Investigation of the structural, electronic, and optical properties of Mn-doped CsPbCl3: theory and experiment

Wide energy gap inorganic halide perovskites have become emerging candidates for potential applications in modern optoelectronics devices. However, to date, these semiconducting compounds have not been explored theoretically to a significant extent. Herein, we performed ab initio computations to explain the structural, electronic and optical behaviour of inorganic CsPbCl3 and Mn-doped CsPbCl3 nanocrystals (NCs). We also synthesized these NCs and further validated our experimental results with density functional theory (DFT) calculations. The results provide insight into the effect of Mn doping on the important properties of CsPbCl3 NCs such as their lattice parameter, electronic band structure, density of states, dielectric constant, absorption coefficient and refractive index. After geometry optimization using the Limited-memory Broyden–Fletcher–Goldfarb–Shanno (LBFGS) algorithm, a reduction in the lattice parameter from 5.605 Å to 5.574 Å was observed after doping Mn in the CsPbCl3 NCs, which is in good agreement with the calculated results from the X-ray diffraction (XRD) pattern (5.610 Å to 5.580 Å) and high-resolution transmission electron microscopy (HRTEM) images (5.603 Å to 5.575 Å). The incorporation of Mn in CsPbCl3 was observed in the electronic band structure in the form of additional states present in the energy gap and an increment in the band gap of the CsPbCl3 NCs. This result is consistent with the photoluminescence (PL) plot, which showed dual color emission in the case of the Mn-doped CsPbCl3, which is attributed to the Mn2+ d-band to d-band transition. The partial density of states (PDOS) of the Mn-doped CsPbCl3 NCs clearly indicates the contribution of the Mn 3d orbitals to the upper valence band and conduction band together with the contribution of the Pb 6p and Cl 3p orbitals. Moreover, a blue-shift phenomenon was observed from the dielectric constant and absorption coefficient spectra, which is due to the incorporation of Mn in CsPbCl3. Also, a significant peak was observed in the absorption coefficient and dielectric constant spectra around 2.08 eV, which is in good agreement with the PL plot. This DFT study with experimental observation provides a way to investigate this type of compound and to tailor its interesting characteristics through doping.


Introduction
Solar energy plays a signicant role in the quest for green and renewable sources of energy worldwide. Solar energy has become a good substitute for the different sources of energy, which can overcome the drawbacks of the traditional sources of energy. 1 To utilize this energy, solar cells play a signicant role in transforming light energy into electrical energy with minimal loss together with a low emission of greenhouse gases. 2 In the design of modern electronic devices such as light-emitting diodes (LEDs) 3 and photovoltaic cells, 4-7 inorganic perovskites have attracted signicant attention due to their low cost and promising utility. Thus, in the solar cell research community, inorganic perovskites [8][9][10][11][12][13][14] have attracted signicant interest in recent years. The general structural formula for these compounds is ABX 3 , where A represents the inorganic cation, B the divalent metallic cation and X the halogen. Inorganic perovskites show unique and interesting properties, such as signicant absorption coefficient and good semiconducting behavior, which make them suitable candidates for widespread utility in photovoltaic and optoelectronic devices. [15][16][17][18][19][20] Cesium lead halide perovskite (CsPbX 3, X¼ Cl, Br and I), 21 which is an inorganic perovskite, has attracted growing attention due to its fascinating properties of extremely efficient PL, which can be tailored over the whole visible spectrum by controlling dimension and anion in its NCs. It has been used as an active material in a wide applications such as light-emitting diodes (LEDs), 22 photovoltaic cells 23 and lasers. 24 Particularly, perovskites can be used as promising materials in photovoltaic applications, in which, within seven years the efficiency has been upgraded from 3.8% 25 to 22.1%. 26 Thus, this perovskite has been developed as a potential material in the light-emitting eld. Light-emitting modern devices in the visible range can be tuned by controlling the NC size. However, hybrid organicinorganic lead halides have a major limitation of sensitivity towards light, heat, and humidity. Accordingly, the CsPbX 3 inorganic perovskite has been proven to be superior to other hybrid organic-inorganic lead halide perovskites due to its ability to show optical properties 27 with higher stability. 28 Also, CsPbX 3 nanocrystals are much more superior than other NCs due to their tunability, leading to their wide application in lasers based on single 27 and multiphoton pumping, 29 LEDs 30 and in optoelectronics.
Nowadays, there are many advantages and applications of cesium lead halide perovskites. However, the highly toxic nature of lead is a serious concern, which has a hazardous impact on the environment and health of human beings on a commercial level. 31 Accordingly, the synthesis of lead-free perovskites is urgent to completely eliminate lead, which causes serious health and environmental issues. Additionally, the optical, electronic and magnetic characteristics of NCs can be controlled by incorporating impurity ions. Recently, there have been notable efforts on doping aimed at the complete elimination/reduction of lead. Doping of impurities has been done in II-VI and III-V NCs to introduce magnetism, 32 impuritybased PL 33 and induction of p-and n-type behavior. 34 The impurity dopants that are widely studied and effectively impart novel properties in semiconductor nanocrystals are Mn 2+ , 35 Co 2+ , 36 Cu 2+ , 37 and Ag + . 38 Among them, Mn 2+ doping has been widely studied 35,[39][40][41][42] due to its high abundance, and potential to induce optical and magnetic properties 43 in the doped host semiconductor nanocrystals.
Herein, we examined the effect of Mn as a dopant in CsPbCl 3 on its structural, electronic, and optical behavior. To obtain information about the crystal structure, XRD was performed and the results compared with the JCPDS database. Further, to study the morphology of the synthesized samples, TEM and high-resolution transmission electron microscopy (HRTEM) were performed. Moreover, optical and electronic characteristics were studied using PL and ultraviolet-visible (UV-vis) spectroscopy. A change in the energy band gap was observed in the case of Mn-doped CsPbCl 3 . These experimentally observed results were compared with the computed results obtained from the DFT-based ab initio study, providing deep insight into the unique and interesting electronic and optical properties of the samples. The DFT-based theoretically computed results show very good agreement with our experimentally observed results.

Computational models and methods
Herein, we explored the structural, electronic and optical properties of CsPbCl 3 and Mn-doped CsPbCl 3 compounds applying ab initio computations based on density functional theory (DFT). The computations were carried out using the Quantumwise Atomistix (ATK) soware package. 44 Here, ab initio computations were performed using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) 45 exchange-correlation functional. The Brillouin zone was sampled using the Monkhorst-Pack grid 46 with 9 Â 9 Â 9 sampling points. To achieve high accuracy in our calculations, an energy cut-off of 75 hartree was used. The crystal structure of the compounds was optimized using the LBFGS optimization code until the residual force on each atom was less than 0.05 eV A À1 . A norm-conserving FHI (Fritz Haber Institute) pseudopotential was used for our calculations. In this work, the double zeta polarized (DZP) basis set was chosen for all atoms, which is comprised of three basis orbitals (analytical split, conned orbital, and polarization orbital for the rst unlled shell of an atom). In our calculation, the Mn atom was doped in CsPbCl 3 at the Pb site and simulations were performed on a 2 Â 2 Â 2 supercell, as shown in Fig. 2(a) and (b).
To investigate the optical behavior of CsPbCl 3 and Mn-doped CsPbCl 3 , the Kubo-Greenwood formalism was used in the DFT framework. The meta-generalized gradient approximation (MGGA) with Tran and Blaha (TB09) 47 as the exchangecorrelation functional was utilized to compute various optical properties, such as dielectric constant, absorption coefficient, and refractive index. A k-point sampling of 15 Â 15 Â 15 was chosen to compute the optical behavior mentioned above for CsPbCl 3 and Mn-doped CsPbCl 3 . We had accounted for the thermal effect while calculating the optical properties by choosing a broadening of 0.01 eV in the simulations.
The susceptibility tensor is given by the Kubo-Greenwood formalism as: where, p nm i represents the i th component of the dipole matrix element between two states n and m, V represents the volume, G represents the broadening and f represents the Fermi function. From the above eqn (i), the relative dielectric constant 3 r is related to the susceptibility according to Griffiths 48 The refractive index, h, is related to the extinction coefficient, k, and the relative dielectric constant, 3 r , as Finally, the optical absorption coefficient is given by a as in ref. 49.

Preparation of Cs-oleate solution.
Following the synthetic method reported in ref. 21, in a 250 mL 2-neck ask, 0.325 g of Cs 2 CO 3 was dissolved in 2 mL of OA and 20 mL of ODE. This mixture was heated at a temperature of 110 C for 60 minutes under a nitrogen (N 2 ) atmosphere. Further, it was heated at 140 C for 30 min. Subsequently, the obtained oleate of Cs was heated at 110 C for the preparation of CsPbCl 3 and Mn-doped CsPbCl 3 . 3 . The synthesis of CsPbCl 3 and Mn-doped CsPbCl 3 was done using the previously reported method in ref. 50. Briey, one mole of PbCl 2 with 2.4 mL of OAm, 2.4 mL of OA, and 10 mL of ODE were added to a 150 mL 3-neck ask and heated at a temperature of 105 C for 30 min under a nitrogen atmosphere. Aerwards, the temperature was increased to 160 C and the precursors in the solvent were heated for 20 min. Subsequently, to solubilize the salt, the mixture was heated at an increased temperature of 200 C. 1.6 mL of Cs-oleate solution was introduced by decreasing the temperature from 200 C to 175 C. An ice-water bath was used to immediately quench reaction mixture to room temperature. Manganese (Mn) was introduced as a dopant in CsPbCl 3 by adding equal mol of MnCl 2 $4H 2 O to the above precursor. To synthesize the Mn-doped CsPbCl 3 (Mn-doped CsPbCl 3 ), all other steps were the same as above. The nanocrystals (NCs) were isolated by centrifugation at 10 000 rpm for 10 min. Further cleaning was performed using acetone, and the NCs were redispersed in hexane for further characterization.

Results and discussion
CsPbCl 3 and Mn-doped CsPbCl 3 were synthesized using the hotinjection method, and the structural, optical, and electronic properties of the synthesized materials were also explored theoretically using DFT. Fig. 1(a) shows the unit cell of the CsPbCl 3 inorganic lead halide perovskite, which consist of Cs (occupying corner position) and Pb (occupying body-centered position) as two cations with different sizes and Cl (occupying face-centered position) as the anion, which forms bonds with both Cs and Pb cations. The unit cell of CsPbCl 3 consists of a PbCl 6 cage (as shown in Fig. 1(a)), which plays a signicant role in determining its electronic behavior.
In this work, we theoretically explored the effect of Mn doping in CsPbCl 3 using the ab initio-based DFT simulation and further validated our calculated results with experimentally observed results. Fig. 1(b) shows the XRD patterns of both the undoped and doped CsPbCl 3 . The XRD patterns of these NCs were measured using an X'pert PANAlytical X-ray diffractometer with Cu Ka radiation (l ¼ 1.5406Å) run a voltage of 40 kV and current of 30 mA. The observed XRD plots for our synthesized samples were compared with the JCPDS database (04-005-6612). The XRD pattern for CsPbCl 3 , as shown in Fig. 1(b), is consistent with the JCPDS database.
The effect of Mn doping can be clearly seen in Fig. 1(b) by analyzing the XRD pattern for the Mn-doped CsPbCl 3 . The XRD peaks for the Mn-doped CsPbCl 3 shows the (110) peak at a higher angle compared to that for CsPbCl 3 . This shi towards a higher angle shows a reduction in the lattice parameter of the Mn-doped CsPbCl 3 crystal structure.
The structural properties were explored using DFT calculations, in which geometry optimization of CsPbCl 3 and Mndoped CsPbCl 3 was performed. Further, to study the ease with which Mn can be introduced within the CsPbCl 3 lattice, the formation energy of Mn-doped CsPbCl 3 was calculated by using the following equation: 51 where, E undoped is the total energy of CsPbCl 3 without Mn doping, E Mn-doped is the total energy of Mn-doped CsPbCl 3 , m Pb is the energy of the Pb atom, and m Mn is the energy of the Mn atom. The energies of the Mn and Pb atoms were calculated from the total energies calculated for Mn molecules and Pb molecules, respectively. The calculated formation energy (E f ) for Mn doped at the Pb site was found to be À4.3856 eV, which indicates the high stability of the Mn-doped CsPbCl 3 crystal structure. Fig. 2(a and b) show the optimized 2 Â 2 Â 2  Table 1.
TEM was performed to investigate the surface morphology and size of the synthesized compounds using a JEOL (JEM-2100F) eld-emission gun transmission electron microscope (FEG-TEM). Fig. 3(a) and (b) show the TEM images of CsPbCl 3 and Mn-doped CsPbCl 3 NCs, respectively. In the TEM images, lattice fringes can be clearly seen all over the NCs, which   3 NCs, respectively, which are consistent with that calculated from the XRD (110) peak and ab initio results. The observed reduction in the lattice parameter in the case of Mndoped CsPbCl 3 is attributed to the contraction in its lattice due to the smaller ionic radius of Mn 2+ compared to that of the Pb 2+ ion. The optical behaviour of the CsPbCl 3 and Mn-doped CsPbCl 3 NCs were investigated by studying the photoluminescence (PL) and ultraviolet and visible absorption (UVvis) spectra of their dispersion in hexane, as depicted in Fig. 4. The ultraviolet and visible absorption (UV-vis) spectra were measured using a PerkinElmer Lambda À950 UV-vis spectrophotometer. Photoluminescence (PL) spectra were collected using a HORIBA Scientic FluroMax 4 Spectrouorometer. The absorption peaks were observed at around 416 nm and 398 nm for CsPbCl 3 and Mn-doped CsPbCl 3 , respectively. The Mndoped NCs showed a dual color broad peak emission, where one emission is located at 398 nm, while the other peak is at 584 nm, which is attributed to the Mn 2+ d-band ( 4 T 1 ) to d-band   By examining the band structure of CsPbCl 3 in Fig. 5(a), the valence band offset lies at the center of the Brillouin zone (G point) with an energy À1.485 eV. The conduction band offset also lies at the G point with an energy 1.504 eV. This clearly indicates the direct band gap nature of the CsPbCl 3 NCs with a band gap of around 2.989 eV. Thus, to investigate the effect of Mn as a dopant on the electronic properties of CsPbCl 3 , the electronic band structure of Mn-doped CsPbCl 3 was calculated, as shown in Fig. 5(b). Due to the introduction of Mn as a dopant in CsPbCl 3 , different states were introduced in the energy gap closer to the VBO with an energy of À1.231 eV. The CBO moves downward with an energy 0.850 eV compared to the undoped CsPbCl 3 . This leads to a d-band to d-band transition due to the incorporation of the Mn 2+ ion in CsPbCl 3 . The energy gap between the two d-bands was found to be 2.08 eV. Both the VBO and CBO for Mn-doped CsPbCl 3 lie at the G, point which indicates their direct band gap nature with an energy gap of 3.12 eV. These energy gaps in Mn-doped CsPbCl 3 are responsible for the two observed emission peaks, which are caused by the band offset emission in the CsPbCl 3 host NCs together with the dband to d-band transition of the Mn 2+ ions. These energy gap values are in accordance with the experimentally calculated This journal is © The Royal Society of Chemistry 2019 values from the optical properties. To examine the effect of the Mn dopant in CsPbCl 3 on the type of semiconductor, we calculated the position of the chemical potential of CsPbCl 3 and Mn-doped CsPbCl 3 using DFT based ab initio calculations. The evaluated chemical potential values for CsPbCl 3 and Mn-doped CsPbCl 3 are À3.871 eV and À3.508 eV, respectively. These values clearly indicate a shi in the chemical potential towards the conduction band due to the incorporation of Mn as a dopant, resulting in n-type behavior in the Mn-doped CsPbCl 3 NCs. Fig. 6(a) and (b) illustrate the partial density of states of the undoped CsPbCl 3 and Mn-doped CsPbCl 3 , respectively. By looking at the PDOS of CsPbCl 3 , as shown in Fig. 6(a), the conduction band is mainly due to the electrons of the Pb (6p) orbitals, while the upper valence band is mainly due to the Cl (3p) orbitals. These contributions lead to the emission at bandedge in the CsPbCl 3 NCs. Cs does not have any signicant contribution to conduction band and valence band.
However, Fig. 6(b) shows the PDOS of Mn-doped CsPbCl 3 NCs, which clearly illustrates the contribution of the Mn (3d) orbitals to the upper valence band and conduction band together with the contribution of Pb (6p) and Cl (3p) orbitals. Moreover, due to the introduction of states by the 3d-orbitals of the Mn 2+ ions, the energy difference between the d-band to dband is less than the energy gap of the host CsPbCl 3 NCs, which can be also seen in the band structure plot in Fig. 5(b).
These calculated PDOS results indicate the dual color emission, one due to the host CsPbCl 3 NCs and the other due to the dband to d-band transition in Mn 2+ ions for the Mn-doped CsPbCl 3 .
The dielectric constant plots as a function of energy explain the interaction between the incident light energy and the crystal structure. The real component of the dielectric constant gives information about the anomalous dispersion effects and polarization. However, the imaginary plot of dielectric constant provides a description about the major absorption energy in a crystal structure as a result of neutral charge excitations. These neutral charge excitations lead to a variation in charge density on account of the creation of excitons. To study these aspects, we investigated the optical properties of CsPbCl 3 and Mn-doped CsPbCl 3 using ab initio DFT calculations. Fig. 7 shows the real and imaginary parts of the evaluated dielectric constant as a measure of photon energy across the three tensors XX, YY, ZZ for both CsPbCl 3 and Mn-doped CsPbCl 3 . The imaginary component of dielectric constant is an important aspect to examine the optical absorption of crystal structures, which explains the plot of Imz(3 2 ) across the XX, YY, and ZZ directions. For the case of CsPbCl 3 , the rst signicant peak lies at around 2.97 eV, while the strongest peak lies at around 3.12 eV. For the case of Mn-doped CsPbCl 3 , the rst signicant peak lies at around 2.08 eV, which is attributed to the Mn 2+ ions (d-band to d-band transition), while the signicant peak due to the host CsPbCl 3 NCs lies at around 3.11 eV. These peaks are in good agreement with the experimentally observed energy gap from the optical properties for undoped CsPbCl 3 and Mn-doped CsPbCl 3 . The values of the static dielectric constant for CsPbCl 3 and Mn-doped CsPbCl 3 are 2.513, and 2.899, respectively, which were evaluated from the real part of the dielectric constant plots. These peaks in the dielectric constant plots indicate that the binding energy of the excitons becomes smaller due to their energy range, resulting in high optical absorption. The effect of Mn doping in CsPbCl 3 can be seen through the blue-shi phenomenon in the imaginary part of the dielectric constant spectrum as a function of energy.
The optical absorption coefficient plots are shown in Fig. 8(a) and (b). The shape of the optical absorption plots shows similar characteristics compared to the imaginary value of dielectric constant plots for both crystal structures. The strongest peak in the optical absorption spectrum lies in the visible region for both CsPbCl 3 and Mn-doped CsPbCl 3 . In the case of Mn-doped CsPbCl 3 , the optical absorption coefficient plot shows another signicant peak at 2.08 eV, which is due to the d-band to d-band transition caused by the Mn 2+ ions. The absorption coefficient edge lies at 2.97 eV and 3.11 eV for CsPbCl 3 and Mn-doped CsPbCl 3 , respectively, which correspond to their band gap.
Using the dielectric constant plot, the refractive index across the XX, YY and ZZ tensors were evaluated for both crystal structures. The peaks observed in Fig. 9 follow the trend of the real component of dielectric constant, as shown in Fig. 7(a) and (c). The refractive index plots are also consistent with the dielectric constant and optical absorption plots. Moreover, from the plot of the above calculated optical properties, it was observed that the XX, YY, and ZZ tensors coincide with each other, showing the isotropic behavior of both crystal structures. This type of ab initio-based DFT investigation combined with experiment opens a new way to gain insight into the properties of these materials and also about the types of doping needed to tailor their unique and interesting behavior for different applications in the domain of optoelectronics, optomagnetics, and photovoltaics.

Conclusion
Herein, we presented a theoretical investigation combined with experimental analysis of the structural, electronic and optical properties of inorganic wide energy gap CsPbCl 3 and Mn-doped CsPbCl 3 compounds. By introducing Mn as an impurity dopant in CsPbCl 3 , a blue-shi phenomenon was observed, which indicates an increase in the energy gap. A dual color emission  was observed in the Mn-doped CsPbCl 3 compound. Also, the change in chemical potential suggests n-type behavior in Mndoped CsPbCl 3 . The incorporation of Mn in CsPbCl 3 led to the introduction of states, which is due to the Mn (3d) orbitals. This behavior can also be easily seen in the plots of the imaginary dielectric constant and absorption coefficient in the form of a signicant peak at around 2.08 eV. These theoretical results show very good agreement with our experimentally observed results. Overall, this work presents insights into CsPbCl 3 and the effect of Mn dopant on its structural, electronic and optical properties, and also suggests a strategy for the synthesis of new compounds with required interesting properties based on various optoelectronics, optomagnetics and photovoltaics devices in halide-based inorganic perovskite compounds.

Conflicts of interest
There are no conicts to declare.