Halide-modulated self-assembly of metal-free perovskite single crystals for bio-friendly X-ray detection

Abstract

•Large, high-quality, metal-free perovskite series, DABCO-NH4X3 SCs are grown•Influence of halide-modulated molecular assembly on physical properties is explored•DABCO-NH4I3 shows superior in-plane charge transport and X-ray detection sensitivity•The feasibility of X-ray imaging based on metal-free perovskites is demonstrated Molecular self-assembly plays a critical role in crystal engineering for the design and fabrication of novel metal-free perovskite compounds. Here, we demonstrate the understandings of halide-modulated molecular assembly via hydrogen bonding in metal-free perovskites and its influence on their crystal packing, band nature, mechanical and electrical properties, and final optoelectronic performance. We found increasing halide radius leads to a gradual transition of band nature and narrower band gaps. The crystal with superior in-plane charge transport exhibits higher carrier mobility and better X-ray detection sensitivity. With a variety of nonmetallic and organic groups readily available for the A, B, and X sites, fine-tuned properties, and free of associated toxicity, this work benefits the understanding of molecular self-assembly behavior and is intended to inspire activities to study an assortment of novel ABX3 perovskite materials for potential biological applications. Not until recently have metal-free halide perovskites become recognized as novel candidates for ferroelectrics and X-ray detection. However, molecular self-assembly of these perovskites and its influence remain unexplored. Here, we prepare large high-quality DABCO-NH4X3 (DABCO = N-N′-diazabicyclo[2.2.2]octonium, X = Cl, Br, I) single crystals and demonstrate the understandings of how halide-modulated molecular assembly affects their crystal packing, band nature, mechanical and electrical properties, and final optoelectronic performance. In this series, the 1D crystal packing and low carrier effective masses endow superior in-plane charge transport for the I-based crystal. As such, higher carrier mobility (110 versus ∼10–20 cm2 V−1 s−1) and longer charge diffusion length (∼90 versus ∼50 μm) are achieved in contrast to the Cl- and Br-based analogs. The excellent charge transport properties finally translate to highly efficient X-ray detection and imaging for the I-based crystal detector, with a sensitivity up to 567 μC Gyair−1 cm−2 and a well-defined “heart” X-ray image. Not until recently have metal-free halide perovskites become recognized as novel candidates for ferroelectrics and X-ray detection. However, molecular self-assembly of these perovskites and its influence remain unexplored. Here, we prepare large high-quality DABCO-NH4X3 (DABCO = N-N′-diazabicyclo[2.2.2]octonium, X = Cl, Br, I) single crystals and demonstrate the understandings of how halide-modulated molecular assembly affects their crystal packing, band nature, mechanical and electrical properties, and final optoelectronic performance. In this series, the 1D crystal packing and low carrier effective masses endow superior in-plane charge transport for the I-based crystal. As such, higher carrier mobility (110 versus ∼10–20 cm2 V−1 s−1) and longer charge diffusion length (∼90 versus ∼50 μm) are achieved in contrast to the Cl- and Br-based analogs. The excellent charge transport properties finally translate to highly efficient X-ray detection and imaging for the I-based crystal detector, with a sensitivity up to 567 μC Gyair−1 cm−2 and a well-defined “heart” X-ray image. Halide perovskites have attracted tremendous attention due to their excellent charge transport properties, in particular high absorption coefficient,1Sun S. Salim T. Mathews N. Duchamp M. Boothroyd C. Xing G. Sum T.C. Lam Y.M. The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells.Energy Environ. Sci. 2014; 7: 399-407Crossref Google Scholar,2Wang Y. Bai S. Cheng L. Wang N. Wang J. Gao F. Huang W. High-efficiency flexible solar cells based on organometal halide perovskites.Adv. Mater. 2016; 28: 4532-4540Crossref PubMed Scopus (86) Google Scholar large carrier mobility,3Dong Q. Fang Y. Shao Y. Mulligan P. Qiu J. Cao L. Huang J. Electron-hole diffusion lengths >175 μm in solution-grown CH3NH3PbI3 single crystals.Science. 2015; 347: 967Crossref PubMed Scopus (3670) Google Scholar and long charge lifetimes (therefore long diffusion lengths).4Stranks S.D. Eperon G.E. Grancini G. Menelaou C. Marcelo J. Alcocer P. Leijtens T. 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Photon. 2016; 10: 333-339Crossref Scopus (856) Google Scholar which has aroused more interest and exploration of halide perovskites for use in nondestructive inspection, medical diagnosis, and homeland security.10Milbrath B.D. Peurrung A.J. Bliss M. Weber W.J. Radiation detector materials: an overview.J. Mater. Res. 2011; 23: 2561-2581Crossref Google Scholar X-ray radiation detection is currently dominated by conventional α-Se, Si, HgI2, CdTe, and Cd1−xZnxTe due to relatively strong stopping power, high bulk resistivity, and sensitive photoconduction,11Wei H. Huang J. Halide lead perovskites for ionizing radiation detection.Nat. Commun. 2019; 10: 1066Crossref PubMed Scopus (295) Google Scholar but the issues of expensive raw materials and complex growth techniques are still problematic. Halide perovskites have emerged as an appealing alternative due to their strong stopping power, high bulk resistivity, as well as radiation hardness.11Wei H. Huang J. Halide lead perovskites for ionizing radiation detection.Nat. Commun. 2019; 10: 1066Crossref PubMed Scopus (295) Google Scholar The first halide perovskite radiation detector, single-crystal CsPbBr3, was reported by Stoumpos et al., in 2013.12Stoumpos C.C. Malliakas C.D. Peters J.A. Liu Z. Sebastian M. Im J. Chasapis T.C. Wibowo A.C. Chung D.Y. Freeman A.J. et al.Crystal growth of the perovskite semiconductor CsPbBr3: a new material for high-energy radiation detection.Cryst. Growth Des. 2013; 13: 2722-2727Crossref Scopus (909) Google Scholar Later, X-ray detectors based on Pb-based perovskite MAPbBr3 single crystals (SCs),9Wei H. Fang Y. Mulligan P. Chuirazzi W. Fang H.-H. Wang C. Ecker B.R. Gao Y. Loi M.A. Cao L. et al.Sensitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals.Nat. Photon. 2016; 10: 333-339Crossref Scopus (856) Google Scholar,13Wei W. Zhang Y. Xu Q. Wei H. Fang Y. Wang Q. Deng Y. Li T. Gruverman A. Cao L. et al.Monolithic integration of hybrid perovskite single crystals with heterogenous substrate for highly sensitive X-ray imaging.Nat. Photon. 2017; 11: 315-321Crossref Scopus (368) Google Scholar MAPbI3 film14Yakunin S. Sytnyk M. Kriegner D. Shrestha S. Richter M. Matt G.J. Azimi H. Brabec C.J. Stangl J. Kovalenko M.V. et al.Detection of X-ray photons by solution-processed organic-inorganic perovskites.Nat. Photon. 2015; 9: 444-449Crossref PubMed Scopus (647) Google Scholar and wafer,15Shrestha S. Fischer R. Matt G.J. Feldner P. Michel T. Osvet A. Levchuk I. Merle B. Golkar S. Chen H. et al.High-performance direct conversion X-ray detectors based on sintered hybrid lead triiodide perovskite wafers.Nat. Photon. 2017; 11: 436-440Crossref Scopus (270) Google Scholar and Bi-based perovskites Cs2AgBiBr68Pan W. Wu H. Luo J. Deng Z. Ge C. Chen C. Jiang X. Yin W.-J. Niu G. Zhu L. et al.Cs2AgBiBr6 single-crystal X-ray detectors with a low detection limit.Nat. Photon. 2017; 11: 726-732Crossref Scopus (598) Google Scholar and (NH4)3Bi2I9 SCs,16Zhuang R. Wang X. Ma W. Wu Y. Chen X. Tang L. Zhu H. Liu J. Wu L. Zhou W. et al.Highly sensitive X-ray detector made of layered perovskite-like (NH4)3Bi2I9 single crystal with anisotropic response.Nat. Photon. 2019; 13: 602-608Crossref Scopus (181) Google Scholar were successfully fabricated. These metal-based halide perovskites indeed demonstrated promising X-ray detection properties. However, the need for energy-efficient, economical, and environmentally friendly (“triple E”) materials and fabrication procedures drives the exploration of novel metal-free semiconductors which should be lightweight, low-cost, nontoxic, and mechanically flexible while retaining similar key features as metal-halide perovskites for X-ray detection, including a high X-ray absorption coefficient, large resistivity, and large μτ product. Metal-free halide perovskites (or at least some of them) adopt a typical ABX3 perovskite structure consisting of organic diamine cations, inorganic ammonium cations, and halide anions. The metal-free perovskite C6N2H14·NH4Cl3 and perovskite hydrate (C4N2H12) (NH4Cl3)·H2O were first reported in 2002, when their structures were shown.17Bremner C.A. Simpson M. Harrison W.T.A. New molecular perovskites: cubic C4N2H12·NH4Cl3·H2O and 2-H hexagonal C6N2H14·NH4Cl3.J. Am. Chem. Soc. 2002; 124: 10960-10961Crossref PubMed Scopus (45) Google Scholar Since then, there have been several papers on that family,18Liu G.-Z. Zhang J. Wang L.-Y. A novel molecular cubic perovskite built from charge-assisted hydrogen bond linkages.Synth. React. Inorg. Metal Org. Nano Metal Chem. 2011; 41: 1091-1094Crossref Scopus (14) Google Scholar ferroelectric,19Ye H.-Y. Tang Y.-Y. Li P.-F. Liao W.-Q. Gao J.-X. Hua X.-N. Cai H. Shi P.-P. You Y.-M. Xiong R.-G. Metal-free three-dimensional perovskite ferroelectrics.Science. 2018; 361: 151Crossref PubMed Scopus (368) Google Scholar dielectric,20Chu L.-L. Zhang T. Zhang W.-Y. Shi P.-P. Gao J.-X. Ye Q. Fu D.-W. Three-dimensional metal-free molecular perovskite with a thermally induced switchable dielectric response.J. Phys. Chem. Lett. 2020; 11: 1668-1674Crossref PubMed Scopus (18) Google Scholar piezoelectric,21Wang H. Liu H. Zhang Z. Liu Z. Lv Z. Li T. Ju W. Li H. Cai X. Han H. Large piezoelectric response in a family of metal-free perovskite ferroelectric compounds from first-principles calculations.NPJ Comput. Mater. 2019; 5: 17Crossref Scopus (24) Google Scholar electrocaloric,22Wang J.-J. Fortino D. Wang B. Zhao X. Chen L.-Q. Extraordinarily large electrocaloric strength of metal-free perovskites.Adv. Mater. 2020; 32: 1906224Crossref Scopus (29) Google Scholar mechanical,23Ehrenreich M.G. Zeng Z. Burger S. Warren M.R. Gaultois M.W. Tan J.-C. Kieslich G. Mechanical properties of the ferroelectric metal-free perovskite [MDABCO](NH4)I3.Chem. Commun. 2019; 55: 3911-3914Crossref PubMed Google Scholar as well as non-linear optical properties.24Kasel T.W. Deng Z. Mroz A.M. Hendon C.H. Butler K.T. Canepa P. Metal-free perovskites for non linear optical materials.Chem. Sci. 2019; 10: 8187-8194Crossref Google Scholar Very recently, we have demonstrated the first example of metal-free halide perovskite optoelectronics based on the DABCO-NH4Br3 (DABCO = N-N′-diazabicyclo[2.2.2]octonium) single crystal.25Song X. Cui Q. Liu Y. Xu Z. Cohen H. Ma C. Fan Y. Zhang Y. Ye H. Peng Z. et al.Metal-free halide perovskite single crystals with very long charge lifetimes for efficient X-ray imaging.Adv. Mater. 2020; 32: 2003353Crossref PubMed Scopus (24) Google Scholar Our preliminary results on the DABCO-NH4Br3 perovskite indicate a carrier diffusion length up to 55 μm, which is comparable with metal-based halide perovskites. The materials also exhibit high bulk resistivity and high X-ray attenuation. In contrast to other lightweight high-energy radiation sensors, such as conjugated organic crystals, metal-free perovskites have a significantly improved lifetime-mobility (μτ) product and much higher attenuation of the X-rays. This results in promising figures-of-merit, including a high sensitivity and a low working voltage. These metal-free halide perovskites will be among the best candidates for lightweight sensors for high-energy radiation detection and imaging. Here, we have demonstrated understandings of halide-modulated molecular self-assembly based on large and high-quality DABCO-NH4X3 (X = Cl, Br, I) SCs, and its influence on mechanical properties, charge transport characteristics, and ultimate performance limits in optoelectronic devices. The crystals exhibit the importance of the nature of halide species, which determines molecular self-assembly from one-dimensional (1D) hexagonal to 3D cubic and crystal symmetries. The transition of band nature from indirect to direct when varying halides from Cl to Br and I was presented based on experimental and simulation results. The halide-dependent mechanical properties were investigated. The charge transport characteristics were further explored in terms of trap densities, carrier mobilities, carrier diffusion lengths, mobility-lifetime (μτ) products, and X-ray attenuation. Excellent X-ray sensitivity and imaging were achieved based on planar detector under X-ray radiation, and the halide influence on X-ray response was also discussed. The ideal ABX3-type molecular structure of the metal-free perovskite DABCO-NH4X3 is shown in Figure 1A. All components are held together largely by ionic and hydrogen-bonding interactions, and hydrogen bonding plays a key role in establishing the global framework. The 3D corner-sharing [(NH4)X6] network encloses cavities occupied by doubly protonated amine cations, with a unique ABX3 perovskite feature that +2 charge cations are located at the “A” position with a +1 charge cation at the “B” position, which is reverse to conventional metal-based halide perovskites.26Saparov B. Mitzi D.B. Organic-inorganic perovskites: structural versatility for functional materials design.Chem. Rev. 2016; 116: 4558-4596Crossref PubMed Scopus (1617) Google Scholar The DABCO-NH4X3 SCs were synthesized from aqueous solutions. The solubilities of the precursors in water were initially evaluated for the three materials. Taking the DABCO-NH4Br3 as an example, we observed a critical temperature at ca. 60°C, below which slower precipitation can be achieved,25Song X. Cui Q. Liu Y. Xu Z. Cohen H. Ma C. Fan Y. Zhang Y. Ye H. Peng Z. et al.Metal-free halide perovskite single crystals with very long charge lifetimes for efficient X-ray imaging.Adv. Mater. 2020; 32: 2003353Crossref PubMed Scopus (24) Google Scholar and there is much less variation of solubility in the lower temperature range (20°C–40°C) (Figure S1). This suggests that SCs can be fabricated via slow cooling from 60°C to 20°C or directly at room temperature (room temperature = 20°C) (Figure 1B). In contrast to the cooling method previously reported,25Song X. Cui Q. Liu Y. Xu Z. Cohen H. Ma C. Fan Y. Zhang Y. Ye H. Peng Z. et al.Metal-free halide perovskite single crystals with very long charge lifetimes for efficient X-ray imaging.Adv. Mater. 2020; 32: 2003353Crossref PubMed Scopus (24) Google Scholar we found that growth via the slow solvent evaporation (SSE) method at room temperature yielded better crystal quality due to a lower energy supply and a much slower growth rate (Figure S2). As such, we achieved SCs of high quality and large DABCO-NH4X3 series successfully at room temperature. Photos of large DABCO-NH4X3 SCs are shown in Figure 1C. Three crystals are highly transparent with a maximum size of 5 × 5 × 2 mm3. The DABCO-NH4X3 SCs exhibit shapes of hexagonal prism, cuboid, and elongated hexagonal prism for the Cl-, Br-, and I-based crystals, respectively. Scanning electronic microscopy (SEM) images indicate a smooth surface of each crystal without noticeable holes or inclusions (Figure 1D). The high-quality surface is an important prerequisite for a good contact to the electrode for optoelectronic devices. Note that the halide species play a critical role in the crystal shape, which is related to the intrinsic crystallographic structure and preferred growth directions. This therefore motivates a probe into the halide-modulated molecular self-assembly and its influence on the crystal structure and band structure for the DABCO-NH4X3 crystals. X-ray diffraction (XRD) analysis was performed on the DABCO-NH4X3 crystals (Figure 2A). The DABCO-NH4Cl3 crystal exhibits two main peaks at 2θ = 23.5° and 48.2°, which are assigned to the (006) and (00 12) planes, and belongs to the R3¯c space group of the trigonal system, with lattice parameters of a = b = 16.196(2) Å, c = 22.443(3) Å, and α = β = 90°, γ = 120° (Table S1). Three main peaks at 2θ = 26.3°, 40.1°, and 54.3° are observed for the DABCO-NH4Br3 crystals, which are assigned to the (204), (306), and (408) planes. This belongs to the P3221 space group of the trigonal system, with lattice parameters determined as a = b = 9.5720 (17) Å, c = 23.204(5) Å, and α = β = 90°, γ = 120° (Table S1). For the DABCO-NH4I3 crystal, four main peaks occurring at 2θ = 21.3°, 32.1°, 43.2°, and 54.5° are indexed to the (200), (300), (400), and (500) reflections,19Ye H.-Y. Tang Y.-Y. Li P.-F. Liao W.-Q. Gao J.-X. Hua X.-N. Cai H. Shi P.-P. You Y.-M. Xiong R.-G. Metal-free three-dimensional perovskite ferroelectrics.Science. 2018; 361: 151Crossref PubMed Scopus (368) Google Scholar and it is related to the P6¯2c space group of the hexagonal system with lattice parameters of a = b = 9.6619(3) Å, c = 8.1594(3) Å, and α = β = 90°, γ = 120° (Table S1). The corresponding main facets of DABCO-NH4X3 SCs are shown and marked with (hkl) indices in the inset of Figure 2A. The halide-dependent structural change was evaluated. Figure 2B illustrates the packing diagrams from a specific viewing direction for the three crystals. It is of interest that only the DABCO-NH4Br3 adopts the corner-sharing [(NH4)Br6] octahedra-formed 3D cubic perovskite structure. Whereas the face-sharing [(NH4)Cl6] octahedra are arranged in the same manner as the NiO6 moieties in hexagonal 2-H BaNiO3,27Lander J. The crystal structures of NiO·3BaO, NiO·BaO, BaNiO3 and intermediate phases with composition near Ba2Ni2O5; with a note on NiO.Acta Crystallogr. 1951; 4: 148-156Crossref Google Scholar which is also observed for 0D (DMEDA)1.5BiI6.28Yao L. Niu G. Yin L. Du X. Lin Y. Den X. Zhang J. Tang J. Bismuth halide perovskite derivatives for direct X-ray detection.J. Mater. Chem. C. 2020; 8: 1239-1243Crossref Google Scholar The face-sharing [(NH4)I6] octahedra are distributed like δ-FAPbI3.29Stoumpos C.C. Malliakas C.D. Kanatzidis M.G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties.Inorg. Chem. 2013; 52: 9019-9038Crossref PubMed Scopus (3728) Google Scholar Both DABCO-NH4Cl3 and DABCO-NH4I3 adopt the 1D hexagonal perovskite structure, which consists of face-sharing octahedra propagating along the [001] direction forming single chains, with the DABCO2+ cations occupying the space and acting as spacers between the chains (Figure S3). This structural evolution also occurs in the DABCO-containing analogs, DABCO-KCl3 and DABCO-RbCl3.30Paton L.A. Harrison W.T. Structural diversity in non-layered hybrid perovskites of the RMCl3 family.Angew. Chem. Int. Ed. 2010; 49: 7684-7687Crossref PubMed Scopus (30) Google Scholar Previous study reveals that the 3D framework is inclined to collapse when the cations cannot be accommodated inside the 3D cavity of the cubic perovskite structure, which then gives rise to the 1D hexagonal perovskite structure.31Stoumpos C.C. Frazer L. Clark D.J. Kim Y.S. Rhim S.H. Freeman A.J. Ketterson J.B. Jang J.I. Kanatzidis M.G. Hybrid germanium iodide perovskite semiconductors: active lone pairs, structural distortions, direct and indirect energy gaps, and strong nonlinear optical properties.J. Am. Chem. Soc. 2015; 137: 6804-6819Crossref PubMed Scopus (526) Google Scholar The ionic size mismatch and the tolerance factor α= (RA + RX)/√2(RB + RX) have been widely evaluated for conventional metal-based perovskites. However, evaluation becomes quite challenging for metal-free perovskites because of two polyatomic ion components whose bond lengths vary under the influence of hydrogen-bonding interactions.32Kieslich G. Sun S. Cheetham A.K. Solid-state principles applied to organic-inorganic perovskites: new tricks for an old dog.Chem. Sci. 2014; 5: 4712-4715Crossref Google Scholar Given the existence of NNH4-H···X hydrogen bonds in the [(NH4)X6] octahedral framework and the NDABCO-H···X hydrogen bonds to two adjacent octahedral chains, we speculate that DABCO2+ and NH4+ exhibit different effective radii in each of these structures: DABCO2+ in the Br-based one can fit within the cage provided by the [(NH4)X6] octahedral framework to form the 3D cubic perovskite structure, whereas they cannot in the others, and the DABCO-NH4Cl3 and DABCO-NH4I3 show the 1D hexagonal perovskite structure. However, the precise calculation of α still depends on a more accurate ionic size calculation method used for polyatomic ions, an area that needs further exploration. Still, we can assume that the halide species combine their ionic sizes and participate in N-H···X hydrogen bonds to change the degree of ionic size mismatch. In other words, hydrogen bonding concerning halogen atoms is the key in driving molecular self-assembly into co-crystal and stabilizing perovskite structure,17Bremner C.A. Simpson M. Harrison W.T.A. New molecular perovskites: cubic C4N2H12·NH4Cl3·H2O and 2-H hexagonal C6N2H14·NH4Cl3.J. Am. Chem. Soc. 2002; 124: 10960-10961Crossref PubMed Scopus (45) Google Scholar,33Corradi E. Meille S.V. Messina M.T. Metrangolo P. Resnati G. Halogen bonding versus hydrogen bonding in driving self-assembly processes.Angew. Chem. Int. Ed. 2000; 39: 1782-1786Crossref PubMed Scopus (441) Google Scholar thus rendering a remarkable variety of perovskite-type structures and crystal symmetries. This indicates that the nature of the halide species is particularly important in the DABCO-containing metal-free perovskites. The full width at half maximum (FWHM) values of the predominant diffraction peaks were determined to be 0.011°, 0.026°, and 0.044° for the Cl-, Br-, and I-based crystals (Figure 2C). The extremely low FWHM values indicate the high quality of the DABCO-NH4X3 SCs. The DABCO-NH4X3 crystals exhibit densities of 1.40, 2.03, and 2.58 g cm−3 for the Cl-, Br-, and I-based crystals, respectively. The densities of these crystals are far lower than those of the metal-based perovskites (CsPbBr3, 4.55g cm−3; MAPbBr3, 3.45 g cm−3; MAPbI3, 3.95 g cm−3),9Wei H. Fang Y. Mulligan P. Chuirazzi W. Fang H.-H. Wang C. Ecker B.R. Gao Y. Loi M.A. Cao L. et al.Sensitive X-ray detectors made of methylammonium lead tribromide perovskite single crystals.Nat. Photon. 2016; 10: 333-339Crossref Scopus (856) Google Scholar indicating the potential for lightweight optoelectronics. Thermogravimetric analysis (TGA) exhibits the onset temperatures of weight loss at ∼210°C, 250°C, and 260°C for the Cl-, Br-, and I-based crystals, respectively, with 100% weight loss at ca. 500°C in a single step, confirming their nonmetallic nature (Figure S4). This suggests that the DABCO-NH4X3 crystals are thermally stable at operational temperatures below 200°C, which is beneficial for device stability. The halide influence on optical properties was further evaluated by UV-vis spectroscopy. The transmittance spectra suggest that the three crystals are highly transparent in the visible-NIR region, with lower transmittance for the I-based crystal (Figure 2D). Small inverted peaks at ≈280, ≈310, and ≈370 nm (possibly due to a defect or a Frenkel exciton) were found, followed by a sharp drop at ≈200, ≈230, and ≈260 nm for the Cl-, Br-, and I-based crystals, respectively. The band gap (indirect/direct) values determined using Tauc plots generated from the transmittance data are 5.89 eV (indirect), 5.25 eV (direct), and 4.71 eV (direct) for the Cl-, Br-, and I-based crystals (Figure 2E), respectively. We further calculated the electronic band structures using density functional theory (DFT). DFT calculations indicate an indirect band gap of 5.02 eV for the Cl-based crystal, an indirect band gap (4.58 eV) and a direct band gap (4.65 eV) for the Br-based crystal (but an indirect band gap is less reliable, see Song et al.25Song X. Cui Q. Liu Y. Xu Z. Cohen H. Ma C. Fan Y. Zhang Y. Ye H. Peng Z. et al.Metal-free halide perovskite single crystals with very long charge lifetimes for efficient X-ray imaging.Adv. Mater. 2020; 32: 2003353Crossref PubMed Scopus (24) Google Scholar), and a direct band gap of 3.71 eV for the I-based one (Figure 2F). These values are typically underestimated compared with the experimental values,16Zhuang R. Wang X. Ma W. Wu Y. Chen X. Tang L. Zhu H. Liu J. Wu L. Zhou W. et al.Highly sensitive X-ray detector made of layered perovskite-like (NH4)3Bi2I9 single crystal with anisotropic response.Nat. Photon. 2019; 13: 602-608Crossref Scopus (181) Google Scholar and suggest transition from indirect band nature to direct band nature with increasing halide size. Furthermore, these results indicate that the valence band maximum (VBM) mainly originates from the 3p orbitals of Cl, 4p orbitals of Br, and 5p orbital of I, while the conduction band minimum (CBM) is dominated by the C of DABCO2+ and N of DABCO2+and/or NH4+ (Figure S5). The above results indicate that the DABCO-NH4X3 crystals are wide-band-gap materials. Note that the CBM for the metal-free halide perovskite is dominated by the organic component, which is in stark contrast to the conventional metal-based halide perovskites, where the CBM is governed by the inorganic component (metal-halide).34Yin W.-J. Yang J.-H. Kang J. Yan Y. Wei S.-H. Halide perovskite materials for solar cells: a theoretical review.J. Mater. Chem. A. 2015; 3: 8926-8942Crossref Google Scholar Therefore, the VBM can be readily changed by different halides. Furthermore, the calculated band diagrams show different electronic band dispersions for the different halide-based structures. DABCO-NH4Cl3 and DABCO-NH4Br3 give relatively flat valence and conduction bands, while the VBM and CBM of the I-based structure are considerably more dispersive, indicating much lower carrier effective masses and better carrier mobilities for the I-based structure. We conducted nanoindentation experiments in load-controlled mode to investigate mechanical properties of the DABCO-NH4X3 perovskite SCs. Note that the predominant facets of the crystals were selected and the hardness (H) and Young’s modulus (E) were measured using a three-sided, sharp pyramidal Berkovich diamond tip.35Li W. Kiran M.S.R.N. Manson J.L. Schlueter J.A. Thirumurugan A. Ramamurty U. Cheetham A.K. Mechanical properties of a metal-organic framework containing hydrogen-bonded bifluoride linkers.Chem. Commun. 2013; 49: 4471-4473Crossref PubMed Scopus (37) Google Scholar,36Tan J.C. Bennett T.D. Cheetham A.K. Chemical structure, network topology, and porosity effects on the mechanical properties of zeolitic imidazolate frameworks.Proc. Natl. Acad. Sci. U S A. 2010; 107: 9938Crossref PubMed Scopus (356) Google Scholar Experimental load–displacement (P–h) curves are presented to show the stress states of DABCO-NH4X3 as the indenter penetrates the sample surfaces (Figure 3A). Figure 3B shows the photograph of a crystal surface after the nanoindentation experiment using a Berkovich indenter. The hardness can be measured because the combination of elastic and plastic deformations takes place in the vicinity of the nanoindenter tip.37Tan J.C. Merrill C.A. Orton J.B. Cheetham A.K. 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