Letter pubs.acs.org/JPCL
Bandgap Narrowing of Lead-Free Perovskite-Type Hybrids for Visible-Light-Absorbing Ferroelectric Semiconductors Chengmin Ji, Zhihua Sun,* Aurang Zeb, Sijie Liu, Jing Zhang, Maochun Hong, and Junhua Luo* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China S Supporting Information *
ABSTRACT: Perovskite-type hybrids (e.g., CH3NH3PbI3) hold great promise in photovoltaics and optoelectronics due to their remarkable semiconducting properties and potential ferroelectricity. However, to date, conclusive evidence for the bulk ferroelectricity of CH3NH3PbI3 is still lacking. In this context, it is highly desirable to assemble concrete perovskite ferroelectric hybrids with the semiconducting feature. Here we report, for the first time, a class of lead-free perovskite halides, (Nmethylpyrrolidinium)3Sb2Cl9−9xBr9x (x = 0−1), showing large ferroelectric polarizations (5.2−7.6 μC/cm2) and pronounced semiconducting performances. In particular, a wide tunability of their optical bandgaps (3.31−2.76 eV) enables superior visible-light-induced photocurrents (∼10 nA/cm2), which allow for assembling of the crystal-based photodetectors. Our work paves a new way to build environmentally benign optoelectronic devices based on low-bandgap ferroelectric hybrids.
F
ferroelectric domains have also been observed in the CH3NH3PbI3 thin films doped with small amounts of methylammonium chloride, which discloses the potential ferroelectricity of CH3NH3PbI3(Cl).23 However, it still lacks conclusive evidence for the bulk ferroelectricity of CH3NH3PbI3, such as dielectric anomaly and electric hysteresis loops. From a structural viewpoint, dynamic motions of organic ammonium cations make predominant contributions to exploring ferroelectricity. Emphatically, such perovskite hybrids show great flexibility in bandgap engineering, of which the optical bandgaps can be rationally tuned by tailoring the inorganic metal−halogen architectures.24−26 As exemplified by layered perovskite ferroelectrics of (CHA)2PbBr4−4xI4x (x ≤ 0.175, CHA = cyclohexylammonium),27 their bandgaps are tunable in the range of 3.05−2.74 eV and strong photovoltaic activities were created in (CHA)2PbBr4.28 This suggests that a lower bandgap may be achieved in metal halides by chemical substitution, as revealed by a red-shift trend in the Cl/Br/I series.29−31 Nevertheless, the potential toxicity of lead in these hybrids arouses great environmental concerns and restricts their widespread application. Up to date, few results have been reported on bandgap engineering of lead-free perovskite-type ferroelectric hybrids, which still remains a great challenge. In the present work, we have demonstrated a family of leadfree hybrid compounds, (N-methylpyrrolidinium)3Sb2Cl9−9xBr9x (x = 0−1), featuring zero-dimension (0D) perovskite-like Sb2X9 dioctahedral clusters, which show large ferroelectric polarizations up to 5.1−7.6 μC/cm2.
erroelectrics are the most important class of electroactive substances that enable storage and switching of spontaneous polarization (Ps). The emergence of ferroelectricity results in diverse optoelectric effects,1 and the coupling of ferroelectric polarity with other physical properties produces new conceptual entries into electronic, spintronic, and optoelectronic applications.2,3 Strikingly, interactions between ferroelectricity and absorbing light are deserving of great interest. One of the most essential issues in this concept is to extend the spectra-responsive range of ferroelectrics to a specific wavelength region, especially toward the visible-light range. For instance, electric conductivities of ferroelectric BiFeO3 polycrystals and thin films can be greatly enhanced by UV light radiation,4,5 termed as photoconductivity, which holds great promise for UV detection. Recently, the inorganic perovskite oxides of (KNbO3) (BaNi1/2Nb1/2O3−σ)x were reported to absorb full-range visible light with an ideal match to the solar spectrum and thus behave as the first visible-lightabsorbing ferroelectrics.6 Nevertheless, their photocurrent density and conversion efficiency need further improvement because considerable ferroelectrics are highly insulating,7 although a high power conversion efficiency of 8.1% had been achieved in the double perovskite multiferroic oxides of Bi2FeCrO6.8 In this context, semiconducting molecule-based ferroelectrics with visible-light-absorbing capability are expected as the complementary part to next-generation optoelectronic devices.9−11 Perovskite hybrids, most notably CH3NH3PbI3, have made a breakthrough in the photovoltaic field.12−18 In addition to the preeminent semiconducting merits (i.e., long carrier diffusion length, high carrier mobility, and large absorption coefficient),19 large ferroelectric polarization (∼38 μC/cm2) was estimated in CH3NH3PbI3 by computational calculations.20−22 Further, © 2017 American Chemical Society
Received: March 21, 2017 Accepted: April 20, 2017 Published: April 20, 2017 2012
DOI: 10.1021/acs.jpclett.7b00673 J. Phys. Chem. Lett. 2017, 8, 2012−2018
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Figure 1. (a) Schematic drawing for crystal structures of NMPX. Dashed lines denote N−H···X hydrogen bonds. Crystal packing of NMPC viewed along the a-axis direction at (b) FEP and (c) PEP, respectively. For clarity, the organic N-methylpyrrolidinium cations are represented by the blue circles in the diagram.
and three protonated organic cations, closely connected through N−H···Cl hydrogen bonds. The central Sb atoms are hexacoordinated with six chloride atoms, and three of them behave as bridging linkers to create a dioctahedron configuration (in Figure 1). This architecture differs from that of [(CH3)2NH2]3[Sb2Cl9], in which the [Sb2Cl9]3− octahedra connect with each other to form the two-dimensional polyanionic layers.34 From a viewpoint of electric polarization, this dioctahedron exhibits a slightly distorted structure geometry (Figure 1c). Terminal chloride atoms of two adjacent octahedra orientate away from the symmetric plane through the Sb1−Sb2 reference; the bridged chloride atoms also locate at sites away from the Sb1−Sb2 plane. As a result, an asymmetric configuration for the dioctahedron is constructed, which favors the appearance of Ps. Meanwhile, the relatively large temperature factors of carbon atoms suggest possible atomic disordering induced by dynamic motional freedom, although the cationic structure is refined as an ordered model. This atomic motional freedom is reminiscent of dynamic disorder for organic cations (in Figure 1c). As expected, a highly disordered state was solidly confirmed for organic cations above Curie temperatures (Tc, as discussed below). In this context, it is supposed that Ps of NMPX originates from the coupling of deformation of anionic frameworks and reorientation disorder of organic moieties. Another striking feature is that the bridging halogen atoms in NMPC can be substituted by Br atoms through chemical tailoring, as shown in Figure 1. This leads to a minor change of the geometric parameters for the inorganic moieties (Figure S3). In NMPCB, the coexistence of Cl and Br coincides with refinement of the site occupancy factor. However, a relatively large distinction between the bridging Br and terminal Cl atoms generates greater distortions of the octahedron (Figure S4), together with changes of physical properties including the
Strikingly, their bandgaps are tunable in the range of 3.31−2.76 eV, which enables absorption of visible light comparable with that of the typical inorganic ferroelectric BiFeO3 (∼2.74 eV) and building of notable photocurrents (∼10 nA/cm2). To our best knowledge, it is the first report on lead-free hybrid perovskite-type ferroelectrics with bandgaps as low as that of BiFeO3. This finding suggests an avenue to design low-bandgap ferroelectric semiconductors for potential photodetecting application. Bulk crystals of (N-methylpyrrolidinium)3Sb2Cl9−9xBr9x (abbreviated as NMPX) hybrids were grown from saturated aqueous solutions by a temperature cooling method (Figure S1). It is interesting that partial and/or full substitution of halogen atoms in this system still preserves the structural phase transitions and bulk ferroelectricity. For convenience, three typical examples were termed NMPC (x = 0), NMPCB (x = 1/ 3), and NMPB (x = 1). Elemental analysis and powder XRD were used to confirm the purities of these crystals (Figure S2). Variable-temperature structural analyses reveal that NMPX crystals belong to the trigonal system with space group R3c (point group 3m) and exhibit almost the same cell constants at room temperature (see Table S1). This is reminiscent of their isomorphic crystal structures. It is well-known that the lone-pair electrons on Sb3+ sterically interfere with ideal octahedral coordination in the Sb2X93− moiety but result in relative movement of the Sb atom away from the crystallographic center of the octahedron.24,32 Therefore, rich structural flexibility could be established by the distortion of inorganic SbX6 frameworks.33 Referring to the dimensionality and connectivity of the SbX6 octahedron, these inorganic moieties have been described as a 0-D perovskite-type structure, which is derived from the classic prototypes of 3-D perovskites. Taking NMPC as an example, the basic unit of its crystal structure consists of an isolated corner-sharing Sb2Cl93− dioctahedron 2013
DOI: 10.1021/acs.jpclett.7b00673 J. Phys. Chem. Lett. 2017, 8, 2012−2018
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Figure 2. Ferroelectric and related properties of NMPX hybrids. (a) DSC traces obtained in heating/cooling cycles. (b) Temperature dependence of SHG effects. (c) Temperature-variable dielectric constants measured along the c-axis at 100 kHz. (d) Ps values obtained from pyroelectric currents. (Inset) Polarization−electric field (P−E) hysteresis loops measured at 50 Hz.
dependence of Ps was also achieved by intergrating the pyroelectric currents (Figures S6−S8). The obtained Ps values are about 7.6, 7.3, and 5.6 μC/cm2 for NMPB, NMPC, and NMPCB, respectively. Such Ps magnitudes agree fairly well with those of SHG effects (Br > Cl > Cl/Br). Especially, it is notable that the temperature-dependent tendency of Ps is linearly consistent with that of the SHG coefficients χ(2); the relationship follows the Landau expression, χ(2)= 6ε0βPs, where ε0 is the vacuum dielectric constant and β is nearly independent of temperature (as shown in Figure 2b,d).36 Furthermore, the typical P−E hysteresis loops achieved using the Sawyer−Tower circuit method provide the most direct evidence of ferroelectricity (the inset in Figure 2d). The measured P−E loops afford saturated Ps values of 7.6, 6.8, and 5.2 μC/cm2, respectively. These Ps values are among the highest results for molecular ferroelectrics and are comparable with those of several other hybrid ferroelectrics, such as (pyrrolidinium)MnCl3 (∼5.5 μC/cm2), (3-pyrrolinium)CdCl3 (∼6.0 μC/cm2), and so forth.37 Such high ferroelectric polarizations would generate a built-in electrostatic field, which enhances the charge-carrier separation and leads to prominent photoelectric behavior.3 For instance, ferroelectric anomalous photovoltaics have a bright feature for solar-energy application,7 and the ultrahigh electric field greatly improves the sensitivity of MoS2 photodetectors.38 For NMPX crystals, their large Ps values could also create high ferroelectric-polarizationinduced electric fields, if used for photoelectric devices. Taking a vertical structure photodetector as an example, the local electric field at the electrode interface can be estimated from the equation σ = εε0E, where σ is the charge density at the electrode surface related to remnant ferroelectric polarization (Pr),39 ε is the dielectric constant, and ε0 is the vacuum
phase transition point and optical bandgap. Full substitution of Cl by Br constructs an isostructural hybrid, NMPB, as revealed from their coordination geometries. As expected, the structural features of metal−halogen frameworks in NMPB still remain similar to those in NMPC and NMPCB, reminiscent of their potential ferroelectric order. Ferroelectricity is closely associated with symmetry breaking from the paraelectric phase (PEP) to the ferroelectric phase (FEP).35 Here, differential scanning calorimetry (DSC) and second-harmonic generation (SHG) measurements confirm phase transitions in NMPX hybrids (Figure 2). Thermal anomalies in the DSC curves disclose that NMPX undergoes reversible phase transitions with different Tc values, which are far below their decomposition temperatures (∼495 K, Figure S5). Variable-temperature SHG results indicate that all NMPX hybrids are SHG-active below Tc. At room temperature, the quadratic coefficients of NMPB, NMPC, and NMPCB are estimated to be ∼0.24, 0.22, and 0.18 pm/V, respectively (Figure 2b). With the temperature approaching above Tc, a sharp change of SHG signals reveals the transition from the SHG-active phase to the SHG nonactive state, reminiscent of the symmetry breaking. As expected, all NMPX crystals adopt the centrosymmetric space group R3̅ c above Tc, suggesting ferroelectric-type symmetry breaking of 3̅mF3m (Table S1). Furthermore, it is notable that NMPX hybrids exhibit distinct physical behaviors, including SHG, dielectric, photoluminescence (PL), and pyroelectric and ferroelectric properties (see Figure 2). As an indicator of the emergence of ferroelectricity, significant dielectric peaks at Tc support the occurrence of phase transitions for NMPX hybrids; that is, the sharp peak values of dielectric constants are a few hundred times larger than that in the stable states. Besides, the temperature 2014
DOI: 10.1021/acs.jpclett.7b00673 J. Phys. Chem. Lett. 2017, 8, 2012−2018
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Figure 3. UV−vis absorption and PL spectra (a) and tunable optcial bandgaps (b) of NMPX hybrids.
permittivity. Hence, an electric field (E) of ∼4 × 106 V/cm is estimated, which greatly motivates us to investigate the possible photoconductive properties of NMPX hybrids. Because light-absorption capacity plays an essential role for photoelectric devices, we first studied the optical bandgaps and PL of NMPX hybrids. Figure 3a reveals that NMPX hybrids show strong PL peaks at around 620−640 nm at 100 K. From the transient PL spectrum of NMPC, both fast (τ1 ≈ 38 ± 4 ns) and slow (τ2 ≈ 160 ± 10 ns) components of the carrier dynamics were confirmed from biexponential fitting of transient PL spectrum (Figure S9). These values are comparable with that presented for other hybrid perovskite crystals, such as CH3NH3PbCl3 (τ1 ≈ 83 ns and τ2 ≈ 66 ns),40 demonstrating the potentials in optoelectronic application. Besides, the optical absorption edge at 375 nm of NMPC corresponds well with the photon energy of 3.31 eV. Its direct bandgap characteristic is indicated by the single slope of its extinction coefficient versus wavelength, as well as the power law of its variation (Figure 3b). Using the same method, optical bandgaps of NMPCB and NMPB are determined to be 2.81 and 2.76 eV. As the bandgap becomes smaller, the color of the crystals changes from colorless to yellow (as shown in Figure S1). Although the bandgap tunability of ΔEg ≈ 0.55 eV is smaller than the previously achieved for ferroelectric materials by the doping of inorganic oxides KNbO3 (Eg = 1.4−3.9 eV),6 the tunable bandgap range (2.76−3.31 eV) for NMPX is almost comparable with other lead halide perovskite ferroelectrics. For example, the optical bandgaps of layered perovskites, (cyclohexylammonium)2PbBr4−4xI4x, were tuned in the range from 3.05 (x = 0) to 2.74 eV (x = 0.175). To the best of our knowledge, NMPX hybrids should be the first report of leadfree inorganic−organic perovskite ferroelectric hybrids, which exhibit a broad bandgap tunability and maintain the harmonious ferroelectricity.27 Resembling that of typical inorganic ferroelectric BiFeO3, the low bandgap of NMPB allows for visible-light absorption of the solar spectrum (∼25%, Figure S10); that is, NMPX behaves as a potential candidate for potential visible-light-absorbing ferroelectric materials. Tunable optical bandgaps of NMPX hybrids enable visiblelight-induced photoconductive responses. Hence, we measured the photocurrents of NMPX crystal samples using a lateral twoprobe architecture with a monochromatic light source tuned in the range of 600−365 nm. Under irradiation of 600 nm light, no obvious photocurrent was observed for all of the NMPX hybrids. In contrast, the photocurrents for NMPB and NMPCB start to rise at 420 nm (∼10 nA/cm2 at 5.0 V, the inset in Figure 4), while NMPC shows undetectable photocurrent even under light radiation of 365 nm. This result coincides well with
Figure 4. Temperature dependence of electric conductivity (σ) measured on NMPX hybrids. (Inset) Photocurrents of NMPCB measured under light illumination at 420 nm.
their optical bandgaps; that is, a higher photon energy is needed for NMPC (Eg = 3.31 eV) to generate photocurrents. In addition, temperature-dependent conductivities also confirm the semiconducting properties of NMPX hybrids.10 As shown in Figure 4, the electrical conductivity of NMPX hybrids increases with the temperature approaching Tc, and the positive slope suggests the semiconducting feature of the transport behaviors. To understand the nature of the tunable optical bangap and structural origin of the semiconducting property, we examined the electronic structures of NMPX hybrids by using the firstprinciples density functional theory (DFT) method. It was found that all NMPX ferroelectrics demonstrate similar band structures. The calculated bandgaps for NMPC, NMPCB, and NMPB are 3.5, 3.1, and 2.8 ev, which coincide with our experimental results (3.31, 2.81, and 2.67 eV, respectively). It is known that stronger cation−anion bonding interactions usually result in more dispersive bands.41 For NMPX, hydrogenbonding interactions link organic cations and the anionic framework together. The absence of remarkable band dispersions in Figure 5a is reminiscent of weak cation−anion interactions, which still needs further studies. The calculated band structure and projected density of states (PDOS) of NMPCB reveals that the valence band maximum (VBM) consists of Br-s/p and Cl-s/p orbits, while the conduction band minimum (CBM) is composed of Sb-s/p orbits. Particularly, the p−p mixing between Sb and Br/Cl makes a dominant contribution to the bandgap of NMPCB (Figure 5). This resembles those of some Pb/Sn-based semiconducting perovskite hybrids, such as (cyclohexylammonium)2PbBr4 and CH3NH3(Pb/Sn)I3.42 Interestingly, the bandgap contrast of 2015
DOI: 10.1021/acs.jpclett.7b00673 J. Phys. Chem. Lett. 2017, 8, 2012−2018
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Figure 5. Calculated band structure (a) and the PDOS for the representative perovskite-type hybrid ferroelectric of NMPCB.
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EXPERIMENTAL SECTION Synthesis and Crystal Growth. NMPC: N-methylpyrrolidinium chloride (30 mmol, 3.68 g), Sb2O3 (10 mmol, 2.915 g) , and HCl aqueous solution (37%, 60 mL) were mixed in a roundbottom flask. After refluxing for 2 h at 353 K, clear solutions were obtained and block transparent crystals were grown by the temperature-lowering method with a cooling rate of 0.5 °C/ day. NMPB crystals were obtained using the same experimental process as that of NMPC crystals. NMPCB: Crystalline materials were prepared by dissolving N-methylpyrrolidinium chloride (30 mmol, 3.68 g) and Sb2O3 (10 mmol, 2.915 g) in HCl aqueous solution (37%, 50 mL) and HBr (40%, 6.07 g) aqueous solution at 353 K. The same growth procedure as that of NMPC crystals was used to grow NMPCB crystals. Elemental analysis (C, H and N) was performed on a Vario EL-Cube elemental analyzer, corresponding to the formula of (N-methylpyrrolidinium)3Sb2Cl6Br3. Calcd: C, 18.88; H, 3.80; N, 4.40. Found: C, 18.93; H, 3.91; N, 4.32. Powder X-ray Dif f raction (PXRD). PXRD experiments were performed using a Rigaku DMAX 2500 PC X-ray diffractometer. Diffraction patterns were collected in the 2θ range of 5−45° with a step size of 0.02°. The experimental PXRD patterns obtained at room temperature match with the calculated results based on the single-crystal structures, which solidly confirms the purity of the as-grown crystals (as shown in Figure S2). DSC Measurement. DSC was performed on a NETZSCH DSC 200F3 DSC instrument in the temperature range of 290− 360 K. Crystalline samples were placed in aluminum crucibles that were heated and cooled at a rate of 5 K/min under a nitrogen atmosphere. SHG Measurement. The SHG experiments were performed on powder samples (particle size range of 72−100 μm), using the fundamental laser beam with a low divergence (pulsed Nd:YAG at a wavelength of 1064 nm, 5 ns pulse duration, 1.6 MW peak power, 10 Hz repetition rate). The instrument model was FLS 920, Edinburgh Instruments, and the temperature system was tuned in the range of 273−380 K, while the laser was Vibrant 355 II, OPOTEK. The measured second-order
NMPX hybrids should be mainly caused by the minor difference of electronegativity (D) for halogen atoms. As shown in the typical inorganic ABO3 perovskites, the excitation across the bandgap is essentially charge transfer from the O-2p state at the VBM to the transition-metal B-d state at the CBM;43,44 a larger electronegativity difference between oxygen and transition-metal atoms leads to wider bandgaps. Here, this thesis has also been very applicable to NMPX hybrids. According to Pauling electronegativity of elements,45,46 the electronegativity difference between Sb (1.9) and Cl (3.0) can be roughly estimated as ΔDSb−Cl = 1.1, while the value between Sb and Br (2.8) is ΔDSb−Br = 0.9. As a result, the Sb−Cl bond should be more ionic compared to the Sb−Br bond, leading to a larger bandgap of NMPC.47,48 This finding agrees well with our experimental results and confirms that chemical tailoring is effective to tune optical absorption of hybrids for potential photodetecting application. In conclusion, we have reported a family of lead-free hybrids with perovskite-type structures, which exhibit excellent ferroelectricity (5.2−7.6 μC/cm2) and notable semiconducting properties, including positive temperature-dependent conductivity and visible-light-responsive photoconductivity. The combination of ferroelectric polarization and fascinating optoelectronic properties in NMPX possibly provides a pathway to next-generation logic, optoelectronic, and lightharvesting devices. The conversion efficiency can be improved by combining photovoltaic properties of lead halide perovskite solar cells and ferroelectricity, which produces an anomalous high open-circuit voltage and efficient charge separation due to the built-in field of ferroelectric polarization. Particularly, their light-absorbing capability has been well modified with the bandgap being tuned in the range of 3.31−2.76 eV, comparable with that of the typical ferroelectric oxide BiFeO3. Owing to the flexibility of structure and bandgap engineering, these ferroelectric hybrids are expected to be a promising complement to inorganic counterparts. This finding opens up opportunities to develop highly desirable ferroelectric semiconductors toward potential photodetecting applications. 2016
DOI: 10.1021/acs.jpclett.7b00673 J. Phys. Chem. Lett. 2017, 8, 2012−2018
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nonlinear optical coefficients were compared with that of the standard KH2PO4 crystal (χ2 = 0.39 pm/V). Dielectric and Ferroelectric Measurements. The processed single crystals deposited with silver conducting paste on surfaces were used for dielectric and ferroelectric measurements. The impedance analyses were performed in the temperature range of 300−360 K on a TongHui TH2828A impedance analyzer. Ferroelectric hysteresis loops were performed along the c-axis of NMPX crystals using the Sawyer−Tower circuit method (Radiant Precision Multiferroic). The thickness (∼1.3 mm) and area (∼0.85 mm2) of the crystals were accurately measured using the optical method. The frequency and amplitude of the applied voltage were 50 Hz and 1200 V, respectively. Calculations of Band Structure. Theoretical calculations the of band structure, density of states, and frontier orbital were performed using the ab initio first-principle DFT method, within the total-energy code CASTEP program. Cell constants and atomic coordinates of the ferroelectric structures of hybrids (below Tc) were used to build the theoretical modes. The exchange and correlation effects were treated by GGA-PBE for solids. The interactions between ionic cores and electrons were described by the norm-conserving pseudopotential. The calculated mode was optimized with a k-point sampling of 3 × 3 × 3 and cutoff energy of 820 eV. The other parameters and convergent criteria were the default values of CASTP code. Single-Crystal Structure Determination. Single-crystal X-ray diffractions of NMPX crystals were performed on a SuperNova diffractometer using the Mo and/or Cu Kα radiation. Data were processed by the Crystalclear software package (Rigaku, 2007). The crystal structures were solved by direct methods and then refined by the full-matrix least-squares refinements on F2 using the SHELXLTL software package. All of the non-hydrogen atoms were located from the trial structure and refined anisotropically with SHELXTL using the full-matrix leastsquares procedure. The hydrogen atom positions were fixed geometrically at the calculated distances and allowed to ride on the parent atoms. Crystallographic data and structure refinements of NMPX crystals at different temperatures are given in Table S1.
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ACKNOWLEDGMENTS was supported by NSFC (21622108, 21525104, 21373220, 51402296, and 51502290), Youth Promotion of CAS (2014262), and the State Key of Luminescence and Applications (SKLA-2016-
This work 91422301, Innovation Laboratory 09).
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00673. Experiments, large-size crystals, and PXRD patterns, crystal structure data, and basic physical properties of NMPX hybrids (PDF) Crystallographic data for NMPC at 293 K (CIF) Crystallographic data for NMPC at 328 K (CIF) Crystallographic data for NMPCB at 100 K (CIF) Crystallographic data for NMPCB at 328 K (CIF) Crystallographic data for NMPB at 293 K (CIF) Crystallographic data for NMPB at 353 K (CIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Z.S.). *E-mail:
[email protected] (J.L.). ORCID
Junhua Luo: 0000-0002-3179-7652 Notes
The authors declare no competing financial interest. 2017
DOI: 10.1021/acs.jpclett.7b00673 J. Phys. Chem. Lett. 2017, 8, 2012−2018
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2018
DOI: 10.1021/acs.jpclett.7b00673 J. Phys. Chem. Lett. 2017, 8, 2012−2018
