Synthesis and Optical Properties of Colloidal M3Bi2I9 (M = Cs, Rb

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C: Physical Processes in Nanomaterials and Nanostructures

Synthesis and Optical Properties of Colloidal MBiI (M = Cs, Rb) Perovskite Nanocrystals 3

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Jaya Pal, Amit Bhunia, Sudip Chakraborty, Suman Manna, Shyamashis Das, Anweshi Diwan, Shouvik Datta, and Angshuman Nag J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03542 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 23, 2018

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The Journal of Physical Chemistry

Synthesis and Optical Properties of Colloidal M3Bi2I9 (M = Cs, Rb) Perovskite Nanocrystals Jaya Pal,†,*Amit Bhunia,⊥ Sudip Chakraborty,§ Suman Manna,† Shyamashis Das,∥ Anweshi Diwan,⊥ Shouvik Datta,⊥, ‡,* Angshuman Nag†,‡,* †

Department of Chemistry, ⊥Department of Physics, ‡Center for Energy Science, Indian Institute of Science Education and Research (IISER), Pune, 411008, India.

§

Materials Theory Division, Department of Physics and Astronomy, Uppsala University, Uppsala, SE- 75120, Sweden

∥Solid

State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.

*Corresponding authors’ e-mails: AN: [email protected] JP: [email protected] SD: [email protected]

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ABSTRACT Bulk Cs3Bi2I9 exhibits 0-D perovskite crystal structure at the molecular level, providing scopes for novel optical properties compared to 3-D perovskite structures. Here 0-D refers to the crystal structure irrespective of the size of the crystal. We have prepared colloidal Cs3Bi2I9 nanocrystals, and elucidated the unique optical properties arising from its 0-D crystal structure. Absorption spectrum at 10 K confirms that the electronic bandgap of Cs3Bi2I9 nanocrystals is at 2.86 eV, along with a sharp excitonic peak at 2.56 eV, resulting into a very high excitonic binding energy, EbX = 300 meV. Interestingly, we observe two peaks in the photoluminescence spectra at room temperature on both sides of the excitonic absorption energy. Since EbX (300 meV) >> effective phonon energy (36 meV), the phonon mediated relaxation of carriers from conduction band minimum to the excitonic state is suppressed to an extent. Consequently, two PL peaks related to both bulk band edge and the excitonic transitions is observed. Furthermore, Rb3Bi2I9 nanocrystals have also been synthesized, but it exhibit 2-D layered structure, unlike the 0-D structure of Cs3Bi2I9.


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1. INTRODUCTION Metal halide perovskite nanocrystals (NCs), particularly Pb-halide ones exhibit interesting optical and optoelectronic properties like high photoluminescence (PL) quantum yield (QY), suppression of PL blinking, high charge carrier mobility, and long carrier diffusion length.1-11 These properties culminate into applications including solar cell, light-emitting diodes, photodetectors and lasers.12-19 Generic formula of three-dimensional (3-D) perovskite is ABX3 (for example CsPbI3), where A is a monovalent cation, B is a bivalent metal cation, and X is a halide anion (Cl−, Br− or I−). ABX3 perovskite structure is electronically 3-D because of the propagation of corner-shared network of [BX6]4− octahedra in all three dimensions. There are also important lower dimensional perovskite structures such as two-dimensional (2-D), onedimensional (1-D), and zero-dimensional (0-D) structure, depending on the connectivity of [BX6]n− octahedral unit.20-25 Chemical formula of 2-D, 1-D and 0-D perovskites differs from ABX3 to maintain the charge neutrality of the compound. [BX6]n− octahedral units form the valence and conduction band. Therefore, different structural dimensionality of [BX6]n− strongly influences the intrinsic electronic, optical and optoelectronic properties of metal halide perovskites.26 Bulk Cs3Bi2I9 is known to exhibit a 0-D structure27 as shown in Figure 1a. Here [BiI6]3− octahedral units share faces to form [Bi2I9]3- dimers. These dimers are electronically isolated from each other, making it electronically 0-D structure with high excitonic binding energy (few hundred meV).27-28 So the electronic confinements in 0-D Cs3Bi2I9 perovskites are associated with its intrinsic 0-D dimer-like structures embedded within its bulk form. Therefore, it is intrinsically different from the usual size dependent quantum confinement observed in quantum dots. 0-D refers to the specific crystal structure of perovskite family, irrespective of the size of the crystal. Compared to 3-D (and also 2-D perovskites in certain directions), a macroscopic crystal or films of 0-D perovskite is expected to exhibit poor



carrier mobility within the crystal. But excitonic binding energies of 0-D perovskites are about an order magnitude larger than the 3-D metal halide perovskites. Consequently, 0-D perovskite have the potential to exhibit novel optical properties including PL, compared to 3D perovskites. A very weak and broad PL was reported from bulk Cs3Bi2I9.27 Only a few papers are reported on colloidal Cs3Bi2I9 NCs, and none of these reports show excitonic emission.29-30 Note that compounds like Cs3Bi2Br9 or Cs3Sb2I9 though appear similar to Cs3Bi2I9 in terms of composition, but both these compounds are 2-D layered structure.31-32 Therefore, excitonic binding energy and charge transport properties of 0-D Cs3Bi2I9 will be significantly different than these 2-D perovskites. Here we report the synthesis of Cs3Bi2I9 NCs, exhibiting unique dual features in both optical absorption and emission. Optical absorption data at 10 K shows that the excitonic binding energy is 300 meV for Cs3Bi2I9 NCs. Absorption due to excitonic transition occurs at 2.56 eV (484 nm), but the electronic bandgap is at 2.86 eV (433 nm). Two PL peaks corresponding to both direct electronic bandgap and excitonic transition are observed at room temperature. Then we replaced Cs+ ions with Rb+ ions, forming colloidal Rb3Bi2I9 NCs. But Rb3Bi2I9 NCs exhibit a 2-D layered perovskite structure, unlike 0-D Cs3Bi2I9. Optical and electronic properties of Rb3Bi2I9 NCs are compared with that of Cs3Bi2I9 NCs using both experiments and spin-polarized first principles electronic structure calculations based on density functional theory (DFT) framework. Temperature dependant structural stability of both the samples has also been studied.

2. Methodology Synthesis of Cs3Bi2I9 NCs. Colloidal Cs3Bi2I9 NCs were synthesized by using hot injection method. Cs-oleate was obtained by dissolving 0.35 g (0.1 M) Cs2CO3 in octadecene (20 mL) and oleic acid (1.25 mL) at 150 oC under N2 atmosphere, following a reported method.33 BiI3


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(100 mg) was taken in 10 mL octadecene solvent and then dissolved by adding 2 mL oleic acid and 0.8 mL oleylamine at 140 °C under N2 atmosphere. Then the solution was heated to 180 °C. Preheated Cs-oleate was then quickly injected into BiI3 precursor solution. After 10 seconds, the reaction was quenched using an ice bath and finally we got bright red coloured solution of Cs3Bi2I9 NCs. Isopropanol was used as non-solvent to precipitate Cs3Bi2I9 NCs. The solution was centrifuged at 7000 rpm for 7 min and then the precipitate was redispersed in toluene solvent for storage and further experiment. A high yield (60-70 %, excluding the contribution from organic ligands on the NC surface) for the synthesis of Cs3Bi2I9 NCs is obtained. The major loss happens in the process of washing of NCs after the synthesis. Synthesis of Colloidal Rb3Bi2I9 Nanocrystals. Colloidal Rb3Bi2I9 NCs were synthesized by adaptation of the synthetic protocol similar to that of Cs3Bi2I9 NCs. The major difference is the use of Rb-oleate instead of Cs-oleate. Rb-oleate was synthesized by using similar synthetic method of Cs-oleate. Characterization. Powder X-ray diffraction (XRD) measurements were performed using a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (1.54 Å). All reference data were analyzed using Crystal Diffract software and the crystallographic information file. Transmission electron microscopy (TEM) experiments were carried out using UHR FEGTEM, JEOL JEM-2100F electron microscope using a 200 kV electron source. Ultravioletvisible (UV-visible) absorption spectra were taken using Thermo scientific (Evolution 300) UV/Vis spectrometer. Steady-state photoluminescence (PL) and PL decay dynamics were carried out on FLS 980 (Edinburgh Instruments). Thermogravimetric analysis (TGA) was performed using Perkin Elmer STA 6000. The samples were heated in the range of 30-800 °C at the heating rate of 10 °C/min under inert atmosphere. Low Temperature Optical Absorption Spectroscopy. Low temperature absorption spectra were collected using a home built set-up. For low temperature measurement we used a CS-



204S-DMX-20 closed cycle cryostat from Advance Research Systems having ultra-pure Helium gas along with a temperature controller from Lakeshore (Model-340). An Acton Research’s SP2555 monochromator succeeding a 1000 Watts quartz-tungsten-halogen lamp was used for optical excitation. We used analog PMT housing (PMH-o2) from Sciencetech having a PMT tube (R2949) from Hamamatsu and Lock-in SR-850 from Stanford Research Systems for photo detection process. Computational Methodology. We have systematically performed first principles spinpolarized electronic structure calculations based on Density Functional Theory (DFT) framework. We have used Vienna Ab-initio Simulation Package (VASP)34-35 throughout all the theoretical calculations with generalized gradient approximation (GGA) type PerdewBurke-Ernzerhof (PBE) exchange correlation functional.36 Our electronic structure calculations are based on hexagonal (P63/mmc) crystal structure of Cs3Bi2I9 and monoclinic (P21/n) crystal structure of Rb3Bi2I9 with similar lattice parameters as obtained from experiments. For both the systems, we have performed a full ionic relaxation until the corresponding Hellman-Feynman forces are getting smaller than 0.001 eV/Å. Considering the symmetry and the unit cell dimension, we have used sufficient number of Monkhorst-Pack kpoints calculations with 5x5x2 grid for Cs3Bi2I9 and 5x5x5 for Rb3Bi2I9 systems. Once we have the minimum energy configurations for both the structures, we have subsequently determined the total and projected density of states (DOS) of the two systems. We have considered denser k-points grid for determining the corresponding total and projected DOS of Cs3Bi2I9 and Rb3Bi2I9.

3. RESULTS AND DISCUSSION XRD patterns in Figure 1b show that Cs3Bi2I9 NCs exhibit hexagonal crystallographic phase with space group P63/mmc (194) [lattice parameters: a = b = 8.4116 Å, c = 21.1820 Å]. The 6 ACS Paragon Plus Environment

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unit cell of these 0-D Cs3Bi2I9 NCs is represented in Figure 1a. XRD pattern of bulk (reference) Cs3Bi2I9 exhibit multiple closely spaced (similar 2θ values) peaks with different intensities. In the XRD pattern of NCs, these closely spaced peaks convolute into a single broad peak, sometime, even with asymmetric shapes. These complexities make the determination of crystallite size of Cs3Bi2I9 NCs more challenging using the broadening of XRD peaks; apart from the usual uncertainties arising from inhomogeneity in size and shape of nanocrystals. Transmission electron microscopy (TEM) images (Figure 1c and Figure S1) of Cs3Bi2I9 depict that the NCs exhibited quasi-spherical shape with an average diameter 5±1 nm. High resolution TEM (HRTEM) image (Figure 1d) of Cs3Bi2I9 NCs exhibits lattice fringes with interplanar distance 4.2 Å, which correspond to (110) planes of Cs3Bi2I9. We performed thermogravimetric analysis (TGA) and high temperature XRD (HTXRD) measurements to check the thermal stability of Cs3Bi2I9 NCs. Figure S2a in supporting information (SI) shows the TGA data of Cs3Bi2I9 NCs. First we observe a minor weight loss around 320 °C because of the decomposition of organic capping ligands, followed by a start of second weight loss around 520 °C due to beginning of the decomposition of Cs3Bi2I9 into CsI and BiI3 (boiling point 542 °C). HTXRD data (Figure 1e) corroborate the TGA analysis showing that Cs3Bi2I9 NCs are thermally stable up to 500 °C. At 600 °C, XRD peaks for Cs3Bi2I9 NCs disappear giving rise to new peaks corresponding to CsI. We have calculated thermal expansion coefficients (Figure S2b,c) for unit cell parameters. Coefficients for length along a-axis (αa) and c-axis (αc) and that of volume (αv) of unite cell are αa = 3.08 × 10−5 K−1, αc = 2.95 × 10−5 K−1, and αv = 0.925 ×10−4 K−1. Also note that, the Cs3Bi2I9 NCs are stable for more than 6 months in ambient conditions as confirmed by the XRD data (Figure S2d).



Figure 1: (a) Crystal structure, (b) XRD pattern, (c) TEM, and (d) HRTEM image of Cs3Bi2I9 NCs. (e) Powder XRD patterns of Cs3Bi2I9 NCs in the temperature range from 30 °C to 600 °C.

Figure 2a depicts the UV-visible absorption and PL spectra of colloidal Cs3Bi2I9 NCs dispersed in toluene, and measured at room temperature. Two separate emission peaks in the PL spectrum are centered at 2.66 eV and 2.28 eV with narrow full-width-at-half-maxima (FWHM). The PL peak (2.66 eV) at energy higher than the lowest-energy absorption feature at 2.28 eV is unusual. PL QY of Cs3Bi2I9 NCs is typically ~1% or less. PL decay at both peaks show similar decay dynamics with 5-6 ns radiative lifetime, along with a strong 8 ACS Paragon Plus Environment

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contribution from ~1 ns nonradiative lifetime (Figure 2b). Radiative lifetime of a few ns and narrow FWHM of both the PL features suggest that these emissions are not defect related ones.37 To verify whether the two PL peaks are originating from different species including different size-shape of NCs, we employed various kinds of washing and size selective precipitation methods. But in all the cases we observed two PL peaks. We also developed a synthesis methodology for Cs3Bi2I9 nanorods. Details of the synthesis procedure are discussed in the SI. Colloidal Cs3Bi2I9 nanorods (Figure S3a,b) exhibit absorption and PL peaks (Figure S3c) similar to those of Cs3Bi2I9 spherical NCs. Therefore, we can rule out the possibility of inhomogeniety in size and shape of NCs as a possible reason for two PL peak. Such independence in bandgap from the size and shape of Cs3Bi2I9 NCs is expected since the excitonic Bohr diameter (0.9 nm) of Cs3Bi2I9 (Table S1) is very small compared to our NC size (5 ± 1 nm). Therefore, absence of size dependent quantum confinement effects is expected from our Cs3Bi2I9 NCs. 1

Absorption PL

Cs3Bi2I9

a

1.5

PL Intensity (a.u.)

Absorbance/PL (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


2.66 eV 2.28 eV fit

Cs3Bi2I9

b

0.1 τ1: 1.6 ns (73%) τ2: 6.2 ns (27%)

0.01 τ1: 1.1 ns (88%) τ2: 5.4 ns (12%)

2.0

2.5

3.0

3.5

4.0

0

10

Energy (eV)

20

30

40

Time (ns)

Figure 2: (a) UV-visible absorption and PL spectra of Cs3Bi2I9 NCs. (b) PL decay profiles of colloidal Cs3Bi2I9 NCs at two different emission wavelengths 466 nm (2.66 eV) and 544 nm (2.28 eV). Pl decays were fitted with a bi-exponential decay, and the best-fit parameters (typically with 95% accuracy) are shown in the figure.

To understand the optical transitions in Cs3Bi2I9 NCs, we measured absorption spectra over the temperature regime of 296 K to 10 K as shown in Figure 3a. Figure 3b shows Tauc plot of



Cs3Bi2I9 NCs at 10 K. A sharp excitonic peak at 2.56 eV is well separated from electronic bandgap at 2.86 eV, corresponding to an excitonic binding energy, EbX = 300 meV for our Cs3Bi2I9 NCs. Similar results were also obtained by bulk Cs3Bi2I9.28 As mentioned before, this large excitonic binding energy is because of the 0-D dimer like structure of Cs3Bi2I9, and not a size-induced phenomenon. Figure 3c schematically presents the optical transitions of Cs3Bi2I9 NCs. Excitonic states (ES) are at 300 meV lower energy than the conduction band (CB) minimum, and therefore, two well-resolved absorption features for electronic bandgap (valence band (VB) maximum to CB minimum), and excitonic gap is observed. Correspondingly, we propose that the room-temperature PL (Figure 2a) peaks at 2.66 eV might originate from de-excitation across electronic bandgap, whereas the PL peak at 2.28 eV is attributed to the excitonic emission.

Figure 3: (a) Temperature dependent UV-visible absorption spectra of Cs3Bi2I9 NC film. (b) Tauc plot of Cs3Bi2I9 NCs at 10 K. (c) Schematic diagram showing the probable excitation and de-excitation processes. (d) Semi-logarithmic variation of absorbance (A) with the optical excitation energy for different temperature. The extrapolated straight lines for different temperature merge to a point, called Urbach focus. (e) Shows the variation of Urbach energy with the temperature. The black line is guide to eye only (f) Urbach steepness parameter σ(T) varies with the temperature. Solid red line is the fit of eq. 2. 10 ACS Paragon Plus Environment

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These dual PL peaks are similar to many of the Mn-doped nanocrystals, where both the bandedge (or excitonic) emission and dopant emissions are observed.38 Also, in many semiconductor NCs, both band-edge emission and red-shifted defect-related emission are observed.39 Both Mn-dopant states and defects do not absorb measurable light compared to bandgap absorption. Furthermore, in a typical inorganic semiconductor, excitonic binding energy is about few tens of meV, therefore, closely overlaps with bandgap absorption. So the absorption spectrum do not show any feature with energy lower than the overlapping bandgap-excitonic transitions. Consequently, both the bandgap (or excitonic) and dopant (or defect related) emission are red-shifted compared to the lowest-energy absorption peaks. But in the present case of Cs3Bi2I9 NCs, 0-D crystal structure separates the bandgap and excitonic emission, and the lower energy excitonic emission has a strong corresponding absorption (unlike Mn-dopants or defect states). Consequently, the bandgap emission is observed at energy higher than the lowest-energy absorption. However, we note that the discussed origin of two PL peaks in our Cs3Bi2I9 NCs is a proposed one, and further study is required to confirm the origin of this unique PL. In any case, prior reports showed broad and red-shifted PL from Cs3Bi2I9 compared to absorption spectrum and attributed it to defect and/or indirect bandgap related PL.29 To the best of our knowledge, the sharp excitonic (and also possibly electronic bandgap) emission from our NCs is reported here for the first time. Now let us analyse the temperature dependent absorption spectra in details. In Figure 3a, we see a blue shift in the excitonic peak energy as the temperature decreases from 296 K to 10 K. Intensity of the absorption peak increases upon cooling the sample. Important to mention here, due to having a quite high excitonic binding energy and strong excitonic features in the absorption spectra, we can term this excitonic transition energy as the ‘excitonic gap’ of the material. Such temperature dependent variation of excitonic gap is usually understood in



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terms of exciton-phonon interactions. We fit corresponding variation of excitonic peak energy with temperature using standard Varshni formula40 (Figure S4). This temperature dependent shift is reversible if the temperature is increased again. Moreover, broad low-energy tails of the absorption spectra indicate the presence of disorder, impurity states below the excitonic gap. In Fig. 3d, we see that optical absorbance (A) below the excitonic gap varies exponentially with the excitation photon energy like Urbach tails. Consequently, we fit temperature (T) and excitation energy ( ℎ ) dependent absorbance (A) with the following equation to estimate the corresponding Urbach energy41 in Figure 3d.

, ℎ = A

= A

(1)

where is the Urbach energy, a measure of energy spread of the exponential band tail. The inverse of the slope from semi-logarithm plot gives an estimation of Urbach energy as

=

"

. # is the steepness parameter of the absorption edge, $% is the Boltzmann

constant. A and are the material specific parameters related to the convergence points of the Urbach bundle. The extrapolated straight lines in Figure 3d meet at a particular point (shown by blue arrow), as predicted from equation 1, called Urbach focus. The obtained Urbach focus is at 2.56 eV for our Cs3Bi2I9 NCs. Ideally Urbach focus is a measure of defectfree zero-temperature optical band gap, which agrees very well with 2.56 eV excitonic gap (or optical band gap) of Cs3Bi2I9 NCs measured at 10 K (see Figure 3b). Interestingly, this band gap value also matches with that obtained from the Varshni formula (see SI section corresponding to Figure S4). In Figure 3e, we plot the temperature dependent variation of Urbach energy. It is noticed that the Urbach energy increases with the increasing temperature. This is because electronic disorder connected with enhanced activation of sub bandgap defect states also increases with increasing temperature. Moreover, the estimated values of Urbach energy are found to be quite high. These high values actually indicate the presence of substantial disorder states 12 ACS Paragon Plus Environment

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below the excitonic gap. At low-temperature, a weak feature is observed in the absorption spectra at 2.2 eV as shown in Figure 3a. These lower energy states are probably responsible for the low (~1%) quantum yield of Cs3Bi2I9 NCs at room temperature. Finally, in Figure 3f, we fit the # following the equation41 '

# = # & ħ) + ,-.ℎ *

ħ)*

'

(2)

where # ~ 0.14 is material specific quantity, ħΩ0 is the effective phonon energy and the prefactor # &

' ħ)*

+~ 0.0007. The corresponding effective phonon energy ħΩ0 ~ 36 meV. This

higher value may be originated from enhanced interaction with bound impurity, disordered states with the optical phonon. However, we are not sure of individual contributions of any specific phonon modes as this is extracted over a wide range of temperature. So, we treat this as the effective phonon energy for the material. It is to be noted that the effective phonon energy is about an order of magnitude smaller than the energy gap between CB minimum and the excitonic level (ES) (see Figure 3c), suggesting inefficient phonon mediated relaxation of charge carriers from CB to ES, and agrees with the observation of dual PL peaks. After successful synthesis of Cs3Bi2I9 NCs, we extended the methodology to synthesize colloidal Rb3Bi2I9 NCs by using Rb-oleate instead of Cs-oleate. To the best of our knowledge, there is no prior report of colloidal Rb3Bi2I9 NCs, though their bulk counterpart is known. XRD patterns in Figure 4a confirm the monoclinic crystallographic phase of Rb3Bi2I9 NCs with space group space group P21/n (lattice parameters: a = 14.6443 Å, b = 8.1787 Å, c = 20.885 Å, β = 90.42°). Here BiI6 octahedral shares three cis vertices to other three octahedral units to form a 2-D disordered layers structure (Figure 4b). This distorted layered structure of Rb3Bi2I9 NCs agrees with reported structure of bulk Rb3Bi2I9,27 and differ from the 0-D structure of Cs3Bi2I9 NCs. TEM image (Figure 4c) of Rb3Bi2I9 NCs shows quasispherical shape with average diameter of 4.5±1 nm. TGA analysis (Figure S5a) and HTXRD



(Figure S5b) data confirm that Rb3Bi2I9 NCs are thermally stable up to 450 °C (details are given in SI). UV-visible absorption spectrum of Rb3Bi2I9 NCs shows two prominent sharp peaks (Figure 4d) 2.28 eV (546 nm) and 2.65 eV (468 nm). Analogous absorption features have also been reported in bulk Rb3Bi2I927 and other 2-D perovskites like Cs3Bi2Br931 and hybrid Ruddlesden Popper42 perovskite. Unfortunately, Rb3Bi2I9 NCs do not show any measurable PL signature at room temperature. Reported bulk Rb3Bi2I9 also do not show PL.27 The absence of PL suggests that these NCs may contain deep defect state in the optical bandgap region. Further understanding of the excitation and de-excitation process of bulk and NCs of Rb3Bi2I9 is required.

Figure 4: (a) XRD patterns, (b) Crystal structure, (c) TEM, and (d) UV-visible absorption spectrum of colloidal Rb3Bi2I9 NCs. (e) Total and projected density of states (DOS) of Rb3Bi2I9. 14 ACS Paragon Plus Environment

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We have determined the electronic structure of Cs3Bi2I9 and Rb3Bi2I9, using the crystal structure and lattice parameters similar to the experimentally obtained ones. From the computational study it is observed that the results are in reasonable agreement with the previous reports based on similar methodology.43-44 To understand the elemental contribution to the total density of states (DOS), we have determined the projected DOS for Cs3Bi2I9 (Figure S6 in SI) and Rb3Bi2I9 (Figure 4e). Both samples show substantial contribution arising from Iodine (I) near the fermi vicinity, while Cs or Rb contributes in the lower energy range of the VB, and CB is contributed by the hybridization of Bi and I. Interestingly, as compared to Cs3Bi2I9, there is a substantial split in the conduction band of Rb3Bi2I9 as shown in Figure 4e. Such differences might arise from the structural difference between 0-D Cs3Bi2I9 and 2-D Rb3Bi2I9, but need to be studied further. We note here that the presence of excitonic gap cannot be captured with the existing electronic structure calculations. For that, one needs to consider Bethe Salpeter Equation (BSE), which is based on the quasiparticle picture while considering electron-hole pair as excitons. The inclusion of BSE picture in our spin-polarized DFT calculations for Cs3Bi2I9 and Rb3Bi2I9 systems would be computationally expensive and hence we keep it beyond our theoretical framework for this work.

4. CONCLUSION Colloidal Cs3Bi2I9 NCs with 0-D dimer structure are synthesized by using hot injection method. Structural, thermal and optical properties of Cs3Bi2I9 NCs have been studied. There are two features in both absorption and emission spectra of Cs3Bi2I9 NCs. UV-visible absorption spectrum of Cs3Bi2I9 NCs at 10 K shows an electronic bandgap of 2.86 eV, along with a large excitonic binding energy of 300 meV. The effective phonon energy ħΩ0 ~ 36 meV, is significantly smaller than the excitonic binding energy, probably suppressing the rate of carrier relaxation process (which otherwise is very fast) from CB minimum to ES, as 15 ACS Paragon Plus Environment


shown in Figure 3c. As results, CB minimum retains a fraction of excited electrons for sufficiently long time to undergo radiative recombination with hole in the VB, and another fraction of carriers emit light via ES states, giving rise to two PL peaks. Such PL mechanism is rather unusual, and needs to be verified further using different experimental methodologies such as transient absorption spectroscopy, single-NC PL, and temperature-dependent PL, which are beyond the scope of present report. In any case, optical properties of structurally 0D inorganic semiconductor NCs are less studied, and this unique PL property of our Cs3Bi2I9 NCs suggests more surprises in the optical properties of such 0-D structures. We also prepared colloidal Rb3Bi2I9 NCs exhibiting 2-D layered structure, but these NCs do not show any PL.

ASSOCIATED CONTENT Supporting Information (SI) Details of materials, synthetic methodology for Cs3Bi2I9 NRs, XRD, UV-Vis absorbance and PL spectra, TEM, TGA data, fitting with Varshni equation, and DOS are given in SI. Author Information The authors declare no competing financial interests. *Corresponding authors’ e-mails: AN: [email protected] JP: [email protected] SD: [email protected] Acknowledgment Authors acknowledge D. D. Sarma and Anirban Dutta from IISc Bangalore, and Nancy Singhal from IISER Pune for carrying out some measurements (which are not part of the manuscript) and useful discussion. Authors acknowledge DST-Nanomission (SR/NM/TP-


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Synthesis and Optical Properties of Colloidal M3Bi2I9 (M = Cs, Rb

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