Study of indium and antimony incorporation into SnS2 single crystals

Accepted Manuscript Study of indium and antimony incorporation into SnS2 single crystals Ankurkumar J. Khimani, Sunil H. Chaki, M.P. Deshpande, Jiten P. Tailor PII: DOI: Reference:

S0022-0248(18)30587-6 https://doi.org/10.1016/j.jcrysgro.2018.11.016 CRYS 24850

To appear in:

Journal of Crystal Growth

Received Date: Revised Date: Accepted Date:

18 September 2018 13 November 2018 15 November 2018

Please cite this article as: A.J. Khimani, S.H. Chaki, M.P. Deshpande, J.P. Tailor, Study of indium and antimony incorporation into SnS2 single crystals, Journal of Crystal Growth (2018), doi: https://doi.org/10.1016/j.jcrysgro. 2018.11.016

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Study of indium and antimony incorporation into SnS2 single crystals Ankurkumar J. Khimani1,2*, Sunil H. Chaki1, M. P. Deshpande1, Jiten P. Tailor3 1 P. G. Department of Physics, Sardar Patel University, Vallabh Vidhyanagar - 388120, Gujarat, India. 2 Shri A. N. Patel PG Institute of Science and Research, Anand - 388001, Gujarat, India. 3 Applied Physics Department, S.V.N.I.T, Surat-395007, Gujarat, India. *Corresponding author: [email protected] ABSTRACT Pure SnS2, 5% In-doped SnS2, 15% In-doped SnS2, 5% Sb-doped SnS2 and 15% Sb-doped SnS2 single crystals are grown in closed sealed quartz ampoule by direct vapour transport technique. The energy dispersive analysis of X-rays analysis of all the five as-grown single crystals showed them to be stoichiometric. The X-ray diffraction analysis showed that all the crystals are single phase possessing a hexagonal structure with (001) preferential orientation. The surface morphology of as-grown single crystals studied by scanning electron microscopy and optical microscopy showed crystal growth is by layer growth mechanism supported by screw dislocation. Selected area electron diffraction showed hexagonal spot pattern confirming the single crystalline nature of the crystals. Optical bandgap of the as-grown crystals determined by UV-Vis-NIR spectroscopy showed that the single crystals possess direct optical bandgap and the value varied between 1.89 and 2.31 eV. The photoluminescence spectra study showed the presence of six peaks. The Raman spectra showed SnS2 type the A1g vibrational mode and shifting in A1g vibrational mode with In and Sb doping. The results are elaborated in details. Keywords: A1. Characterization, A1. Crystal morphology, A1. Crystal structure, A1. Doping, A1. X-ray diffraction, A2. Single crystal growth.

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1. Introduction The materials in two-dimensional (2D) forms has put forward a new state both in exploring fundamental physics and breaking through technological restricted access such as the scaling boundary of bulk [1,2]. The first showing of 2D material was graphene, it has unique mechanical, thermal and electrical properties, but the deficiency of a sizable energy gap limits its application in devices [3]. The fascinating properties of a variety of 2D materials like transition metal dichalcogenides (TMDCs) [4], hexagonal boron nitride (h-BN) [5], black phosphorus (BP) [6], etc. made them to be broadly studied. The 2D TMDCs show great promise due to their monolayers attached by weak van der Waals (vdW) interactions. The increased mobility in 2D nanostructures of TMDCs bestows them with great electronic performances. The dormant short-channel effects and the indirect-direct bandgap transition between the bulk and monolayer in some TMDCs give diverse optoelectronic applications like photo-detectors and light emitters [7–19]. The 2D layered TMDCs have the general formula of MX2 formed by a metal atom (M = Mo, W, Ga, Sn, Zr, Nb, etc.) sandwiched between two chalcogen layers (X = S, Se, Te), Figure 1(a) [20,21]. SnS2 is a member of the 2D TMDC semiconductors with a hexagonal CdI2 type crystal structure possessing direct band gap between 2.34 to 2.88 eV [22,23]. Recently SnS2 has demonstrated good performance in many applications such as cathode active material for aluminium-ion batteries [24], high performance sodium ion batteries [25– 27], photocatalyst [28–30], high-performance supercapacitor [31], photodetector [32,33], saturable absorber in passively mode-locked ytterbium-doped fiber laser [34], visible light driven photoelectrochemical immunosensor for detection of prostate-specific antigen [35], solar cell [36], gas sensor [37,38], field effect transistor [39], humidity and alcohol sensor [40], etc. The doped SnS2 with different properties finds potential applications in various devices. According to authors opinion, the favorable dopants are indium(III), In3+, and antimony(V), Sb3+, because of the closeness of its ionic radii to cation Sn4+ of SnS2 [41]. The atomic radius mismatched between tin (140.5 pm) and indium (155 pm) is estimated to be ~10.32 %; tin (140.5 pm) and antimony (140 pm) is estimated to be ~0.35%. Thus the In and Sb atom can do substitution doping at Sn-atomic site because of their comparable atomic size. The Figure 1(a, b) shows the incorporation of In and Sb in pristine SnS 2. Literature states there exists no comparative study on single crystals grown by direct vapour transport 2

(DVT) technique between pure SnS2, In-doped SnS2 and Sb-doped SnS2 single crystals. This work report the study of the effect of In and Sb doping on crystal structure, surface topography, growth mechanism and optical properties of base material SnS 2.

(a)

(b)

Figure 1 The (a) schematics of 2H-SnS2 unit cell with hexagonal symmetry [21] and (b) schematics of layered structure of pure and In and Sb doped SnS2. 2. Experimental details 2.1 Single crystals growth By direct vapour transport (DVT) technique the single crystals of pure SnS2, 5% Indoped SnS2, 15% In-doped SnS2, 5% Sb-doped SnS2 and 15% Sb-doped SnS2 were grown. For simplicity the samples are labelled; pure SnS2 as P1, 5% In-doped SnS2 as P2, 15% Indoped SnS2 as P3, 5% Sb-doped SnS2 as P4 and 15% Sb-doped SnS2 as P5. The process and conditions for growth of pure SnS2 (P1) is been discussed and rest others have similar growth procedure. Firstly compound preparation of pure SnS2 is carried out. A quartz ampoule is loaded with stoichiometric proportion of pure precursor elements Sn (minimum assay 99.99%, Alpha Aesar, U.S.A) and S (minimum assay 99.99%, Alpha Aesar, U.S.A). The overall weight of the mixture of Sn and S filled into the quartz ampoule is taken to be nearly 8 g. The quartz ampoule is sealed at a vacuum of 10-5 torr. The sealed quartz ampoule is thoroughly shaken to properly mix the constituent elements. The mixture is spread throughout the entire ampoule and is placed in a tubular horizontal furnace. The temperature of the tubular horizontal furnace is gently increased and elevated up to the compound preparation temperature. The quartz ampoule with loaded power mixture is kept at the fixed compound formation temperature and held at the fixed temperature for a finite time. Later, the temperature of quartz ampoule is gently decreased to room temperature. From the quartz 3

ampoule, the synthesized compound is removed and grounded to the fine powder using agate mortar. This synthesized powder compound is transferred into another growth ampoule and sealed under a high vacuum of 10-5 torr. At one end of the ampoule, the whole loaded powder compound is kept. This powder loaded end is called as source zone and is maintained at high temperature. The other empty end is known as growth end and maintained at the low temperature for single crystals growth. The Figure 2 shows the schematic of the growth ampoule.

Figure 2 Schematic of growth ampoule. The entire growth run for pure SnS2 single crystals is tabulated in Table 1. Table 1 The growth parameters of single crystals. Samples

P1 P2 P3 P4 P5

Compound Preparation

Single crystals growth

Temperatur e (K)

Time (h)

873 873 873 873 873

24 30 30 30 30

Temperature distribution (K) Charge Growth zone zone 908 868 893 863 893 863 983 933 983 933

Heating rate (°Ch-1)

30 30 30 30 30

Growth Ampoule time (h) dimension s (mm2)

120 120 120 144 144

220 × 20 (ID)

Identical single crystal growth is followed for In (minimum assay 99.90%, John Baker Inc. Colorado, U.S.A) and Sb (minimum assay 99.90%, HiMedia Laboratories Pvt. Ltd., Mumbai, India) doped SnS2. In the case of In-doped SnS2, the compound is prepared by sealing In (5% and 15%), Sn and S in respective evacuated quartz ampoules in desired stoichiometric proportions. Whereas for Sb-doped SnS2, the compound preparation is done by sealing Sb (5% and 15%), Sn and S in respective evacuated quartz ampoules in desired stoichiometric proportions. In all the single crystals growth firstly the compound preparation

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of the crystals is carried out before single crystals growth. The Table 1 tabulate the entire single crystal growth run for pure SnS2, In- and Sb-doped SnS2. The Figure 3 shows the photographs of the as-grown five crystals. Detailed observations of the as-grown crystals, Figure 3(a – e), shows that flakes of the single crystals has grown on each other. The average large size pure single crystal dimensions of the respective samples are mentioned at the bottom of the respective photographs.

Figure 3 The photographs of the average large size as-grown single crystals P1, P2, P3, P4 and P5. All the as-gown five single crystals are luster, shining, transparent, dull orange color thin platelets. The five as-grown single crystals are characterized for stoichiometry by energy dispersive analysis of X-rays (EDAX) employing Field Emission Gun Nano Nova Scanning Electron

Microscope

(FEG-SEM)

450

with

EDAX

attachment.

The

structural

characterization of all the as-grown single crystals is done by X-ray diffraction (XRD) using Phillips, X’PERT MPD, using CuKα radiation in the 2 range of 100 and 850. The surface micro-topographic studies of all the five as-grown single crystals surfaces are done using an optical microscope, Axiotech 100, Carl Zeiss. The surface morphology of as-grown single crystals is also studied using Field Emission Gun Nano Nova Scanning Electron Microscope (FEG-SEM) 450, the one used for EDAX analysis. The transmission electron microscopy and selected area electron diffraction (SAED) studies are done with the help of electron microscope Philips Technai 20 at an accelerating voltage of 200 kV. The optical absorption 5

study are done with the help of a Double Beam, Double Monochromator ratio recording, Perkin Elmer Lambda 19 UV-Vis-NIR Spectrophotometer, The photoluminescence (PL) spectra of all five as-grown single crystals are recorded using LS-50B Perkin Elmer spectrofluorometer. The Raman spectra of as-grown single crystals are recorded using NXR FT-Raman.

3. Results and discussion 3.1 Energy dispersive analysis of X-ray (EDAX)

Figure 4 The EDAX spectra of as-grown single crystals samples P1, P2, P3, P4 and P5.

The stoichiometric chemical compositions of five as-grown single crystal samples are determined using EDAX technique. The EDAX spectra, Figure 4, revealed no extra peaks other than the respective elements indicating the presence of no impurities in the samples, thus suggesting the grown samples are pure.

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Table 2 The EDAX data of as-grown single crystals. Samples Observed Wt % Standard values EDAX Sn S In Sb Sn S In Sb 64.92 35.08 --64.14 35.86 --P1 61.67 35.08 3.25 -60.07 36.48 3.45 -P2 55.19 35.08 9.73 -56.06 34.99 8.95 -P3 61.67 35.08 -3.25 60.51 35.54 -3.95 P4 55.19 35.08 -9.73 55.56 36.02 -8.42 P5

Sn 60.82 54.12 60.83 54.45

Vegard’s law S In -35.94 3.24 36.22 9.66 35.94 -35.87 --

The observed weight percentage data from EDAX study and the standard weight percentage of Sn, S and respective dopants In and Sb are tabulated in Table 2. The data showed that all the samples are nearly stochiometric. 3.2 X-ray Diffraction

Figure 5(a) The X-ray diffraction pattern of as-grown single crystals. The Figure 5(a) shows the XRD patterns of all the five as-grown single crystal samples. The analysis of the recorded XRD patterns of all the as-grown single crystals represents that (001) is the prominent diffraction plane lying at the 2 value range of 14.50  16.00. It indicates that the growth of all single crystals has occurred along (001) direction.

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Sb ---3.23 9.68

Figure 5(b) The shift of the X-ray diffraction peak lying in the 2 range of 14.5016.00. All the diffraction peaks are indexed as of hexagonal structure of SnS2 with lattice parameters in good agreement with the standard data (JCPDS Card No. 23-0677), Table 3. All the XRD peaks are intense and sharp without any diffraction halo stating formation of a single crystalline phase and the nonexistence of amorphous phase in the grown crystals. Table 3 The crystallographic data of P1, P2, P3, P4 and P5 samples. Samples

2θ (deg.) for (001) plane

Lattice parameters

JCPDS23-0677

15.13

a (Å) 3.647

P1 P2 P3 P4 P5

15.09 15.10 15.19 15.21 15.12

3.64 3.64 3.64 3.63 3.62

c/a

V (Å3)

Crystallite size t (nm)

Strain ε (×10-4) (Lin-2m-4)

Dislocation density δ (×1014) (Lin m-2)

c (Å) 5.898

1.61

67.93

-

-

-

5.89 5.91 5.92 5.88 5.88

1.61 1.62 1.62 1.61 1.62

67.58 67.81 67.92 67.09 66.72

65.76 45.02 43.74 41.06 42.68

6.44 8.97 8.92 9.81 10.21

2.49 5.00 5.16 6.09 6.10

The Figure 5(b) show the magnified images of main peaks around 2 of 14.50  16.00. The figure shows that due to the doping of In and Sb the peaks shift appears. This shift is 8

attributed to Vegard’s law. The law states the dopant elements by themselves cannot generate individual peak other than the host peak, but can produce a finite shift in the position of host peak [42]. No other peaks consequent to dopants or their sulfides are observed suggesting perfect doping of In and Sb into SnS2. The above observation clearly states that in the growth of doped SnS2 single crystals, the cation Sn4+ are replaced by dopants In3+ and Sb3+ ions. The stoichiometric proportion of the elements present in In and Sb doped SnS2 are estimated from the Vegard’s law by using the XRD peak shift; , , and , Where

= 3.64 Å and

other parameters are

= 5.89 Å are the lattice constants determined from XRD. The

= atomic radius of Sn,

= atomic radius of In and

= atomic

radius of Sb [43,44]. The obtained wt% data using Vegard’s law are tabulated in Table 2. The obtained values are in match with the EDAX data, thus further substantiating the stoichiometry and doping of the grown single crystals. The XRD data tabulated in Table 3; shows that in case of 5% and 15% In-doped SnS2 the lattice parameters increases leading to increase in the unit cell volume compared to pure SnS2. The radii of dopant cation In is larger compared to host cation Sn resulting in the increase in lattice parameters and the unit cell volume. While in case of 5% and 15% Sbdoped SnS2, the volumes decrease due to smaller radii of Sb compared to host Sn ion. The crystallite sizes determine from the XRD by Debye-Scherrer’s formula [45] is tabulated in Table 3. It clearly states that the crystallite size drastically decreases in both In and Sb-doped SnS2 single crystal samples compared to pure SnS2. These crystallite size results are in concurrence to G. Yan-Hui et al. [46], N. Anitha et al. [47] which state that crystallinity always gets spoilt when dopants of smaller or larger ionic radii replace host lattice ion.

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The comparative study of crystallite sizes of In- and Sb-doped SnS2, Table 3, show that the values of crystallite size are larger in case of In-doped SnS2 than Sb-doped SnS2. This is owing to the match of In dopant ionic radii with host Sn ion whereas in the Sb dopant the mismatches with host Sn ion is more. The larger variance leads to strain in Sb-doped SnS2 compared to In-doped SnS2 as seen from the strain values given in Table 3. As the strain increases the dislocation density increases as observed by its values in Table 3. Thus it can be accomplished that Sb-doped SnS2 has a larger value of strain and dislocation density compared to In-doped SnS2 due to larger variance between dopant cation Sb and host cation Sn. This increased values of strain and dislocation in Sb-doped SnS2 decreases the crystallite size compared to In-doped SnS2. The determined dislocations exist in the layers of the pure and doped SnS2 single crystals. 3.3 SEM and Optical Microscopy

Figure 6(a) The eccentric spiral layers observed in SEM images of the P1, P2, P3, P4 and P5 samples surfaces.

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Figure 6(b) Spiral screw dislocations observed under the optical microscope on the surfaces of P1, P2, P3, P4 and P5 samples. The surface study is one of an important tool that give condition and mechanism of crystal growth. The surface investigation of as-grown single crystals surfaces are done by SEM technique. The SnS2 single crystals grow in platelet forms, Figure 3. In TMDC the (a, b) plane lies along the platelet and c-axis is perpendicular to the platelet surface [48]. The XRD states (001) to be the plane of single crystals growth thus confirming the surfaces of the asgrown crystals to be (0001) plane. The eccentric spiral layers, Figure 6(a), are the common features visible on the as-grown single crystal surfaces under different magnifications. The eccentric spiral layers are observed on the surfaces of all the five as-grown single crystal surfaces. The layers have steps which are far off at the centre and gets closer to each others at the outer periphery. The supersaturation gradient and the anisotropy of environmental growth conditions during crystal growth are responsible for the formation of the eccentric spiral layers [49]. Literature states the eccentric spiral layers on the surfaces are formed due to congregation of mono molecular layers [48]. The mono molecular layers originate from the screw dislocations which are present on the crystal face. The presence of screw dislocations are confirmed by the optical microscope observations of the as-grown crystal surfaces, Figure 6(b). The spiral features occur due to the termini of gaseous inclusion initiating centre of a spiral [50,51]. These observations of the optical microscopy and SEM images conclude that

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the growth of single crystals happened by layer growth mechanism supported by screw dislocation. 3.4 Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) Sonication technique is employed to exfoliate single layer of the as-grown single crystal samples for TEM analysis. The sonication is performed in acetone solvent. A nanoflakes of the samples obtained are examined under TEM. The observed TEM images are shown in Figure 7(a). Detailed analysis of the images showed overlap of layers, stating exfoliation by sonication does not give perfect single layer. Further analysis of the images show that exfoliated crystals has taken nearly the hexagonal shape. This is due to the presence of screw dislocations on the crystals surfaces that modify the layers into eccentric spirals of hexagonal shapes. These observations further corroborate the SEM observation. The recorded SAED patterns of the samples are shown in Figure 7(b). The patterns show the perfect hexagonal arrangement of points inferring single crystalline nature of the sample as well as hexagonal lattice structure of as-grown crystals. The hexagonal nature observed in SAED patterns substantiates the observation of the XRD structure and SEM analysis. The SAED of P1 sample, Figure 7(b), show overlapping of two spot patterns. This arises due to the presence of twin defect in this sample.

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Figure 7(a) The TEM images of P1, P2, P3, P4 and P5 samples.

Figure 7(b) The SAED patterns of P1, P2, P3, P4 and P5 samples.

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3.5 UV-Vis-NIR spectroscopy

Figure 8 The (a) absorbance spectra and (b) the plot of (αhυ)2 versus hυ of P1, P2, P3, P4 and P5 samples. The optical absorption spectra recorded for the as-grown single crystal samples analyzed in the wavelength range of 200 nm and 850 nm is shown in Figure 8(a). Absorbance were measured normal to the basal plane (ab-plane) of the as-grown samples. The spectra showed that the samples P1, P3 and P5 have sharp absorbance in the wavelength range of 500 nm and 590 nm. While, samples P2 and P4 show broad absorbance in the wavelength range of 540 – 650 nm and 370 – 550 nm, respectively. Optical bandgap energy values of as-grown samples are evaluated by absorbance spectra using the near band edge absorption relation [45,52,53]. (1) Where n illustrate the transitions, A is the optical transition dependent constant, E g is the optical energy bandgap,

is the frequency of incident beam, h is Planck’s constant and α is

the absorption coefficient. In case of direct allowed and forbidden transitions, n is 2 and 2/3 respectively, while for indirect allowed and forbidden transitions, n is 1/2 and 1/3 respectively. The analysis of equation (1) depicts that n = 2 fits for all the samples. This substantiate that all as-grown single crystals possess direct allowed optical bandgaps [54]. The intercept of the straight line of graphs between ( h )2 versus h , Figure 8(b), on the energy axis gives the direct optical bandgap values as 2.20 eV, 1.89 eV, 2.12 eV, 2.10 eV and 2.31 eV for P1, P2, P3, P4 and P5 samples respectively. The obtained optical direct bandgap values are in good agreement with the reported optical bandgap value for SnS2 that lies between 1.82 and 2.41 eV [23,55].

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3.6 PL spectroscopy

Figure 9 The photoluminescence spectra of P1, P2, P3, P4, and P5 samples. The PL analysis of semiconductor samples usually provides insight into the migration, transfer and recombination processes of photo-generated charge carriers. Room temperature photoluminescence spectra of the as-grown single crystal samples are recorded for an excitation wavelength of 350 nm. The obtained PL spectra of the as-grown single crystal samples are shown in Figure 9. All the spectra exhibits six peaks positioned at wavelengths of 421 nm (2.94 eV), 459 nm (2.70 eV), 495 nm (2.50 eV), 522 nm (2.37), 574 nm (2.16 eV), and 599 nm (2.06). The peaks arising at 421 nm, 459 nm and 495 nm are owing to the interstitial sulfur (S) lattice defect [56]. The PL peaks at 522 nm, 574 nm and 599 nm are attributed to direct bandgap transitions, whose energy matches the determined direct optical energy bandgap values [57,58].

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3.7 Raman spectroscopy

Figure 10 The Raman spectra of P1, P2, P3, P4 and P5 samples. The Figure 10 shows the recorded Raman spectra of all five as-grown single crystal samples. The spectra showed only one peak positioned at around 315 cm-1 for all the five single crystal samples. This peak is assigned A1g mode arsing due to S-Sn bond [39]. A slight blue shift is observed in the Raman spectra for the P4 and P5 compared to other samples. The shift is due to the attenuation of the lattice constant [59]. In Raman spectra of standard 2HSnS2 crystal, there arises an intra-layer Eg mode at 200–205 cm-1, which is not been detected in the present samples. The nonexistence of Eg mode may be due to weak Rayleigh scattered radiation not being able to be detected by the Raman sensor or by the selection rules for scattering geometry [60].

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4. Conclusions The single crystals of pure SnS2, In (5% and 15%) doped SnS2 and Sb (5% and 15%) doped SnS2 are grown by direct vapor transport (DVT) technique. The EDAX analysis showed that the as-grown single crystal samples are near perfect stoichiometric. The XRD analysis showed that the crystals possess the hexagonal structure with (001) as the prominent diffraction plane. The shifting of the prominent peaks of the doped SnS2 samples confirms doping of In and Sb to have taken place in SnS 2. The crystallite sizes calculated by the XRD data illustrate that the crystallite size decreases with an increase in dopant concentration in both In and Sb-doped SnS2 compared to pure SnS2. The surface morphology study by SEM and optical microscopy showed eccentric layers on flat surfaces of the crystals. The morphology study by optical microscopy showed spirals stating layer growth being supported by spiral growth mechanism. The optical analysis of the crystals showed they possess direct optical bandgap. The bandgap values are in agreement with the reported data. The room temperature PL spectra of the as-grown single crystal samples exhibited six peaks positioned at wavelengths of 421 nm (2.94 eV), 459 nm (2.70 eV), 495 nm (2.50 eV), 522 nm (2.37), 574 nm (2.16 eV), and 599 nm (2.06). The peaks arising at 421 nm, 459 nm and 495 nm are owing to the interstitial sulfur (S) lattice defect. While the peaks at 522 nm, 574 nm and 599 nm are due to direct bandgap transitions. The Raman analysis of the five as-grown single crystal samples showed only one peak assigned as the A1g vibrational mode of SnS2.

Acknowledgements All the authors are thankful to the Sophisticated Instrumentation Centre for Applied Research & Testing (SICART), Vallabh Vidyanagar, Gujarat, India for EDAX, SEM, TEM and UVVis-NIR Spectrophotometer analysis; Shah-Schulman Centre for Surface Science and Nanotechnology (SSCSSN), Dharmsinh Desai University (DDU), Nadiad, Gujarat, India for XRD analysis.

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Highlights    

Pure, In and Sb doped SnS2 single crystals grown by DVT technique. The single crystals were stoichiometric and of single phase possessing hexagonal crystal structure. The single crystals possessed direct optical bandgap. Optical microscopy showed layer growth mechanism.

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