Synthetic Control on Structure/Dimensionality and Photophysical Properties of Low Dimensional Organic Lead Bromide Perovskite

Low dimensional lead halide perovskites have attracted huge research interest due to their structural diversity and remarkable photophysical properties. The ability to controllably change dimensionality/structure of perovskites remains highly challenging. Here, we report synthetic control on structure/dimensionality of ethylenediammonium (ED) lead bromide perovskite from a two dimensionally networked (2DN) sheet to a one dimensionally networked (1DN) chain structure. Intercalation of solvent molecules into the perovskite plays a crucial role in directing the final dimensionality/structure. This change in dimensionality reflects strongly in the observed differences in photophysical properties. Upon UV excitation, the 1DN structure emits white light due to easily formed ``self-trapped'' excitons. 2DN perovskites show band edge blue emission (similar to 410 nm). Interestingly, Mn2+ incorporated 2DN perovskites show a highly red-shifted Mn2+ emission peak at similar to 670 nm. Such a long wavelength Mn2+ emission peak is unprecedented in the perovskite family. This report highlights the synthetic ability to control the dimensionality/structure of perovskite and consequently its photophysical properties

Energy Transfer Mechanisms and Optical Thermometry of BaMgF4:Yb3+,Er3+ Phosphor

Motivated from our previous studies on the upconversion properties of BaMgF4:Yb3+,Tb3+ phosphor, here we investigated the upconversion properties of BaMgF4:Yb3+,Er3+ phosphor. We demonstrate a two-way versatile approach for the fine-tuning of emission from green to the red region, by varying the dopant concentration and adjusting the pulse width of an infrared laser. The mechanism involved in tuning the emission color by laser power and pulse width variation was illustrated in detail. The temperature dependent upconversion spectra were studied by analyzing the fluorescence intensity ratio of the thermally coupled levels. The maximum sensitivity obtained is 83.29 x 10(-4) K-1 at 583 K, which is much higher than the temperature sensitivity reported for other fluoride based materials. Moreover, the influence of the excitation power density on the ability of the phosphor for temperature sensing was also investigated. We obtained a maximum (similar to 415 K) temperature detection at 2563 mW laser power. The obtained results illustrate the potential use of BaMgF4:Yb3+,Er3+ phosphor in an optical thermometer due to its highly sensitive temperature detection ability

Behavior of Methylammonium Dipoles in MAPbX(3) (X = Br and I)

Dielectric constants of MAPbX(3) (X = Br, I) in the 1 kHz-1 MHz range show strong temperature dependence near room temperature, in contrast to the nearly temperature -independent dielectric constant of CsPbBr3. This strong temperature dependence for MAPbX(3) in the tetragonal phase is attributed to the MA+ dipoles rotating freely within the probing time scale. This interpretation is supported by ab initio molecular dynamics simulations on MAPbI(3) that establish these dipoles as randomly oriented with a rotational relaxation time scale of similar to 7 ps at 300 K. Further, we probe the intriguing possibility of transient polarization of these dipoles following a photo excitation process with important consequences on the photovoltaic efficiency, using a photoexcitation pump and second harmonic generation efficiency as a probe with delay times spanning 100 fs-1.8 ns. The absence of a second harmonic signal at any delay time rules out the possibility of any transient ferroelectric state under photoexcitation

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Critical Comparison of FAPbX₃ and MAPbX₃ (X = Br and Cl): How Do They Differ?

Dielectric measurements on formamidinium lead halide perovskites, FAPbCl₃ and FAPbBr₃, compared to those of MAPbCl₃ and previously reported MAPbBr₃, reveal the strongly suppressed temperature dependence of dielectric constants in FA compounds in the temperature range of approximately 140–300 K. Although the behavior of dielectric constants of FA compounds for temperatures <140 K resembles that of the MAPbX₃ system, the absence of any strong temperature dependence in sharp contrast to MA analogues in the higher temperature range up to room temperature suggests that the formamidinium (FA) dipoles are in a deep-frozen glassy state unlike the MA dipoles that rotate nearly freely in the temperature range relevant for any photovoltaic application. This observation is further supported by the temperature-dependent single-crystal X-ray diffraction (XRD) results

Energy Transfer Mechanisms and Optical Thermometry of BaMgF₄:Yb³⁺,Er³⁺ Phosphor

Motivated from our previous studies on the upconversion properties of BaMgF₄:Yb³⁺,Tb³⁺ phosphor, here we investigated the upconversion properties of BaMgF₄:Yb³⁺,Er³⁺ phosphor. We demonstrate a two-way versatile approach for the fine-tuning of emission from green to the red region, by varying the dopant concentration and adjusting the pulse width of an infrared laser. The mechanism involved in tuning the emission color by laser power and pulse width variation was illustrated in detail. The temperature dependent upconversion spectra were studied by analyzing the fluorescence intensity ratio of the thermally coupled levels. The maximum sensitivity obtained is 83.29 × 10^–4 K^–1 at 583 K, which is much higher than the temperature sensitivity reported for other fluoride based materials. Moreover, the influence of the excitation power density on the ability of the phosphor for temperature sensing was also investigated. We obtained a maximum (∼415 K) temperature detection at 2563 mW laser power. The obtained results illustrate the potential use of BaMgF₄:Yb³⁺,Er³⁺ phosphor in an optical thermometer due to its highly sensitive temperature detection ability

Is CH3NH3PbI3 Polar?

In view of the continued controversy concerning the polar/nonpolar nature of the hybrid perovskite system, CH3NH3PbI3, we report the first investigation of a time resolved pump probe measurement of the second harmonic generation efficiency as well as using its more traditional form as a sensitive probe of the absence/presence of the center of inversion in the system both in its excited and ground states, respectively. Our results clearly show that SHG efficiency, if nonzero, is below the limit of detection, strongly indicative of a nonpolar or centrosymmetric structure. Our results on the same samples, based on temperature dependent single crystal X-ray diffraction and P-E loop measurements, are entirely consistent with the above conclusion of a centrosymmetric structure for this compound in all three phases, namely the high temperature cubic phase, the intermediate temperature tetragonal phase and the low temperature orthorhombic phase. It is important to note that all our experimental probes are volume averaging and performed on bulk materials, suggesting that basic material properties of CH3NH3PbI3 are consistent with a centrosymmetric, nonpolar structure