostly focusing on the double-layer MOF@MOF as well as their derived carbonaceous materials, [26][27][28][29] the construction of triple-layer even multiple-layer MOF-on-MOF hybrids (i.e., "Matryoshka doll" structure) has limited success, let alone their derived metal sulfides with delicate control of structure design and the exposed vacancy sites due to the etching effect of S 2À . Especially, upon the integration of MOFs with MOFs, another intriguing question arises regarding interfacial connectivity and structures. [30] Except for the regularly ordered growth of one-layer MOF on anotherlayer MOF to construct a "Matryoshka doll" structure, random self-assembly of different kinds of individual MOFs would give rise to another interesting "Chaotic" structure, which is similar to a traffic jam in a country road with respect to the unblocked traffic in an expressway for the former. Consequently, not only the resulting heterostructures but also related lattice defect, sulfur vacancy, and conductivity may result in huge but quantized variations in EM parameters and EMW absorption performance. Nevertheless, to the best of our knowledge, few studies have examined novel MOFs architecture engineering to reveal the relationship of multiple-layer heterostructures to quantized modulation of EM parameters, and little has been done to explicitly disclose the underlying involved EM response and attenuation mechanisms.
Encouraged by these considerations, we are ingenious to fabricate three types of MOFs hybrids (i.e., "Chaotic" structure (Structure 1), "Matryoshka doll" structure (Structure 2), and cation-doped "Matryoshka doll" structure (Structure 3) and corresponding metal sulfide by a stepwise crystal epitaxial growth strategy and solvothermal sulfuration. Due to abundant heterointerfaces in core-shell structure, Structure 2 exhibits improved lattice strain and defect level than Structure 1, and the subsequent ion exchange process can largely strengthen defect sites and conductivity ability for "Matryoshka doll" structured Zn-Co Sulfides, benefiting the improved EMW attenuation ability. Moreover, to further optimize the impedance matching and boost EMW absorption performance, metal ions are specially introduced in a cation-doped "Matryoshka doll" structure (Structure 3). Consequently, the obtained sulfides own broad effective absorption bandwidth (EAB) of 7.8 GHz at 2.6 mm due to improved matched impedance. Our work would provide valuable guidelines for rational design and accurate construction of multiple-layer heterostructures for obtaining high-performance EMWAMs.

Preparation of "Chaotic" and "Matryoshka Doll" Structured ZIF Precursors
Conjugation of two or more different MOFs into one MOF-on-MOF structure offers an intriguing strategy for the synthesis of complicated MOF hybrids with unprecedented nanostructures, [31] and the zeolitic imidazolate frameworks (ZIFs), such as Co-containing ZIF-67 and Zn-containing ZIF-8, are chosen as building blocks. The schematic illustrations of controllable synthesis of traditional "Chaotic" (Structure 1) and well-designed "Matryoshka doll" (Structure 2) structured ZIF precursors are presented in Figure 1a-f. For the synthesis of the "Chaotic" structue, it starts with the simultaneous addition of cobalt ion (Co 2þ ) and zinc ion (Zn 2þ ) into a 2-MeIM solution, where these two metal ions would be randomly bonded with 2-methylimidazole (2-MeIm) linkers as shown in Figure 1a. As a result, the asobtained ZIF-67/ZIF-8 (namely precursor-2L-DM) rhombic dodecahedrons ( Figure 1a1) shows the unordered (i.e., uniform) metal elements distribution of Co and Zn (Figure 1a2), which suggests the successful fabrication of "Chaotic" structured Co-Zn ZIFs precursor. In contrast, a stepwise crystal epitaxial growth strategy enables us to elaborately design the number of layers and components of ZIF precursors to harvest a welldesigned "Matryoshka doll" structure (Figure 1b-f ). Briefly, the ZIF-67 crystals are first prepared (Figure 1b) by the reaction between Co 2þ and 2-MeIM linkers, and then served as seeds in a fresh solution containing Zn 2þ and 2-MeIM linkers. Under the crystallization conditions, the ZIF-8 favors heterogeneous nucleation in the presence of ZIF-67 seeds (see Figure 1c and S1a, Supporting Information) to obtain a core-shell ZIF-67@ZIF-8 structure (denoted as precursor-2L) in an epitaxial manner due to their isostructural character with similar unit cell and coordination mode. [24] Following the consecutive epitaxial growth of ZIF-8 (or ZIF-67) layers based on their isostructural topology feature, we are able to achieve further seeded growth for the preparation of precursor-3L (Figure 1d), precursor-4L (Figure 1e), and precursor-5L (Figure 1f ), and their "Matryoshka doll" structures could be clearly confirmed by the energy-dispersive X-ray spectroscopy (EDS) mapping images in Figure 1b2-f2 and S1b-d. Similar to Structure 1, Structure 2 still retains well-defined rhombic dodecahedral structures but with slightly rough surfaces and larger sizes (Figure 1a1-f1). Even so, the X-ray diffraction (XRD) ( Figure S2a, Supporting Information) reveals that they possess the same crystal structure because of the analogous unit cell parameters of ZIF-8 (a = b = c = 16.9910 Å) and ZIF-67 (a = b = c = 16.9589 Å). [32] Although components of precursors are all composed of ZIF-67 and ZIF-8, we can achieve the controllable modulation of the defect level by simply changing the structural design of ZIF-8 and ZIF-67 (i.e., whether it is a "Chaotic" or "Matryoshka doll" structure, and layers a number of "Matryoshka doll" structure as shown in Figure S2b, Supporting Information). Therein, the lower intensity of photoluminescence (PL) spectra in "Matryoshka doll" structures exhibit a higher defect level than "Chaotic" structure, implying the ordered layer-by-layer coreshell assembly of ZIF-67 and ZIF-8 is superior to their random construction, which is possibly arising from the lattice strain arising from the core-shell heterostructures. Accordingly, their EM parameters ( Figure S3, Supporting Information) and EMW absorption properties ( Figure S4, Supporting Information) have shown a slight improvement relative to these of the "Chaotic" structure. Despite the obvious structural advantages, the EM parameters of these precursors are too low to ensure excellent EMWattenuation capability. Aiming to improve EM parameters and defect level while keeping the structural advantages, a solvothermal sulfuration method is subsequently adapted to design a series of "Chaotic" and "Matryoshka doll" structured Zn-Co sulfides upon the obtained ZIFs by the in situ transform process (see Experimental section). In this process, thioacetamide as a sulfur resource can decompose to give rise to sulfur ions (S 2À ) under alkaline conditions. The mobile S 2À will gradually adsorb to the surface of the dodecahedron, and gradually infiltrate through anion exchange to finally obtain Zn-Co ZIFs-derived Zn-Co sulfides, which are denoted as 1L, 2L, 3L, 4L, 5L, and 2L-DM, corresponding to ZIFs of precursor-1L, precursor-2L, precursor-3L, precursor-4L, precursor-5L, and precursor-2L-DM, respectively.
As expected, these sulfides almost inherit the shape of their ZIFs parent with some slight deformation caused by the etching effect of S 2À ( Figure S5, Supporting Information). As for 1L, revealed by X-ray diffraction (XRD) patterns in Figure 2a and High-resolution TEM (HR-TEM) in Figure S6, Supporting Information, it is composed of Co 3 S 4 (PDF#47-1738). With respect to 2L-DM, it is observed from Figure 2b1,b2 that the lattice spacing of 0.315 nm could be assigned to the Zn 0.76 Co 0.24 S, which is further confirmed by the selected area electron diffraction (SAED) and XRD analysis. It is worth noting that the Co and  Zn elements are homogeneously distributed over rhombic dodecahedron in EDS mapping images (Figure 2b3), this result clearly reveals the "Chaotic" structure of Precursor-2L-DM is retained for 2L-DM even after the high-pressure solvothermal sulfuration process. Similar to 2L-DM, the diffraction peak of Zn 0.76 Co 0.24 S (PDF#47-1656) without accompanied by any obvious phase is solely detected for 2L, 3L, 4L, and 5L ( Figure 2a and Table S1, Supporting Information). Generally, the sulfuration treatment, especially the high-pressure solvothermal sulfuration process, will induce lattice defects such as lattice dislocation and regional vacancy. [33] These regional vacancies and dislocation can change the stoichiometric ratio of some regions, leading to local electric field distortion and electron density redistribution and then enhancing dielectric polarization loss capacity. [34,35] To this end, the 3L is taken as an example to explore the differences in lattice information between "Chaotic" and "Matryoshka doll" structured Zn-Co sulfides, as shown in Figure 2c. A similar thing is that 3L (Figure 2c1 (Figure 2c3), lattice dislocation, discontinuous lattice fringe (Figure 2c5), and what's more, vacancy sites ( Figure 2c6) are obviously generated for 3L, indicating that "Matryoshka doll" structured Zn-Co sulfides own the higher defect level than that of "Chaotic" structured Zn-Co sulfide. The similar defect-rich feature found in 5L ( Figure S7, Supporting Information) and 2L ( Figure S8, Supporting Information) further verify the aforementioned result. All these information delivers that the "Matryoshka doll" structured Zn-Co sulfides possesses a highly defective nature, which will be responsible for EMW dissipation due to intense dielectric polarization loss. [36,37] Otherwise, EDS mapping images along with line scanning profiles results (Figure 2c9) once again signify the successful reservation of the "Matryoshka doll" structure for 3L, in which the S element is homogeneously distributed over the sample while the Co element is mainly concentrated in core and outermost shell, and the middle-shell is occupied by Zn   (Figure 2c7,c8). Considering that TEM and HR-TEM can only characterize the regional lattice defects, the PL is again applied to track and analyze the defect feature of "Chaotic" and "Matryoshka doll" structured Zn-Co sulfides. Evidently, as observed in Figure 2d, the lower spectra intensity representing the larger defect level follows the order: 3L > 5L > 2L > 4L > 2L-DM > 1L, suggesting the higher defect level could be obtained in "Matryoshka doll" structured Zn-Co sulfides than the "Chaotic" structured one, which is analogous to those of precursor ZIFs in Figure S2b, Supporting Information, further exhibiting the structural advantages of "Matryoshka doll" structure.
Here two things should be emphasized. For one thing, achieved by ion exchange, the "Matryoshka doll" structured Zn-Co sulfides show a much high defect level relative to the "Matryoshka doll" structured precursors ( Figure S9, Supporting Information) even if both of them possess comparable structure. This demonstrates the positive effect of the sulfuration strategy in improving the defect level, and also implies that sulfuration-related sulfur vacancies may be a significant role in boosting the defect level. For another thing, the higher defect level could be created in 3L than 2L, as well as 5L than 4L, which delivers a distinct information that compared to ZIF-8, the outermost layer of ZIF-67 in the "Matryoshka doll" structure is more favorable to generate lattice defects and sulfur vacancies by interacting with S 2À during solvothermal sulfuration process. This is due to the higher stability of ZIF-8 than ZIF-67 under solvothermal conditions. [32] To sum up, this "Matryoshka doll" structure can make us achieve improvement in defect level compared to the "Chaotic" structure. And what's more, the precursorto-sulfides transformation can undoubtedly enable us to precisely modulate the defect level of "Matryoshka doll" structured Zn-Co sulfides by simply adjusting the layer number of the ZIF-67 or ZIF-8.
Regular arrangement of ZIF-67 and ZIF-8 in the "Matryoshka doll" structure, rather than the random collocation among ZIF-67 and ZIF-8 in the "Chaotic" structure, has shown advantage not only can enrich heterointerfaces and promoting defects level, but also modulate the electronic structure of the synthesized Zn-Co sulfides. It can be clearly seen from X-ray photoelectron spectroscopy (XPS) spectra that a series of periodic binding energy shifts in S 2p (Figure 3a, as marked by a black dash line) and Zn 2p ( Figure S10a, Supporting Information, as marked by a black dash line) as well as Co 2p ( Figure S10b, Supporting Information, as marked by a black dash line) can be observed directly for "Matryoshka doll" structured Zn-Co sulfides (from 2L to 5L) compared to 1L and 2L-DM, revealing their strong electronic interactions among S, Co, and Zn after the regular introduction of ZIF-67 and ZIF-8 into 1L. It has been reported that some severe surface structural disorder and anion vacancy could be built resulting from such strong electron interactions. [35,38] Thus, focusing on the high-resolution S 2p XPS spectra in Figure 3a and Table S2, Supporting Information, a more obvious S 2p 1/2 peak that symbolizes the character of sulfur vacancy could be found in 3L (40.6%) and 5L (36.4%), followed by 2L (34.6%) and 4L (33.2%) compared to 2L-DM (31.7%) and 1L (30.1%), which is in good agreement with EPR results (Figure 3b) and high-resolution TEM evidences (Figure 2c5,c6). Previous reports have well revealed that sulfur vacancies can shape the band structure by introducing intra-band gap states, and benefits to the improved electronic conductivity. [35] The electronic impedance spectra (EIS) is conducted to investigate the electronic transfer resistance (R ct ) of samples, where the lower R ct , the higher the electronic conductivity. As shown in Figure 3c, the variation trend of conductivity is well coincided with that of V s results, suggesting that the improvement of sulfur vacancy is beneficial to the enhanced conductivity. These results indicate that the enhanced vacancy sites, together with multiple lattice defects (lattice dislocation and lattice distortion in Figure 2c), can produce a large number of additional active sites, thereby improving rapid charge transfer and electronic conductivity (Figure 3c). [39,40] This analysis is further validated by the improvement of the complex permittivity parameter (Figure 3d), which is a positive correlation to the conductivity of metal sulfides.
To better understand the underlying relationship among these factors, the complex permittivity parameters, sulfur vacancy concentration (V s ), as well as charge transfer resistance (R ct ), are collected and compared in Figure 3e and Table S3, Supporting Information. Initially, the as-synthesized Co 3 S 4 (1L) possesses low V s based on a simple-layer "Matryoshka doll" structure but has high conductivity, accordingly, it has the highest complex permittivity parameters ( Figure S11a, Supporting Information). Such high values are figuratively defined as the "1 state." When introducing the Zn-containing ZIF-8 into 1L for the synthesis of 2L, the situation becomes fascinating. On the one hand, compared to the simple-layer structure of 1L, the ordered arrangement of ZIF-67 and ZIF-8 in such a unique double-layer "Matryoshka doll" structure benefits to induce more drastic anion exchange effect between S 2À and 2-MeIM in heterogeneous core-shell structure, which leads to the enhanced lattice defects and enriched V s and thus improves EM parameters as demonstrated previously. On the other hand, compared to Co 3 S 4 (1L), the inferior conductivity of Zn 0.76 Co 0.24 S (2L) make a sharp decline of complex permittivity parameters. Such two opposite roles synergistically account for the reduction of EM parameters of 2L ( Figure S11b,c, Supporting Information). In comparison with 1L, the heterogeneous core-shell construction in 2L brings about lower values of V s , conductivity as well as complex permittivity parameters, which is symbolically defined as the "0 state." However, the addition of the Co-containing ZIF-67 into 2L exhibits a different case. The regular construction of ZIF-67@ZIF-8@ZIF-67 in a triple-layer "Matryoshka doll" structure (precursor-3L) gives rise to more heterogeneous interfaces (Figure 2d-d2), and benefits to higher defects level than precursor-2L (Figure 2d). This multilayer heterointerface merit can be further amplified in the subsequent sulfuration process via the anion exchange effect, which inevitably leads to sufficient exposed defect sites and enhanced conductivity, thus improving EM parameters for 3L (Figure 3e). Likewise, we define these huge improvements in 3L as "1 state" relative to "0 state" of 2L. Noted that although the introduced Zn-containing ZIF-8 into precursor-3L increases heterogeneous interfaces for precursor-4L, the anion exchange effect is instead weakened owing to the higher stability of Zn-containing ZIF-8 than Co-containing ZIF-67 under solvothermal conditions as we mentioned previously. Unsurprisingly, the subsequent 4L shows the decreased tendency compared to 3L, which is once again defined as a "0 state". However, the arrival of ZIF-67 of precursor-5L can offset the aforementioned shortcomings, and in return, the heterointerface advantage in such a five-layer "Matryoshka doll" structure is well inherited and strengthened in the subsequent ion exchange process. As a result, vacancy level and conductivity together with complex permittivity parameters are largely increased for 5L, and it is defined as "1 state" as well. In other words, in terms of complex permittivity parameters, V s and Rct, an interesting "quantized state" between "0 state" and "1 state" in a set of "Matryoshka doll" structured Zn-Co sulfides could be obtained by well designing the regular arrangement of ZIF-67 and ZIF-8 in the multiple-layer heterostructures.
It should be highly emphasized that even if the components of 2L-DM are identical to 3 L, a lacking of orientation design of ZIF-67 and ZIF-8 will not produce rich lattice strain, defect level, conductivity as well as the desirable EM parameters (Figure 3f and S11, Supporting Information). In a world, our findings suggest that not lattice strain alone in the "Matryoshka doll" structured precursor ZIFs, but the conjoint factors, such as structural defects or sulfur vacancy changes in "Matryoshka doll" structured Zn-Co sulfides during ZIFs precursors-to-sulfides transformation, may significantly account for observed enhancements of EM parameters in architecture-engineered EMWAM. Figure 3. a) High-resolution S 2p XPS spectra; b) ESR spectra; c) Nyquist plots; e) Comparisons in the complex permittivity parameters (ε 0 and ε 00 ,) range, sulfur vacancy concentration (V s ), and charge transfer resistance (R ct ), showing an interesting "quantized state" between "0 state" and "1 state"; Comparison of 3L and 2L-DM, showing the structural advantage in improving lattice strain, defect, sulfur vacancy, and conductivity; g) Comparisons in attenuation ability and impedance matching as well as h) resulting electromagnetic wave (EMW) absorption performances.
www.advancedsciencenews.com www.small-structures.com What's more interesting, as the layers number of the "Matryoshka doll" structure increases, the EM parameters improvement is not continuous linear but a "quantized state" linear ( Figure 3d,e), giving us more room to modulate EM parameters and satisfying performance. To this end, the EMW absorption ability is then investigated. In principle, the improvement of EM parameters contributes to elevating EMW dissipation ability. Evidenced by the attenuation factor (α) in Figure 3g, we discovered that the attenuation ability of "Matryoshka doll" structured Zn-Co sulfides is 1L > 3L > 5L > 2L > 4L, and far greater than 2L-DM, which is almost in accordance to the order of defect and vacancy level (Figure 2d and 3b), implying that defect-induced polarization loss is mainly responsible for their outstanding EMW attenuation ability. Nevertheless, the strong attenuation ability does not result in a broadband EMW absorption performance ( Figure S12, Supporting Information), which is attributed to its inferior impedance matching (Figure 3g), especially for 3L resulting from unsatisfactory complex permittivity parameters caused by the exorbitant defect level (Figure 3e). Even so, as presented in Figure 3h, the "Matryoshka doll" structure (from 2L to 5L) gets a significant victory over "Chaotic" structured Zn-Co sulfides (2L-DM) in broadening EMW absorption, firmly showing the structural advantages of "Matryoshka doll" heterostructures than "Chaotic" structure. Thereinto, the 5L has an EAB of 4.4 GHz with only a thin thickness of 1.56 mm, showing huge potential application.

Cation-Doped "Matryoshka Doll" Structured Zn-Co Sulfides (Structure 3)
The aforementioned analysis demonstrates the advantages of "Matryoshka doll" heterostructures in regulating defect level, however, a much high defect level, especially the sulfur vacancy, favors substantial enhancement of EM parameters but damages the impedance matching and EMW absorption performance for 3L compared to 4L and 5L. Thus, aside from increasing the number of layers in the triple-layer "Matryoshka doll" structure (3L), is there any other way to optimize defect level and EM parameters, and further, improve the EMW absorption performance based on such "Matryoshka doll" heterostructures? Motivated by the above promising prediction, a series of cation-doped "Matryoshka doll" structured precursors (Structure 3) obtained by extra additions of metal cations (Cu 2þ , Ni 2þ , or both of them) are specially introduced ( Figure S13 and Experimental Section, Supporting information). EDS mapping ( Figure S14 and S15, Supporting Information) and XPS spectra ( Figure S16, Supporting Information) clearly verify the successful achievement of cation installation into our desirable "Matryoshka doll" structured precursors and their derived sulfides.
It is worth mentioning that after succedent solvothermal sulfuration, compared to undoped 3L, these metal cations introduction can result in the generation of a new phase of Co 3 S 4 besides Zn 0.76 Co 0.24 S (Figure 4a and Table S4, Supporting Information) for 3L-Cu, 3L-Ni, and 3L-CuNi, respectively, which is further confirmed by TEM and HR-TEM results (Figure 4b-d). Considering the differences in formation energy and migration barrier of different metal cations, [41,42] as illustrated in Figure 4e and taking 3L-CuNi as an instance, we speculate that the Cu 2þ and Ni 2þ introduced into the "Matryoshka doll" structured precursors may serve as a barrier to slow down the migration and reaction ability among the Co 2þ located at the outermost shell and the Zn 2þ concentrated at the intermediate shell with S 2À during the sulfuration process. As a result, the S 2À reacts preferentially with outermost Co 2þ to produce Co 3 S 4 , and then with residual Co 2þ and Zn 2þ to form Zn 0.76 Co 0.24 S, as revealed by the HR-TEM in Figure 4b. This result brings about two aspects of profits: 1) With respect to the composition regulator of introduced cations, such multicomponents of Zn 0.76 Co 0.24 S/Co 3 S 4 composites benefit to provide abundant heterointerfaces, thereby elevating interfacial polarization ability; 2) As for defect regulator of introduced cations, the weakened anion exchange of S 2À , accompanied with the introduction of vacancy-scarcity Co 3 S 4 phase (Figure 2d), both contribute to reducing the defect level ( Figure S17a, Supporting Ifnormation), sulfur vacancy ( Figure 4f ) and the related conductivity ( Figure S17b and Table S5, Supporting Information) as well as EM parameters ( Figure S18, Supporting Infrmation) for 3L-Cu, 3L-Ni, and particularly 3L-CuNi relative to 3L. Owing to the above beneficial synergistic effects induced by cations introduction, compared to 3L, cation-doped "Matryoshka doll" structured Zn-Co sulfides can keep a good balance between EMW attenuation capacity ( Figure S19, Supporting Information) and impedance matching ( Figure S20, Supporting Information), thus elevating ultimate EMW absorption performances (Figure 4g and S21, Supporting Information). Surprisingly, 3L-CuNi holds the best EMW absorption performance accompanied by an EAB as large as 7.80 GHz at 2.60 mm (Figure 4j), overwhelmingly surpassing all advanced metal sulfide absorbents reported previously as collected in Table S6, Supporting Information.
To sum up, this work brought forward four interesting findings: 1) New structures. Three structures, including the "Chaotic" structure (Structure 1), "Matryoshka doll" structure (Structure 2), and cation-doped "Matryoshka doll" structure (Structure 3) those have been rarely reported, are successfully fabricated; 2) New phenomenon. The regular arrangement of ZIF-67 and ZIF-8 in Structure 2 rather than a random assembly of these ZIFs in Structure 1 benefits to construct multiple-layer core-shell heterostructures, by which we can quantize "0 state" or "1 state" of EM parameters; 3) New mechanism. Anion exchange process based on such multiple-layer heterostructures in Structure 2 contributes to "quantization" modulation of lattice strain, exposed defect sites, conductivity, and EM parameters by changing the number of layers, and thus harvesting the controlled EMW absorption performance; 4) New performance. The EAB of 7.80 GHz at 2.60 mm is obtained for 3L-CuNi while 2L-DM exhibits no response to EMW, demonstrating the distinctive structural advantage of the "Matryoshka doll" structure, which provides a new inspiration for the design of EMWAMs by EM parameters "quantization" based on multiple-layer heterostructures. In summary, for the first time, we find an effective approach to quantize EM parameters, and this "quantization" modulation can be stepped in synergy with other microcosmic factors (such as lattice defect, sulfur vacancy, and conductivity) to tailor the desirable EMW absorption capacity.

Conclusion
Aiming to disclose multiple-layer heterostructures to the quantized modulation of EM parameters, we conceptually develop a series of MOFs hybrids and derived metal sulfides based on MOF architecture engineering and solvothermal sulfuration. Benefiting from the structural advantage of producing ample heterointerfaces for the "Matryoshka doll" structure (Structure 2), more lattice strain and higher defect level are generated compared to the "Chaotic" structure (Structure 1). A precursors-tosulfides transformation resulting from the anion exchange process is found to further intensify the above merits along with enriching lattice defects and sulfur vacancy as well as improving conductivity for the "Matryoshka doll" structure Zn-Co sulfides.
Noted that an interesting "quantized state" phenomenon between "0 state" and "1 state" not only in such above microcosmic factors, but also in EM parameters could be achieved by tailoring the layers number of "Matryoshka doll" structure, due to the differences in composition and chemical properties for ZIF-8 and ZIF-67 as building blocks in multiple-layer heterostructures. As a result, the EAB of optimized 5L reaches 4.4 GHz at only 1.56 mm, while 2L-DM exhibits EMW transmission characteristics, showing a distinctive structural advantage of multiple-layer "Matryoshka doll" heterostructures. Besides, the synergistic effects, including enriched heterointerfaces by the formation of Co 3 S 4 , and reduction of defect level, vacancy sites, and conductivity by cations (Cu 2þ and Ni 2þ ) introduction, lead to the optimized EM parameters, well-matched impedance, and . a) X-ray diffraction (XRD) patterns of undoped (3L) and cation-doped "Matryoshka doll" structured Zn-Co sulfides (3L-Cu, 3L-Ni, and 3L-CuNi); b) HR-TEM, c) SEAD and d) EDS mapping images of 3L-CuNi; e) Schematic illustration of the difference in migration and reaction abilities among the Co 2þ , Zn 2þ , and S 2À before and after Cu 2þ and Ni 2þ introduction for 3L and 3L-CuNi; f ) ESR spectra; g-j) 2D RL plots.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.