Excitonic and Confinement Effects of 2D Layered (C10H21NH3

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Excitonic and Confinement Effects of 2D Layered (C10H21NH3)2PbBr4 Single Crystals Huatao Yin, Lei Jin, Yuqin Qian, Xia Li, Yuhao Wu, Michael Steven Bowen, Deirdre Kaan, Chao He, Derek Ian Wozniak, Bolei Xu, Asa James Lewis, Weiwei Shen, Ke Chen, Graham E. Dobereiner, Yan Zhao, Bradford B. Wayland, and Yi Rao ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00265 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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ACS Applied Energy Materials

Excitonic and Confinement Effects of 2D Layered (C10H21NH3)2PbBr4 Single Crystals Huatao Yin1, 5 ‡, Lei Jin1‡, Yuqin Qian2, Xia Li2, Yuhao Wu1, Michael S. Bowen1, Deirdre Kaan1, Chao He1, Derek I. Wozniak1, Bolei Xu1, Asa J. Lewis1, Weiwei Shen3, Ke Chen4, Graham E. Dobereiner1, Yan Zhao5, Bradford B. Wayland1, and Yi Rao2* 1

Department of Chemistry, Temple University, Philadelphia, PA 19122, USA Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA 3 Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA 4 Department of Physics, Temple University, Philadelphia, PA 19122, USA 5 College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650500, China. 2

Abstract Recognition of unusual optoelectronic properties for two-dimensional (2D) layered organicinorganic lead (II) halide materials (CnH2n+1NH3)2PbX4 (X=I, Br, and Cl) has attracted intense renewed interest in this class of materials. Single crystals of the 2D layered materials (C10H21NH3)2PbBr4 and pseudo alloy (C10H21NH3)2PbI2Br2 were grown for photo-physical evaluation. A 10-carbon alkyl ammonium cation was selected for investigation to provide strong dielectric screening in order to highlight quantum confinement effects of the anionic (PbX42-) semiconductor layer. Single crystals of the 2D layered ((C10H21NH3)2PbBr4 compound display a characteristic free exciton with a binding energy of 280 meV. Observation of a short photoluminescence lifetime of 2.8 ± 0.2 ns suggests that this electronic transition for the PbBr4based layered material has only singlet character. Sheets of (C10H21NH3)2PbBr4 with thicknesses of a few layers were fabricated and the dimensions verified by AFM experiments. Excitonic emissions from (C10H21NH3)2PbBr4 and (C10H21NH3)2PbI4 exhibit relatively small spectral shifts from the bulk down to a thickness of five layers indicative of the strong confinement effect of the 10-carbon alkyl ammonium spacers. Single crystals of the pseudo alloy (C10H21NH3)2PbBr2I2 give an excitonic absorption peak close to that of the tetra bromide (C10H21NH3)2PbBr4 and an emission peak with a large stokes shift to a position similar to that of the tetra iodide (C10H21NH3)2PbI4.

Keywords: 2D layered materials; Excitons; Exciton binding energy; Quantum confinement; Single crystals; Pseudo-alloy crystals

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Introduction Two-dimensional (2D) layered metal (II) halides with the general formula (CnH2n+1NH3)2MX4 (M=Ge2+, Sn2+, Pb2+; X=I-, Br-, Cl-; n > 2) consist of alternating layers of organic cations (CnH2n+1NH3+) and inorganic (MX42-) semiconductor sheets. The semiconductor layer consists of an infinite array of six coordinate MX6 sites where the metal (II) centers are bound by four twocentered bridging halides and two terminal halides which produce the net MX42- stoichiometry for the sheet.1-25 These 2D layered materials are observed to manifest an unusual scope of linear and nonlinear optical properties such as high quantum yield luminescence and large third-order susceptibility.26-27 The semiconductor properties for these materials suggest potential applications as photodetectors and light-emitting diodes that are usually associated with traditional inorganic semiconductors.26-38 The unusual electronic properties for these semiconductors result in part from layers of organic cations that function as insulators and support charge confinement in the inorganic metal halide semiconductor layers. Prior reported research on this class of 2D layered semiconductors was focused on the lead (II) iodide materials (CnH2n+1NH3)2PbI4),28, 35 but increased interest in the lead bromide derivatives has resulted from recent reports that (CnH2n+1NH3)2PbBr4 semiconductors manifest properties different from the iodide derivatives such as structural relaxation, layer-dependent photoluminescent spectral shifts and color-tunability.11 Thin films of (CnH2n+1NH3)2PbBr4 (n=4, 5, 7, and 12) were found to exhibit variations of the excitonic structures dependent on the length of alkyl chain,18 which is an important tuning capability not observed for the 2D lead iodide materials . This article reports on a systematic case study of the optical and excitonic properties for single

crystals

of

the

previously

unreported

ten-carbon

lead

bromide

derivative

(C10H21NH3)2PbBr4. The relatively large alkyl hydrocarbon chain should minimize interactions between the inorganic PbBr4 semiconductor layers and thus optimize the confinement and stabilization

of

free

excitons

in

the

two-dimensional

layers.

Single

crystals

of

(C10H21NH3)2PbBr4 were prepared by using a vapor diffusion method and characterized by XRay Diffraction (XRD). Different thicknesses of the 2D layered sheets were produced in order to evaluate the dependence of optical properties on the number of layers. Tauc plot analyses were

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used to determine the exciton binding energy of (C10H21NH3)2PbBr4. Single crystals of the pseudo alloy (C10H21NH3)2PbBr2I2 give an excitonic absorption peak close to that of (C10H21NH3)2PbBr4 and an emission peak with a large stokes shift to a position similar to that of (C10H21NH3)2PbI4.

Results and Discussion The powder X-ray diffraction pattern for micro-crystalline (C10H21NH3)2PbBr4 is shown in Figure 1(A). Intense Bragg reflections and well-separated diffraction peaks appear in the pattern, indicating that the (C10H21NH3)2PbBr4 crystals are well layered along the c-axis which is defined to be perpendicular the inorganic layer. These diffraction peaks correspond to the (0 0 2l) reflections of (C10H21NH3)2PbBr4 single crystals, in which the neighbor peaks occur with dspacing of 11.91, 7.97, 5.98, 4.78, 3.99, 3.42, 2.99, 2.66, 2.40 Å, respectively. The diffraction peaks were indexed as (0 0 2l) (l=2-10) as labeled in Figure 1 (A). The lattice constant for the caxis of the (C10H21NH3)2PbBr4 single crystals was evaluated to be 47.8 Å, which is in accordance with those for 2D PbBr4–based materials of other alkyl chain length and inorganic layer height.18 Differential Scanning Calorimetry (DSC) was used to characterize thermally induced structural phases. Traces for both the cooling and heating cycles are shown in Figure 1 (B). Phase changes for crystals of (C10H21NH3)2PbBr4 were found to occur at 259 K and 318 K upon heating and at 267 K and 328 K during cooling. The temperatures for the two reversible phase transitions in (C10H21NH3)2PbBr4 occur at 15-20 K lower in temperature than the corresponding values for (C12H25NH3)2PbBr4.6 The phase transition that occurs at lower-temperature is thought to be associated with the rotational freedom of the organic alkylammonium chain in crystals of (C10H21NH3)2PbBr4. The optical absorption for bulk micro-crystalline (C10H21NH3)2PbBr4 and photoluminescence spectra for single crystals are given in Figure 2 (A). The absorption spectra were taken for microcrystalline samples and the emission spectra were carried out for single crystals. A sharp absorption peak with a Lorentzian line shape located at 393 nm is observed for powders of (C10H21NH3)2PbBr4, while the photoluminescence peak exhibits a very small Stokes shift to 395 nm for the single crystals. Observation of relatively small bandwidths of 105 meV (Full width at half maximum (FWHM)) for both the absorption (393 nm) and photoluminescence (395 nm)

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transitions is characteristic of a free exciton11 and the broad peak centered at 450 nm ( Figure 2(A)) is attributed to the formation of a self-trapped exciton.11, 39 Evaluation of exciton binding energies is usually accomplished by either measuring the photoluminescence intensity as a function of temperature or through analysis of Tauc plots of UV absorption spectra. The exciton binding energy (Eb) is related to the photoluminescence intensity by the relationship I ∝ exp(Eb/kBT),10, 18, 40-41 where I is the integrated fluorescence intensity, kB is the Boltzman constant, and T is the temperature. The DSC experiments in Figure 1(B) show a large enthalpy change near 263 K for a phase transition of (C10H21NH3)2PbBr4, which complicates the extraction of the activation energy Eb from temperature-dependent photoluminescence measurements. Therefore, it is necessary to use the Tauc plot analysis to extract the direct band gap from optical absorption spectra.42 In the Tauc plot analysis, the product of energy-dependent absorption coefficient α(E) and energy E is expressed as, 42 α(E)E = A*(E-Egdirect)1/2+B*(E - Egindirect ± Ephonon)2

Eq. 1

where two parameters A and B, the direct and indirect energy gaps Eg, and the phonon energy Ephonon. The Tauc plot, ((α(E)E)2 versus E in Figure 2 (B), yields the band gap Eg of 3.42 eV from the intersection point of the straight-line segments with the horizontal E axis. The energy difference between band gap (Eg) and exciton absorption transition (Ea) gives the exciton binding energy (Ex= Eg- Ea), which was found to be approximately 280 meV for (C10H21NH3)2PbBr4. This value is close to those reported for other (CnH2n+1NH3)2PbBr4 derivatives (~300 meV).18 Spectra-resolved

and

time-resolved

photoluminescence

(PL)

experiments

for

(C10H21NH3)2PbBr4 were performed in an effort to obtain more experimental evidence for the intrinsic properties of both the free and self-trapped excitons. Normalized pseudo-color 2D PL spectra as a function of time for bulk (C10H21NH3)2PbBr4 crystals are shown in figure 3(A). Time-resolved photoluminescence (PL) for (C10H21NH3)2PbBr4 was also carried out to obtain electronic properties of the exciton emission as shown in Figure 3 (B). The short exciton lifetime (2.8 ± 0.2 ns) for (C10H21NH3)2PbBr4 was found, which is consistent with the corresponding value of 3.3 ns reported for the shorter chain (C4H9NH3)2PbBr4.11 Although previous studies have indicated that polycrystalline (CnH2n+1NH3)2PbBr4 materials exhibit both triplet and singlet

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excitons,18 our experimental results indicate that only singlet free excitons are observed for single crystals of (C10H21NH3)2PbBr4. Analysis of the normalized pseudo-color 2D PL spectra in Figure 3 indicates that the lifetimes for the free and self-trapped excitons are comparable. The unnormalized pseudo-color 2D PL spectra are seen in Supporting Information. The rate of geminate recombination for the free and self-trapped excitons are effectively equal even though their energies differ by ~0.57 eV. The difference in line widths between the free and self-trapped excitons thus does not result from a lifetime difference, but probably arises from a wide energy disorder distribution of the trapping sites and environments. Confinement effects for (C10H21NH3)2PbBr4, were further investigated by observing the dependence of the measured optical properties on thickness of the sample along the axis normal to the 2D sheets. Samples with different numbers of layers deposited on Si and SiO2 substrates were prepared by using a conventional mechanical exfoliation method.43 Figure 4 (A) shows the AFM images of areas where several different thin layers are located. The corresponding height profiles obtained from AFM scans in the inset (red and green), indicate that the thickness of (C10H21NH3)2PbBr4 is on the order of tens of nanometers. Each layer has a thickness of 23.8 Å, which permits identifying samples that have 5 (red) and 20 (green) layers as shown in the inset of Figure 4 A. An AFM image for an 11-layer sample is included in Supporting Information. The layer-dependent PL for 2D (C10H21NH3)2PbBr4 shows a blue shift of 5 nm from the bulk to a 20layer (50 nm) sample (Figure 4 (B)) which is smaller than the 10 nm shift previously observed for the shorter 4-carbon derivative (C4H9NH3)2PbBr4.11 Only a very small spectral shift (~1 nm) is seen as the crystal thickness is further reduced from eleven to three layers. These results fulfill the expectation that the 10-carbon alkylammonium derivative (C10H21NH3)2PbBr4 produces larger confinement effects than the 4-carbon derivative (C4H9NH3)2PbBr4. The optical properties of a mixed bromide and iodide pseudo alloy material with an approximate stoichiometry of (C10H21NH3)2PbI2Br2, are shown in Figure 5. Absorption and emission spectra of (C10H21NH3)2PbI2Br2 are shown in comparison with PL spectra for samples of pure (C10H21NH3)2PbBr4, and (C10H21NH3)2PbI4. The absorption peak of the pseudo alloy is found to occur at an energy close to that of the pure PbBr4 derivative, while the PL peak shows a large Stokes shift that approaches the energy of the emission peak for the pure PbI4 crystal in Figure 5. The spectral broadening in both the absorption and luminescence spectra for the pseudo-alloy (C10H21NH3)2PbI2Br2 crystal suggests that the excitons formed in the mixed anion

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crystals are more disordered than those in the pure anion crystals. These energy inhomogeneities and site disorder might arise from statistical fluctuations of I and Br, lattice strains, and structural relaxation

in

the

pseudo-alloy

crystal.23(

Nano

Lett.

2016,

16,

3335−3340,

DOI:

10.1021/acs.nanolett.6b00964)

The observed spectra suggest that segregated layers of PbBr4 and PbI4 are not present, but that authentic mixed halide layers occur. The difference in absorbance and emission results from within individual mixed halide semiconductor layers. This spectroscopic behavior is very likely a manifestation of intra-layer exciton migration to lower energy emissive sites because inter-layer exciton migration is effectively precluded by the organic insulating layers. There are equal numbers of halides occupying two terminals and four two-centered bridging Pb-X sites in (C10H21NH3)2PbX4 compounds. In mixed Br-/I- halide materials, the equilibrium distribution of halides depends on the equilibrium constants for halide binding in the bridging and terminal sites. In the 2D lead halide materials mixed halide occupation of the terminal sites is achieved, but the bridging sites that give structural convergence may have only one type of halide or minimal substitution. The conduction band is primarily composed of Pb 6p which is raised in energy (anti-bonding) by interaction with the 4p orbitals of Br- and 5p orbitals of I-. There are 4 in-plane bridging Pb-X interactions and two terminal out-of-plane Pb-X interactions. The ordering of the anti-bonding sub-bands is uncertain but probably has the following order: terminal Pb-I < bridging Pb-I ~< terminal Pb-Br ~ bridging Pb-Br. Each sub-band would vary in energy with the change in the number of Br and I in bridging and terminal sites around the Pb2+ center. The lowest energy portion of the conduction band is associated with iodide-rich sites and the highest energy associated with bromide-rich sites. Photo-excitation from the valence band terminates at a wide range of energies in the conduction band and produces excitons with a variety of energies. The excitons can then migrate to more iodide-rich sites at lower energy before geminate recombination and photon emission. This is effectively the same mechanism as that proposed by the literature to explain the large difference in the photon absorption and emission energies for closely related 2D layered materials.6 In order to probe this mechanism further, 2D single crystals C10H21NH3)2PbCl4 and the pseudo alloys C10H21NH3)2PbCl2Br2 as well as C10H21NH3)2PbCl2I2 were prepared and reported in the Supporting Information. The inhomogeneity and stokes shift occurs primarily for lower energy iodide-rich sites. On the other hand, the higher energy sites of Pb-Br and Pb-Cl tend to exhibit structural ordering in the plane.

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The photoluminescence lifetime of the (C10H21NH3)2PbI2Br2 pseudo-alloy is compared with those for (C10H21NH3)2PbBr4 and (C10H21NH3)2PbI4 in Figure 5 (B). The lifetime of the excited state of (C10H21NH3)2PbI2Br2 was found to be similar to that of (C10H21NH3)2PbI4 (~1 ns), which has a relaxation rate that is three times larger than that of (C10H21NH3)2PbBr4. In general, the exciton recombination rate (Bx) is a function of intrinsic parameters of exciton transition intensity, emission photon energy, and the exciton Bohr radius which is given by, 44-45 Bx ∝ (Eg − Ex ) Pcv

2

1

Eq. 2

πa x3

where Ex is the exciton binding energy, Pcv is the band-to-band transition matrix element, ax is the exciton Bohr radius. The exciton binding energy of 301 meV for (C10H21NH3)2PbI4 reported previously is comparable to that of 280 meV for (C10H21NH3)2PbBr4.6, 46 The Bohr radius of the free excitons in (C10H21NH3)2PbBr4 was estimated to be 1.30 nm, which is close to the value of 1.27 nm reported for C10H21NH3)2PbI4.46 This suggests that the excitonic wave function covers a few unit cells in the layer plane where the nearest-neighbor distance of Pb ions is 0.6 nm.11 According to the relationship described in Eq. 2, a smaller interband transition matrix element ( Pcv ) is expected to account for the observed smaller exciton recombination rate (Bx) for (C10H21NH3)2PbBr4 compared to the value for (C10H21NH3)2PbI4. This could be qualitatively explained by the fact that the orbital overlap between I (5p) and Pb (6p) in the electronic transition is larger than the Br (4p) with Pb (6p) overlap. For the mixed I/Br alloy crystals, the excitons formed in the higher energy Pb-Br sites relax to the lower Pb-I sites, which is likely responsible for the larger recombination rate of the mixed Br/I (C10H21NH3)2PbI2Br2 crystals. Conclusions Single crystals of the 2D layered 10-carbon lead bromide semiconductor (C10H21NH3)2PbBr4 were prepared and the optical and excitonic properties were systematically examined. The 2D bulk materials feature singlet free excitons with a large binding energy of 280 meV. Sheets of (C10H21NH3)2PbBr4 with thicknesses of few layers were fabricated and excitonic emissions from (C10H21NH3)2PbBr4 exhibit relatively small spectral shifts (~6 nm) from the bulk down to a thickness of five layers indicative of the effective confinement of the PbBr4- semiconductor layers by 10-carbon alkyl ammonium spacers. The mixed halide (C10H21NH3)2PbBr2I2 material shows unusual optical properties where the excitonic absorption peak is similar in energy to that

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of (C10H21NH3)2PbBr4 and the emission peak shows a large Stokes shift to a position approaching that of (C10H21NH3)2PbI4. The unusual excitonic properties of the mixed halide semiconductors merit further investigation as material for applications in tunable LED devices.

Supporting Information Supporting Information Available: Preparation of (C10H21NH3)2PbBr4 and its derivations single crystals; Experimental methods for characterization; 2D photoluminescence spectra; Optical images of 2D (C10H21NH3)2PbBr4; UV and PL spectra of pseudo alloy crystals (C10H21NH3)2PbCl2Br2 and (C10H21NH3)2PbCl2I2. Corresponding Author *Author to whom correspondence should be addressed. Electronic address: [email protected]. Author Contributions The manuscript was written through contributions of all authors. H.T.Y, X.L., D.K. synthesized the samples. H.T.Y, D.W., X.L. took XRD data. H.T.Y, K.C. did the AFM measurement. H.T.Y, L.J., C.H., B.X., Y.W., Y.H.W., A. L., M.B. did UV and fluorescence measurements. Y. R, B. W., and G. D. designed the experiments. Y.R. and H.T.Y. wrote the paper. Y.R. and B.W. guided the project and edited the paper. All authors discussed the results and commented on the manuscript. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources This work was supported by a Targeted Research Grant from the Temple University Office of the Vice Provost for Research before June 30th, 2017, and by Utah State University after June 30th, 2017. Acknowledgments The authors thank Drs. Stephanie L. Wunder and Parameswara Rao Chinnam for the DSC measurements.

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004

Cooling 006 014 016 010

012

008

018

Heating

020

2θ (B)

(A)

Figure 1. (A) Powder X-ray diffraction pattern of (C10H21NH3)2PbBr4 single crystals. The wellseparated diffraction peaks corresponding to the (0 0 2l) for (l=2-10) reflections. The inset is a schematic of 2D layered (C10H21NH3)2PbBr4. (B) Differential Scanning Calorimetry (DSC) trace for (C10H21NH3)2PbBr4 single crystals with two reversible phase transitions.

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(α(E) E)2


(A)

(B)

Figure 2. (A) Absorption and photoluminescence spectra of (C10H21NH3)2PbBr4. (B) Tauc plot of the absorption spectrum for (C10H21NH3)2PbBr4 with a bandgap of 3.42 eV from extrapolation to the energy axis.

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(A)

(B)

Figure 3. (A) Normalized pseudo-color 2D photoluminescence spectra as a function of time for bulk (C10H21NH3)2 PbBr4 single crystals. The 2D spectra show that the lifetimes for the free and self-trapped excitons are almost the same. Un-normalized pseudo-color 2D spectra are included in Supporting Information. (B) Time dependent PL traces for the peaks centered at 395 nm and 450 nm, yielding a similar lifetime of 2.8 ± 0.2 ns.

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(A)

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(B)

Figure 4. (A) AFM image of areas of the substrate where two thin sections are located. The corresponding height profiles are shown in the inset (red and green). The thickness of (C10H21NH3)2PbBr4 was found to be on the order of tens of nanometers. Given that each layer has a thickness of 2.38 Å, the red and green lines correspond to samples of 5 layers and 20 layers, respectively. (B) PL spectra for samples of (C10H21NH3)2PbBr4 with thicknesses of 5 (red), 11 (blue), 20 (green) layers, and bulk are shown. AFM image of a sample of 11-layers is seen in Supporting Information.

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(A)

(B)

Figure 5. (A) UV and PL spectra of pseudo alloy crystals (C10H21NH3)2PbBr2I2 as compared with 2D single crystals (C10H21NH3)2PbI4 and (C10H21NH3)2PbBr4. (B) Time-dependent PL for the 2D single crystals and pseudo alloy crystals.

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Excitonic and Confinement Effects of 2D Layered (C10H21NH3

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