本篇文章來自第一作者Agnieszka Kuc, 第二作者為Maximilian A.Springer, 通訊作者為Thomas Heine,發表期刊《Advanced Functional Materials》,題目為「Proximity Effect in Crystalline Framework Materials: Stacking-Induced Functionality in MOFs and COFs」,DOI:10.1002/adfm.201908004
MOF-COF堆積結構已經有先前的研究,主要應用於小分子物質的分離。金屬有機框架(MOF)和共價有機框架(COF)由分子結構單元組成,這些結構單元通過牢固的鍵連接在一起,大多數MOF和COF的電子特性是其構成基礎的電子特性的疊加。晶體(MOF-COF)則可以觀察到固態現象,例如電導率、電子帶、吸收帶變寬、準分子態的形成、移動電荷載流子和間接帶隙(原理內容可以詳見於《固體物理導論》)。這些情況通常是由堆疊的芳族結構單元之間的範德華相互作用引起的鄰近效應而出現的。
在此篇文章內通過綜述揭示出了此鄰近效應是怎樣施加功能性作用——即通過以使得在MOF和COF中出現非常巧妙的方式堆疊芳香分子來施加功能性作用。同時本文在討論了石墨烯相關材料中的鄰近效應後,展示了其對分層COF和MOF的重要性。對於具有明確定義的結構的MOF,可以通過更改MOF拓撲,晶格常數以及連接空間控制單元來控制堆棧芳香基結構單元。最後,文章總結了用於有序表面組裝MOF的逐層生長技術,對預測和分析這些效果的理論方法進行了概述。
Figure 1. (Top) Structures, (middle) electronic structures, and (bottom) Brillouin zones of a) graphene, b) staggered graphite (ABAB stacking, Bernal structure[49]), and c) eclipsed graphite (AAAA stacking, hypothetical). Single layer of graphene, which exhibits Dirac points (linear dispersion relation) at K points and a band gap of about 1 µeV, changes its electronic properties when stacked differently into 3D structures: ABAB graphite has quadratic and nearly linear dispersion relations at the K and H points, respectively, and band gap at H of 5 meV; AAAA graphite has two Dirac points (at K and H points) above and below Fermi level (blue dashed horizontal lines). Simulations at the DFT/PBE-D3(BJ) level of theory with TZP basis set and spin–orbit coupling as implemented in AMS/BAND software. Pictures of structures made with VESTA.
Figure 2. a,b) Moiré period and pattern obtained from two graphene layers twisted with respect to one another. High symmetry local stackings are denoted (AB stacking in (b) denoted with blue dots). a) Smallest positive energy of the interlayer Hamiltonian. The energy vanishes for local AB or BA coordination and reaches a maximum for local AA coordination. c) Energy dispersion for the 14 bands closest to the Dirac point plotted along the k-space trajectory KΓ -K′1-K′2-K and the corresponding density of states (DOS). Adapted with permission. Copyright 2011, National Academy of Sciences.
Figure 3. a) Relative energy when shifting layers of COF-5 with respect to one another in armchair (left) and zigzag (right) directions. The energies of the two high symmetry stackings (AAAA and ABAB) are indicated. The low-energy minimum corresponds to the equilibrium structures, shown in the insets. b) Building blocks of COF-5. c) Experimental and calculated PXRD patterns of all the stacking arrangements. d–g) (Top) structures and (bottom) electronic band structures (lowest two panels show zoom-ins of conduction band maximum [CBM] and valence band minimum [VBM]) of a) monolayer (1L), b) eclipsed (AAAA), c) inclined (AA′A″A′″), and d) serrated (AA′AA′) stackings of COF-5. Single layer COF-5 has fairly flat bands close to the Fermi level (blue dashed horizontal lines). Prominent dispersion of bands in the direction perpendicular to the layers can be observed upon stacking single layer to multilayer COF-5, the strongest in case of AAAA (eclipsed) stacking. Calculations performed on the optimized structures from ref. at the DFT/PBE level of theory with TZP basis set as implemented in AMS/BAND software. a,c) Adapted with permission. Copyright 2011, Wiley. Pictures of structures made with VESTA.
Figure 4. Simplified schemes illustrating important coupling reactions for the formation of 2D polymers and 2D COFs. a) Ullmann coupling, b) boron esterification, c) Knoevenagel-like condensation, d) borazine formation, and e) nitrile cyclotrimerization.
Figure 5. Building blocks together with the top and side views of the atomically thin layered MOFs: a) K3Fe2[PcFe-O8] (Pc, phthalocyanine), b) Ni3HITP2 (HITP, 2,3,6,7,10,11-hexaiminotriphenylene), and [Cu3(C6Se6)]n. All systems are in AA′AA′ stacking. Pictures of structures made with VESTA.
Figure 6. (Top and middle panel) Top and side views of different stackings in SURMOF-2 based on Cu-paddle wheel and 1,4-benzene dicarboxylate linkers. (Bottom panel) corresponding band structures calculated on the optimized structures from ref. at the DFT/PBE0-D3 level of theory with all electron basis set (double zeta with polarization on light elements and triple zeta with polarization for Cu atoms) as implemented in Crystal14. The Cu atoms were antiferromagnetically coupled in the present simulations (bands corresponding to α and β electrons are identical). Slipped and inclined stacking of SURMOF-2 layers induces out-of-plane dispersion in band structure, due to increased van der Waals and preferable Coulomb interactions. Pictures of structures made with VESTA.
Figure 7. a) Building block (15-diphenyl-10, 20-di(4-carboxyphenyl)porphyrin, Pd–porphyrin) together with the top and side views of Pd–porphyrinbased Zn-SURMOF, and b) the corresponding band structure with zoom-in to the top of valence and bottom of conduction bands. Adapted with permission. Bands are fairly flat, however, small out-of-plane dispersion occurs in the direction perpendicular to the layers. The dispersion is in the limit of a couple of meV. Pictures of structures made with VESTA.
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