Band Gap Engineering in Cs2(NaxAg1-x)BiCl6 Double Perovskite

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Band Gap Engineering in Cs(NaAg )BiCl Double Perovskite Nanocrystals Raman singh, Pooja Basera, Saswata Bhattacharya, and Sameer Sapra J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02168 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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The Journal of Physical Chemistry Letters

Band Gap Engineering in Cs2(NaxAg1-x)BiCl6 Double Perovskite Nanocrystals Raman Singh Lamba a, Pooja Basera b, Saswata Bhattacharya #b, Sameer Sapra *a aDepartment

of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New

Delhi 110016, India bDepartment

of Physics, Indian Institute of Technology Delhi, Hauz Khas, New

Delhi 110016, India *Email: [email protected] #Email: [email protected]

Abstract: Lead-free double perovskites materials, A2M(I)Mˈ(III)X6, have recently attracted attention as environment-friendly alternatives to lead-based perovskites, APbX3, because of both rich fundamental science and potential applications. We report band gap tuning via alloying of Cs2AgBiCl6 nanocrystals (NCs) with non-toxic, abundant Na. It results into a series of Cs2NaxAg1xBiCl6

(x = 0, 0.25, 0.5, 0.75, and 1) double perovskite NCs, leading to increase in optical band

gap from 3.39 eV (x = 0) to 3.82 eV (x = 1) and 30-fold increment in weak photoluminescence. The tuning of band gap has been further explored by electronic structure calculation under the framework of density functional theory (DFT). The latter confirms that the increase in band gap is due to reduction of Ag contribution near valence band maxima (VBM) on incorporation of Na ion in place of Ag. These alloyed double perovskites can have useful potential applications in optoelectronic devices.

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In recent years, metal halide perovskites have become burgeoning field of research owing to their applications in solar cells,1–3 light emitting diodes (LEDs),4–6 photodetectors,7–9 and lasers.10,11 These large number of applications of perovskites are results of their useful properties like long charge carrier diffusion length,12,13 high charge carrier mobility,13,14 high photoluminescence quantum yield (PLQY),10,15 broad absorption spectrum,16 high absorption coefficient,16,17 tunable band gap,15,18,19 and defect tolerance.20,21 Furthermore, the band gap of perovskites can be tuned by varying their composition through the control of the percentages of dopants.22 All-inorganic metal halide perovskites nanocrystals (NCs) with the general formula APbX3 (where A = Cs, and X = Cl, Br, I) were first reported from the group of Kovalenko via colloidal synthesis.15 During this period, researchers focused on tuning the band gap of perovskites by doping or alloying through direct or indirect synthetic routes e.g. anion and cation exchange. Thus, CsPbX3 NCs (X = Cl, Br) and a series of CsPb1-xMxBr3 (0 ≤ x ≤ 0.1) NCs (M2+ = Sn2+, Zn2+, Cd2+) have been synthesized using anion and cation exchange synthetic routes.18,23 Besides, Mn2+doped CsPbX3 (X = Cl, Br) NCs have also been studied.24 In spite of a plethora of applications of these perovskite semiconductor materials, there are two major obstacles, viz. stability of the perovskite structure against moisture and toxicity owing to the presence of Pb2+, which pose constraints on 2 ACS Paragon Plus Environment

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the commercial application of these materials.22 Subsequently, further efforts were made to substitute Pb2+ with Sn2+,25,26 although Sn2+ gets easily oxidized to Sn4+ in ambient conditions, thereby resulting in degradation of Sn2+ based perovskites.27 Mixed Pb/Sn perovskites are also known with power conversion efficiency of >19%,28 but these are also unstable and toxic due to the presence of Sn2+ and Pb2+, respectively.29–31 To resolve these main issues regarding instability and toxicity of lead halide perovskites, a recent surge in research has been observed towards alternative lead-free double perovskites having cubic crystal structure and band gaps in visible region. In CsPbX3 NCs, two Pb2+ cations can be transmuted with two other cations one in +1 oxidation state and other in +3 oxidation state, resulting in double perovskite structure with formula Cs2M(I)Mˈ(III)X6, where M and Mˈ may be homometallic or heterometallic. Roman et. al. have reported the synthesis of Cs2Au(I)Au(III)Br6 NCs by complete cation exchange in CsPbBr3.32 Recently, first report on heterometallic Cs2M(I)Mˈ(III)X6 has introduced us to Cs2AgBiX6 (X = Cl, Br) NCs, which were synthesized by hot injection method. These NCs were further used to synthesize Cs2AgBiI6 NCs by anion exchange method.33 Also, Cs2AgBiBr6 NCs are useful in photocatalytic CO2 reduction owing to their tolerance towards external stimuli (temperature, moisture, and light).34 Double perovskite materials have environmental friendly composition, stable lattice and solution processability that makes them strong candidates in applications involving solar cells,35–37 X-ray photodetectors,38 and LEDs.39,40 Since Cs2AgInBr6 may have absorption coefficient comparable to or even more than Si, solar cells based on these materials may reach efficiency of 28%.41,42 But most of these double perovskites have direct or indirect wide forbidden band gap, which is not suitable for solar cell applications.43 To solve this problem, several efforts have been made to tune their band gap, and to enhance the PLQY of these double perovskites. Deng et. al. demonstrated the synthesis of



Cs2SbAgCl6 with indirect band gap of 2.6 eV,44 whereas Tran et. al. reported the alloying of Cs2SbAgCl6 with In to synthesize a series of Cs2(SbxIn1-x)AgCl6 (0 ≤ x ≤1) double perovskites. The indirect to direct band gap modification was explained in Cs2(SbxIn1-x)AgCl6 double perovskites with increase in percentage of In.45 Doping of Cu2+ in Cs2SbAgCl6 leads to the narrowing of band gap from 2.6 eV to 1.05 eV.46 Similarly, Yang et. al. modified the band gap of Cs2AgBiCl6 NCs by alloying with In and synthesized a series of Cs2AgInxBi1-xCl6 (x = 0, 0.25, 0.50, 0.75, 0.90) NCs with maximum PLQY of 36.6% for Cs2AgIn0.9Bi0.1Cl6 NCs.47 Mn2+ doped Cs2NaBiCl6 bulk material shows PLQY of 15% and Mn2+ doped Cs2AgInCl6 in bulk as well as in NCs show PLQY of (3-5) % and 16% respectively.48–50 Zhou et. al. demonstrated the synthesis of Mn2+ doped Cs2NaBi1-xInxCl6 crystals and studied band gap modification and their corresponding PL.43 Recently Li et. al. explored the Pb-based materials, where the synthesis of Na doped CsPbBr3 with 85% PLQY for 1.25 Na/Pb ratio was reported and the band gap of CsPbBr3 was tuned by varying the Na/Pb ratio.51 However, there is no report dealing with the variation of Na in these materials. As an isolated example, in bulk phase, Cs2Na0.4Ag0.6InCl6 is a promising phosphor, covering nearly the entire visible region with a very high PLQY of 86 ± 5%.52 Here, we report the alloying of Cs2AgBiCl6 NCs with Na, which results in an increase in the band gap in the ultraviolet region. These results are consistent with the electronic structure obtained from first-principles based density functional theory (DFT), which confirms increase in the bandgap, with systematic increase in Na%. We find that these alloyed double perovskites have cubic crystal structure and broad photoluminescence (PL) ranging from 440 nm to 850 nm. Cs2NaxAg1-xBiCl6 alloyed double perovskites were prepared as per the method described in the materials section. A representative crystal structure of the alloyed double perovskite (50% Na) is 4 ACS Paragon Plus Environment

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shown in Figure 1a. The crystal structure remains intact upon alloying and does not get changed as seen from the diffraction patterns in Figure 1b. This matches with the elpasolite cubic crystal structure of Cs2AgBiCl6 crystallizing in the 𝐹𝑚3𝑚 space group.33 For nominal alloying values of 0, 25, 50, 75 and 100% Na in the alloyed structure, there is no change in the gross diffraction patterns. What does change is summed up in the two observations, viz. (i) the change in the intensity of the (111) peak marked with the asterisk (see Figure 1b) and (ii) the shift in the 2θ values for the (220) planes as seen in Figure 1c. The smaller size of Na+ cation (1.02 Å)41 compared to the Ag+ cation (1.15 Å)41 would urge us to conclude the shrinkage of lattice upon alloying with Na. On the contrary, we have noticed lattice expansion, as the diffraction peak shifts to lower 2θ values. This is due to Ag-Cl covalent character, which results in a much stronger bond (bond length = 2.708 Å)53 compared to the highly ionic Na-Cl bond (bond length = 2.736 Å)48. This results in the expansion of the lattice and also weakens the Cs-Cl bond.48 From the shift in the highly intense (220) reflection (see Figure 1c), we can estimate the d values for the lattice, which are shown in Figure 1d, plotted against the nominal Na values. If we assume Vegard’s Law behavior for the alloyed structure, the calculated d values can be used to estimate the actual alloying percentage. The trends are shown in the Figure 1d. Nominal alloying of 25, 50 and 75% actually result in 22, 39 and 77% Na incorporation, respectively. However, one must notice that the variations in the d-spacings are in the second decimal place – the total change upon complete replacement of Ag with Na being only 0.02 Å – and thus the reported deviations from the nominal values have a lot of uncertainty and therefore should not be regarded sacrosanct. The second observation, i.e. the increase in the (111) intensity corresponds to an increased Na/Ag ratio in the lattice. The latter is in agreement with a recent report,52 which further confirms the formation of



the alloyed structure. The intensity of (111) reflection is known to depend on the dispersion of M(I) and Mˈ(III) cations in the lattice.54

(b)

(a)

(220)

+ +

Cs Cs

(111)

3-

*

Counts

[AgCl6]

* *

[NaCl6]5-]5[NaCl

*

6

Cs Na Ag BiCl6

(220)

Cs2AgBiCl6

(111) (200)

Cs2Na0.25Ag0.75BiCl6 Cs2Na0.5Ag0.5BiCl6 10 Cs2Na0.75Ag0.25BiCl6 Cs2NaBiCl6

(c)

(422) (400) (311) (420) (440) (600) (620)(444) (222) (331) (642) (511) (531)

*

[AgCl6]5- 5-

2 0.5 Cs2Na0.5 Ag0.50.5BiCl 6

(200)

Cs2AgBiCl6 std

[BiCl [BiCl6]63-]

Cs NaBiCl (std)

(400) 2 6 (311) (422) (222) (511) (440) (620) (642) (331) (444) (531)

20

30

40

50

60

2 (degrees)

(d) 3.84

77%

d (Å)

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3.83

39%

22.5

23.0

23.5

22%

3.82

inc. Na %

24.0

0

25

50

75

100

Na (%)

2 (degrees)

Figure 1: (a) Crystal structure of Cs2Na0.5Ag0.5BiCl6 double perovskite NCs. (b) Powder X-ray diffraction patterns for Cs2NaxAg1-xBiCl6 NCs (x = 0, 0.25, 0.50, 0.75, 1). The reference pattern is also shown for x = 0 (Ref. 53) and x = 1 (Ref. 48) samples. (c) Enlarged view of the (220) reflection


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showing the peak shift as a function of Na alloying. The solid line is a guide to the eye. (d) Graphical representation of increase in d spacing with increase in Na percentages in alloyed material Cs2NaxAg1-xBiCl6. The d values correspond to the (220) reflections. TEM images with scale of 20 nm are shown in Figure 2a and 2b corresponding to Cs2Na0.75Ag0.25BiCl6 and Cs2AgBiCl6, respectively. These show the cubic shape of pure Cs2AgBiCl6 and alloyed Cs2Na0.75Ag0.25BiCl6 NCs with edge length of 12.9 ± 1.7 nm and 12.7 ± 1.7 nm, respectively. The size of the crystals does not change much upon alloying. This is also evident from the peak-widths of the x-ray diffraction patterns (see Figure 1a). To determine the crystallite size, the Scherrer equation55 was used. The estimated size lies between 8.8-10.8 nm for the various alloys. To estimate the particle sizes for the 75% and 0% Na-alloys, the number of cubic particles analyzed were 141 and 77, respectively. Histogram corresponding to TEM images 2a and 2b are shown in Figure 2c and 2d, respectively. Black circular dots observed in the TEM images decorating the cubes have been postulated to be Ag nanoparticles.56 Thus, reduction and diffusion of the silver ions are the main factors that have been identified for the structural instability of Ag based double perovskites. The degradation pathway and side products obtained after degradation for Cs2AgBiBr6 have been explained as per following equation:56

3Cs2AgBiBr6

Cs3Bi2Br9 + Cs3BiBr6 + 3Ag + 3/2Br2



(a)

(b)

(c)

(d) Lavg = 12.7nm

Lavg = 12.9nm

20

SD = 1.7nm

SD = 1.7nm

30

Counts

15

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

10

10

5

0

0 8

10

12

14

16

18

8

10

12

14

16

18

Edge length (nm)

Edge length(nm)

Figure 2: (a, c) Low resolution TEM image and histogram of Cs2Na0.75Ag0.25BiCl6 cubes having average size of 12.7 ± 1.7 nm (b, d) Low resolution TEM image and histogram of Cs2AgBiCl6 cubes having average size of 12.9 ± 1.7 nm.

Figure 3a shows the HRTEM image of Cs2Na0.75Ag0.25BiCl6 NCs in which lattice fringes are clearly visible. The PXRD pattern shown in Figure 1b confirmed that, marked lattice spacing in HRTEM image (see Figure 3a) corresponds to highly intense (220) reflection. From electron diffraction pattern of Cs2Na0.75Ag0.25BiCl6 as shown in Figure 3b, bright spots confirm the high 8 ACS Paragon Plus Environment

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crystallinity of the alloyed double perovskite material with 75% Na content. Planes are assigned to the corresponding bright spots. Figure 3c-f show TEM-EDS images of Cs2Na0.75Ag0.25BiCl6 NCs, which qualitatively confirms the presence of all the elements viz. Cs, Na, Ag and Bi in the double perovskite NCs. The presence of these elements in the series of alloyed double perovskites has also been confirmed using ICP-OES results. The obtained values for the five samples are 𝑥 = 0, 0.35, 0.66, 0.76, 1 where the nominal values were 0, 0.25, 0.5, 0.75, 1in Cs2NaxAg1xBiCl6.

These values show that Na content in the lattice of alloyed double perovskites increases

with increase in amount of Na, added during synthesis. There are slight deviations though at lower Na contents. This could be possibly due to the lower solubility of AgCl compared to NaCl.57 Therefore, while preparing the samples for ICP-OES analysis, it is quite possible that at high concentrations of AgCl, formed upon the digestion of the double perovskite material, some of the AgCl is left insoluble. This leads to an underestimation of Ag compared to Na, at lower Na content. However, this anomaly is not observed at higher Na content of 75%, as the amount of Ag is much lower and therefore dissolves completely and is subsequently detected in the ICP-OES results. It can be noted from the results in Figure 1d that the solid solution incorporates slightly lower amount of Na than what is added in the synthesis. Therefore, the ICP-OES data gives an even higher estimate of the Na-content than what we discussed above using the nominal alloying values.



(a)

(c)

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(b)

(d)

(e)

(f)

Figure 3: (a). High resolution TEM image of Cs2Na0.75Ag0.25BiCl6 NCs in which lattice fringes correspond to reflection due to 220 planes are marked. (b) Electron diffraction pattern of Cs2Na0.75Ag0.25BiCl6 NCs showing its crystalline nature. (c-f) Elemental analysis of Cs2Na0.75Ag0.25BiCl6 by TEM-EDS imaging, depicting the presence of Cs, Na, Ag, and Bi, in Cs2Na0.75Ag0.25BiCl6 NCs.

Band gap of Cs2AgBiCl6 NCs can be tuned in the ultra-violet region by alloying with an alkali metal as seen in Figure 4a that depicts the optical absorption spectra of alloyed double perovskites with different Na content. Figure 4a depicts the blue shift in absorbance with increase in Na content. Absorbance maxima shifts to 325 nm for Cs2NaBiCl6 from 366 nm for Cs2AgBiCl6; 10 ACS Paragon Plus Environment

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an increase in the bandgap by more than 0.4 eV. The bandgap, as estimated from the excitonic peaks seen in Figure 4a, plotted as a function of the Na% is shown in Figure 4b. These materials are luminescent, though the emission intensities are very low, we observe ~30-fold increase in the emission of Cs2Na0.75Ag0.25BiCl6 NCs as compared to Cs2AgBiCl6 NCs having λem in the orange region. Despite this increase, the PL is feeble and the quantum yield stays much below 1%. PL spectra normalized with absorbance values at the corresponding excitation wavelength (350 nm for Ag containing, 325 nm for pure Na structure) are presented in Figure 4c. Emission spectra of all the samples were collected in hexane using a 455 nm, high bandpass filter to cut off the scattering in the PL signal at 700 nm. This broad PL in case of double perovskites has been attributed to self-trapped excitons .58 In the case of Cs2Ag0.6Na0.4InCl6, it is known that Na itself contributes neither to valence band maxima (VBM) nor to the conduction band minima (CBm) but it changes the parity of wave function of self-trapped excitons at Ag atoms. This breaks the dark transitions and converts them into radiative transitions, which results in increase in PLQY of Naalloyed double perovskite compared to the pristine Cs2AgInCl6 perovskites.52 A similar reasoning could well be operative in the present system also suggested by the time-resolved photoluminescence (TRPL) data shown in Figure 4d. The average lifetimes of the PL decay increases from 40 – 120 ns upon alloying the Cs2AgBiCl6 NCs with Na. As one notices from the plot, the long-lived emission component increases upon increasing the Na content, indicating the enhancement in the generation of self-trapped excitonic states that manifests in the increase of the PL quantum yield as the percentage of Na increases. A changeover to Cs2NaBiCl6 removes all the trap states that were responsible for the emission and brings in a completely different system. In order to gain a deeper insight into the bandgap tuning, we have employed first-principles calculations under the framework of DFT for different concentration of Na in Cs2NaxAg1-xBiCl6 11 ACS Paragon Plus Environment


(x = 0.25, 0.50, 0.75). Note that, the pristine system Cs2AgBiCl659 and Cs2NaBiCl660,43 are already very well studied. Therefore, we have not considered them into our studies. For Cs2NaxAg1-xBiCl6 (x = 0.25), as shown in Figure (5a), the VBM that lies at the Γ point comprises of mainly Cl (3p) and Ag (4d), whereas near CBm, the primary contribution consists of Bi (6p) and Cl (3p) located at the L point. This will lead to an indirect bandgap of 1.8 eV (from Γ— L). For Cs2NaxAg1-xBiCl6

(a)

Cs2AgBiCl6 Cs2Na0.25Ag0.75BiCl6

3.8

(b)

Cs2Na0.50Ag0.50BiCl6 Cs2NaBiCl6

Eg (eV)

Absorbance

Cs2Na0.75Ag0.25BiCl6 3.7 3.6

3.5

3.4 300

350

400

450

0

25

Wavelength (nm)

50

75

100

Na (%)

(c)

(d) 103

Counts

Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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102

101

500

600

700

800

0

Wavelength (nm)

50

100

time (ns)


150

200

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Figure 4: (a) Absorption spectra of Cs2NaxAg1-xBiCl6 (x = 0, 0.25, 0.50, 0.75, 1) NCs exhibiting the blue shift in absorbance maxima with increase in percentage of Na, (b) shows increase in the bandgap with increase in percentage of Na in these NCs. (c) Steady-state PL spectra and (d) time-resolved PL traces along with three exponential fits for the double perovskite NCs. The dashed line marks the position of 2𝜆𝑒𝑥 in panel c. The color codes are same for all the four panels. (x = 0.50), we observe that, with an increase in the concentration of Na, the bandgap is raised successively by 0.1 eV. Initially for x = 0.50, the gap is calculated to be 1.9 eV (indirect bandgap). Similarly, for the case of Cs2NaxAg1-xBiCl6 (i.e.; x = 0.75), the VBM slightly shifts to lower energy, resulting in an increase in the bandgap to 2.06 eV (indirect bandgap). Therefore, we conclude that with an increase in the concentration of Na (x = 0.25, 0.50, 0.75), the bandgap also increases. In order to better describe and compare some of the features of the electronic states around the Fermi level of Cs2NaxAg1-xBiCl6, the partial density of states (PDOS) are calculated. The PDOS plots clearly indicate the contribution of Cl (3p) and Ag (4d) character at VBM. From Figure 5(d) inset, we observe a sharp slope near VBM at 25% Na, whereas at 50% Na (see Figure 5(e) inset) the slope gets reduced because of the low concentration of Ag as compared to former case. At higher concentration of Na (75%) the slope is almost negligible, indicating that the contribution of Ag is very small near VBM. Hence, with an increase in the concentration of Na, corresponding contribution of Ag decreases. Note that the latter is the main contributor in the VB edges. This will tend to flatten the peak near VBM and as a result the bandgap increases. Note that, the small contribution of Na is also observed deep inside the VB and CB. There is no significant contribution of Na in VBM and CBm. This is obvious because Na has orbitals deep inside the bands, which effect only the deeper levels and not the band edges. It must be emphasized that the difference 13 ACS Paragon Plus Environment


observed between experimental and theoretical calculations is due to the tendency to underestimate the bandgap using the DFT-PBE exchange and correlation functional. However, it should be noted that the main goal of this study is to gain qualitative insight and understand the trend as observed in experiment with an increase in the concentration of Na, rather than to be very exact. Also, we have cross checked our calculations with advanced hybrid functional HSE06 and observed the same trend as observed with PBE functional. However, bandgap obtained from HSE06 is shifted by 1.45 eV in comparison with PBE functional as shown in Figure 6. Therefore, with increase in concentration of Na, the trend observed either with experiment or PBE/HSE06 functional are same as shown in Figure 6. Further, we have also checked the effect of spin orbit coupling (SOC). We found that the bandgap is significantly reduced by 0.2-0.3 eV, due to the SOC induced splitting of the defect levels. Still, the trend remains intact as shown in Figure 6.


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(b)

(c)

4

4

4

Energy (eV)

(a)

2

2 1.80 eV

2

1.90 eV

0

0

0

-2

-2

-2

-4

-4

-4

(d)

L



X W L K

16



2

Γ

No. of states/eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8



X W L K



2

16

Total DOS Na Cl 1 Ag Cs Bi

12

L (e)

4

0 -2

0

Energy (eV)

2

4

2

1

10 0

6

0

8 -2

-1

0

-2

-1

0

6

4

4

2

2

0 -4



12

8 0

X W L K

14

1

10

-1



16

12

-2

L

18

14

0

(f)

2.06 eV

0 -4

-2

0

2

Energy (eV)

4

-4

-2

0

2

Energy (eV)

4

Figure 5. Calculated electronic band structures and PDOS of Cs2NaxAg1-xBiCl6 for different values of x (a) and (d) corresponds to x = 0.25, (b) and (e) x = 0.50, (c) and (f) x = 0.75 from the PBE calculations, and the corresponding bandgaps are also marked with arrows. In conclusion, we have investigated a series of Cs2NaxAg1-xBiCl6 (x = 0, 0.25, 0.50, 0.75, 1) alloyed double perovskite NCs. The PXRD, TEM and electron diffraction studies confirmed cubic crystal structure and crystalline nature of these double perovskites. TEM-EDS imaging and ICP-OES studies confirmed the presence of Cs, Na, Ag, Bi in these double perovskite NCs. Our key observation with these stable alloyed double perovskite structures is that band gap of Cs2AgBiCl6 can be tuned in UV-region by alloying with most abundant, nontoxic alkali metal Na leading to multiple fold increment in weak PL. Both our experimental and theoretical results



5

Experimental HSE06 PBE PBE+SOC

4

Eg (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3

2

1 25

50

75

Na (%)

Figure 6. Comparison of bandgap obtained from experimental data and HSE06, PBE, and PBE+SOC calculations. . confirmed tuning of bandgap with increase in Na%. Also, PDOS show flatten peak near VBM, due to the low concentration of Ag, which results in increase in bandgap. These crystalline alloyed double perovskites materials with higher band gaps than the pristine Cs2AgBiCl6 NCs may find useful futuristic applications in UV-photodetectors and lasers.

Experimental Methods Bi(OAc)3 (>99.99%, Sigma-Aldrich), Ag(OAc) (99%, Sigma-Aldrich), Cs(OAc) (99%, Spectrochem), Na(OAc) (98%, Fischer-Scientific), 1-Octadecene (90%, Sigma-Aldrich, ODE), Oleylamine (70%, Sigma-Aldrich, OLAm), Oleic acid (>90%, Sigma-Aldrich, OA), trimethylsilylchloride (98%, Spectrochem, TMSCl), and Toluene (99.5%, Qualigens), were used as received without further purification. Hexane was purchased from Merck chemicals and used after distillation.


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Cs2NaxAg1-xBiCl6 (x = 0, 0.25, 0.5, 0.75, and 1) NCs were synthesized using hot injection method by varying the molar ratios of Na/Ag in the precursor following the synthetic route reported by Creutz et. al. with slight modifications.33 For the synthesis of Cs2AgBiCl6 NCs, metal carboxylate precursors CsOAc (0.71 mmol), AgOAc (0.5 mmol), and Bi(OAc)3 (0.5 mmol) were dissolved in the combination of 1-Octadecene (10 mL), Oleic acid (2.5 g, 2.8 mL) and Oleylamine (0.5 g, 615 µL) and this precursor solution was heated at 110 °C for 45 minutes under vacuum. The precursor solution, which was initially colorless, subsequently turned brown in a continuous manner. The precursor solution was then heated to 140 °C under an argon atmosphere, and Trimethylsilyl Chloride (TMSCl, 2.7 mmol, 340 µL) was quickly injected. After 15 seconds, the reaction mixture was cooled to room temperature over an ice-water bath. After that, the reaction mixture was centrifuged at 6000 rpm for 30 minutes and the precipitate was collected. The precipitate was suspended in 15 mL toluene with sonication and centrifuged again at 6000 rpm for 30 minutes. Precipitate collected after the second centrifugation was resuspended in distilled hexane for subsequent characterizations. The syntheses of Cs2NaxAg1-xBiCl6 (x = 0, 0.25, 0.5, 0.75, and 1) NCs were carried out according to the above-mentioned procedure. Synthesis of Cs2NaxAg1-xBiCl6 (x = 0, 0.25, 0.5, 0.75, and 1) NCs is summarized in scheme shown below,

OA, OLAm, ODE, Ar atm CsOAc + (1 ― x)AgOAc + xNaOAc + Bi(OAc)3 110 °C, degassed 45 min Cs2NaxAg1 ― xBiCl6NCs 140 °C, injected TMSCl

Steady-state absorption measurement was carried out using Perkin Elmer (UV/Visible/NIR) Lambda 1050 double-beam spectrophotometer with slit widths of 2 nm. Steady State PL spectra



Page 18 of 29

were collected using Edinburgh instrument FLSP 920 with constant monochromator bandwidths of 8 and 10 nm for excitation and emission, respectively, at an excitation wavelength of 350 nm for Cs2NaxAg1-xBiCl6 (x = 0, 0.25, 0.5, 0.75,) and 325 nm for x = 1. Photoluminescence lifetime decay curves were collected by time-correlated single photon counting (TCSPC) using the same instrument. A pulsed laser diode (377 nm) with a pulse repetition rate of 2000 kHz was employed for the lifetime measurements. For all these optical measurements, dilute solution of samples in hexane were placed in 1 cm quartz cuvettes. The instrument response function was determined using a Ludox scattering solution in water. All the decay curves were fitted with a sum of three ―𝑡

exponentials using the equation, 𝐼(𝑡) = ∑𝑖𝛼𝑖𝑒

𝜏𝑖

, where 𝐼(𝑡) is the total intensity remaining at

time 𝑡. 𝛼𝑖 and 𝜏𝑖 are the amplitude and decay time of ith component, respectively. The average lifetime of the sample is measured using the equation 𝜏𝑎𝑣

∑𝑖

𝛼𝑖𝜏2𝑖

(

𝑛

=

)

𝛼𝑖𝜏𝑖 . Both Absorption and

PL spectra were collected by dissolving samples in hexane at room temperature. Powder X-ray diffraction (PXRD) was performed on Rigaku (ultima IV) X-ray diffractometer equipped with Cu Kα (λ = 1.5418 Å) X-ray tubes. All samples were dried and ground properly before collecting their XRD. The transmission electron microscopy (TEM) images were collected using JEOL JEM-1400 microscope with an accelerating voltage of 120 kV. High resolution TEM images were captured using a JEOL JEM-2200FS field emission transmission microscope operating at 200 kV. Samples were prepared on 200-mesh carbon coated Cu grids by dropping dilute solution of NCs suspended in hexane. Elemental composition study of samples was performed using Agilent technologies 5110 ICP-OES instrument. Samples were prepared by digesting solid sample in conc. nitric acid followed by dilution with HPLC grade water. The standards were prepared using salts of Cs, Na, Ag, and Bi metals such as CsCl, NaCl, Ag(OAc) and Bi(OAc)3 respectively. 18 ACS Paragon Plus Environment

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Density functional theory (DFT) calculations were performed using the projector augmented plane-wave (PAW) by employing Vienna ab initio simulation package (VASP).61,62 A model structure of double perovskite Cs2NaxAg1-xBiCl6 (x = 0.25, 0.50, 0.75) consisting of 40 atoms with periodic boundary condition was constructed by 2x2x2 replication of the cubic unit cell (space group). The supercell size ensures enough spatial separation between the periodic images of the doped impurities under periodic boundary conditions.63,64 In order to ensure that our findings are not just an artifact of DFT functionals, we have used two different exchange and correlation (xc) functionals viz., generalized gradient approximation (GGA) with PBE65 and cross checked our calculations with advanced hybrid functional i.e., HSE0666. All the structures were fully relaxed (atomic position) upto 0.001 eV/Å force minimization using conjugate gradient minimization with 4×4×4 K-mesh. For electronic structure energy calculations, the brillouin zone is sampled with a 8×8×8 Monkhorst-Pack67 K-mesh with 0.01 meV energy tolerance. In all our calculations, the plane wave energy cut-off is set to 600 eV.

Acknowledgment RSL acknowledges CSIR and PB is thankful to UGC for the research fellowship. SS acknowledges Nanoscale Research Facility (NRF) and Central Research Facility (CRF), IIT Delhi for instrument facilities and DST CERI grant no. DST/TMD/CERI/C166(G) for partial financial assistance. SB acknowledges the financial support from YSS-SERB research grant, SERB India (grant no. YSS/2015/001209). PB and SB are thankful to High Performance Computing (HPC) facility at IIT Delhi for computational resources. The authors are grateful to Vikash Kumar Ravi and Angshuman Nag at IISER Pune for help with the HRTEM images.



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Band Gap Engineering in Cs2(NaxAg1-x)BiCl6 Double Perovskite

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