Bandgap Engineering of InSe Single Crystals through S Substitution

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Bandgap engineering of InSe single crystals through S substitution Hui Li, Xu Han, Ding Pan, Xin Yan, Huan-Wen Wang, Changming Wu, Guanghui Cheng, Huachen Zhang, Shuo Yang, Baikui Li, Hongtao He, and Jiannong Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01751 • Publication Date (Web): 21 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Crystal Growth & Design

Bandgap engineering of InSe single crystals through S substitution Hui Li1, Xu Han1, Ding Pan1,2,3, Xin Yan1, Huan-Wen Wang1, Changming Wu1, Guanghui Cheng1, Huachen Zhang1, Shuo Yang1, Baikui Li1,4, Hongtao He5, Jiannong Wang1* 1

Department of Physics, the Hong Kong University of Science and Technology, Clear

Water Bay, Hong Kong, China 2

Department of Chemistry, the Hong Kong University of Science and Technology, Clear

Water Bay, Hong Kong, China 3

HKUST Fok Ying Tung Research Institute, Guangzhou, China

4

College of Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong

518060, China 5

Department of Physics, Southern University of Science and Technology, Shenzhen,

Guangdong 518055, China

** Correspondence and requests for materials should be addressed to J. W. (email: [email protected]).

Abstract Bandgap engineering offers opportunities for tailoring the properties of semiconductor materials for desired applications in microelectronics and optoelectronics. Alloys of different semiconductor materials can lead to the continuously tuning of the bandgap. Here, we report the bandgap engineering in layered InSe single crystals by substituting the Se atoms with S atoms. The formation of InSxSe1-x single crystal alloy 1

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with x ≤ 0.3 is evidenced by the X-ray diffraction and resonant Raman spectra. The photoluminescence (PL) spectra peak position blue shifts from ~1.27 eV to ~1.42 eV as S composition increases from 0 to 0.3 in the alloys, which is consistent with the bandgap shifts calculated by density functional theory. Temperature dependence of the PL spectra indicate that the presence of S atoms decreases the strength of the electron-phonon interaction but increases the average phonon energy in InSxSe1-x alloys. Our findings will open an intriguing avenue in understanding the fundamental physics in the III-VI layered semiconductor materials and their potential applications in optoelectronic devices.

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The two-dimensional (2D) layered transition metal dichalcogenides semiconductors (TMDS) have sparked considerable attention due to their both fundamental and practical interests.[1-5] Unlike graphene, the TMDS have a direct bandgap and a high on/off current ratio for field effect transistors (FET) in the monolayer limit, which are considered as promising candidates for the applications in optoelectronic and microelectronic devices.[610]

For example, the MoS2 monolayer has a direct bandgap of 1.56 eV and shows a high

on/off current ratio of ∼107 in FET.[11-13] For applications in the photonic devices, such as photo-detectors, it is important to develop 2D TMDS with continuously tunable bandgaps. Alloying provides an effective strategy to engineer the bandgap of the TMDS.[14,15] By substituting dopant in the lattice of 2D TMDS, the lattice structures could be eventually changed, rendering a wide range bandgap tunability, and the emergency of novel electronic and optical properties in the TMDS alloys. For example, a tunable emission wavelength in a wide spectral range is reported in MoS2(1-x)Se2x alloys by substituting Se atoms with isoelectronic chalcogens S atoms,[11,16,17] a 2H-phase to an orthorhombic 1T’-phase transition is obtained in Mo1xWxTe2

alloy by substituting the W atoms with Mo atoms.[18,19]

Here, we demonstrate the bandgap engineering of the layered single crystal InSxSe1-x (x=0 - 0.3) alloys grown by chemical vapor transport (CVT) method. The photoluminescence (PL) measurements show that the alloys exhibit compositiondependent emission with the peak position blue shifts by about 150 meV as S composition increases from 0 to 0.3. The density functional theory calculation confirms the observed evolution of the bandgap in InSxSe1-x alloys. These results shed light on the 3



applications in tunable optoelectronic devices in a wide spectral range using III-VI layered semiconductor materials.

Results and discussion Single crystal InSxSe1-x (x=0, 0.1, 0.2 and 0.3) alloys were grown by the chemical vapor transport (CVT) technique. The stoichiometry of each InSxSe1-x single crystal was determined by energy dispersive X-ray mapping spectroscopy (EDS) analysis, as shown in Figure S1 and Table S1 in the Supporting Information. Figure 1(a) shows the layered crystal structure of the InSxSe1-x alloys (top panel) with each layer of hexagonal structure (bottom panel). The Se sites are randomly occupied by S atoms during the alloying process. X-ray diffraction patterns for the single crystal InSxSe1-x alloys with different S composition are depicted in Figure 1(b). All of the (00n) diffraction peaks for n=2, 4, 6, 8 of the hexagonal structured InSe single crystal were indexed with the lattice constants of a = 4.005 Å and c = 16.64 Å (JCPDS-34-1431).[20] Substitutions of Se atoms with S atoms do not change the hexagonal lattice structure. However, the diffraction peaks are shifted to a higher diffraction angle (see the enlarge view in the inset of Figure 1(b)) corresponding to the decreases of lattice constant c, which gradually decreases from about 16.64 Å to about 16.53 Å as S composition increases from x=0 to 0.3 (see Figure 1(c)). Such a decrease tendency is also consistent with our density functional theory calculations, where the lattice constant c also decreases from 16.45 Å to 16.30 Å when the S composition increases from 0 to 0.25 (see Figure S2 in the Supporting Information). Such a decrease is due to shrinkage of the unit cell after substituting Se atoms with S atoms of smaller ionic radius.[21] It is noted that the maximum S composition for the 4


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hexagonal structured InSxSe1-x alloys is about 30%. Further increase of S composition results in the formation of orthorhombic InS clusters in the alloy as evidenced from the XRD pattern marked with asterisk (*) in Figure 1(b) for InS0.3Se0.7 and Figure S3 for InS0.4Se0.6. The crystal structures of the as-grown single crystal InSxSe1-x alloys were further investigated by the transmission electron microscopy (TEM). Figure 1(d) shows the morphology of thin InSe flake transferred on Cu grids by mechanical exfoliation and wet transfer method (see section IV in Supporting Information for details). The selected area electron diffraction pattern (see Figure 1(e)) measured in the marked area (see the red rectangle in Figure 1(d)) shows a 6-fold symmetry indicating the single-crystalline InSe flake orientated along the [001] zone axis. The high resolution TEM image in Figure 1(f) shows clearly hexagonal structure of the synthesized InSe with well recognized a and b axis and the angel between the two axis is 120o. The lattice constant is measured to be about 0.34 nm, which is corresponding to d-spacing (100) lattice planes of β or ε phase of InSe.[22] The same TEM investigations are also carried out on InSxSe1-x alloys as shown in Figure S4-S6 in the Supporting Information. The InSxSe1-x alloys all demonstrate the hexagonal structure with the same lattice constant a and b as that of InSe flake even when the S composition reaches to 30%. Figure 2(a) shows the normalized resonant Raman spectra of the as-grown InSxSe1-x alloys of about 80 nm in thickness with S composition x increases from 0 to 0.3, respectively. Four Raman modes namely E11g at 41 cm-1, A11g at 115 cm-1, E21g at 177 cm-1 and A21g at 226 cm-1 are clearly seen for InSxSe1-x flakes. As illustrated in Figure 2(b), E1g 5



corresponds to the in-plane vibration modes, while A1g to the out-of-plane vibration modes of the Se–In–In–Se planes. No Raman peak at about 202 cm-1 is observed, indicating the as-grown InSe single crystal is of β-phase.[22,23] Besides, two additional Raman peaks marked with asterisk (*) at about 98 cm-1 and 220 cm-1 are observed for InSxSe1-x alloys. These two peaks are ascribed to A11g and A21g modes of In-S bonds and indicate the successful substitutions of Se atoms by S atoms. In particular, A21g mode of In-S bonds is significantly enhanced with increasing S composition.[24] Figure 2(c) shows the relative Raman frequency shift as a function of S composition. For A21g and E11g modes, the Raman frequency changes little as S composition increases. In contrast, the Raman frequency of A11g and E21g modes blue shifts as S composition increases. A maximum shift about 7.5 cm-1 and about 1.5 cm-1 for A11g and E21g modes, respectively, is observed when S composition increases to 0.3. Such a blue shift is due to the change of cell volume after substitutions of Se atoms with S atoms of smaller ionic radius and the different relevant interatomic force constant between In-S bonds and In-Se bonds.[24,25] Figure 2(d) presents the full width at half maximum (FWHM) of each Raman mode. The FWHM is calculated by Lorentz fitting of the Raman peaks. With increasing S composition, the FWHM of all Raman modes gradually increases. Generally, in a perfect single crystal, the FWHM of Raman peak is in proportion to the inverse phonon lifetime, including both electron-phonon and phonon-phonon (anharmonic effects) interactions.[26] However, when dopants are introduced, these dopants would serve as the scattering centers to scatter phonons and give rise to another contribution to FWHM by coupling phonons of wave vector q0 and q0+δq, resulting in the broadening of Raman peaks. [26] In 6


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our experiments, the scattering effect on phonons would be eventually enhanced as S composition increases, which leads to the broaden of Raman peaks. Figure 3(a) shows the normalized PL spectra of InSxSe1-x alloys at 10 K. For InSe, a prominent peak at ∼1.27 eV is observed originating from the direct gap free-exciton recombination.[27,28] For InSxSe1-x alloys, with increasing S composition the peak blue shifts systematically. When x = 0.3, a maximum shift of about 150 meV is reached, as shown in Figure 3(b). Such a blue shift is attributed to the change of the band structures for InSxSe1-x alloys by replacing Se atoms with S atoms. Figure 3(c) shows the calculated band structures of InSe (left) and InS0.25Se0.75 (right) by density functional theory. The energy reference point (0 eV) is set at the middle of the band gaps. Density functional theory calculations show that the bandgap increases as S composition increases. The conduction band minimum (CBM) of InSe at the Γ point is dominated by the In s-orbitals and the valence band maximum (VBM) at the Γ point mainly composes of the Se pzorbitals.[29,30] As the electronegativity of the S atoms is larger than that of the Se atoms, when the Se atoms are replaced with the S atoms, the hybridization of pz orbital of S and Se atoms at the Γ point would pull down the VBM. Therefore, we attribute the increase of band gap for InSxSe1-x alloy mainly to the downshift of the VBM. Figure 3(d) shows the calculated relative bandgap change of InSxSe1-x alloys with S composition x. It can be seen that the bandgap increases monotonically with increasing x and by about 50 meV when x=0.125, which are consistent with the experimental observations (see Figure 3(b)). The spin-orbit coupling (SOC) effect on the variation of the bandgap of InSxSe1-x alloys is also considered in the calculations but it shows little effect, see the red dash line in Figure 3(d), which is also consistent with the previous calculations.[29,30] Moreover, the 7



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InSxSe1-x alloys undergo a direct-to-indirect gap transition when the layer thickness is reduced to below about 10 nm similar to that for InSe.[31] Figure 4(a) shows the PL spectra of InS0.1Se0.9 flake of about 80 nm in thickness at temperatures indicated. The same PL measurements have also been performed on InSxSe1-x alloys as shown in Figure S7 in the Supporting Information. Figure 4(b) plots the temperature dependence of the PL peak position for all of InSxSe1-x alloys. As it can be seen, the peak positions gradually increase as temperature decreases from 300 K to 130 K, and saturate when temperature further decreases below 130 K (see the open squares in Figure 4(b)). The temperature dependence of the bandgap evolution of semiconductors follows a standard hyperbolic cotangent relation given by: [32] () = (0) − 〈ℏ〉 coth

〈ℏ〉

− 1

(1)

where Eg(0) is the bandgap energy at 0 K, 〈ℏ〉 is the average phonon energy, kB is the Boltzmann’s constant, T is temperature, A is a dimensionless coupling constant describing the relative strength of the electron-phonon interaction, and the coth function term relates to effective number of phonons at certain temperature.[33,34] As illustrated by the solid red lines in Figure 4(b), the temperature dependent PL peak positions of InSxSe1-x alloys can be well fitted by Eq. (1), indicating that the observed PL spectra in InSxSe1-x alloys is associated with direct gap free-exciton emissions.[35] The calculated Eg(0), A, and 〈ℏ〉 for InSxSe1-x alloys with different S composition are summarized in Table 1. It can be seen that the value of the coupling constant A decreases with increasing S composition, demonstrating that the coupling strength of electron-phonon interaction becomes weaker after substitutions of Se atoms with S atoms. Such a decrease of A is 8


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ascribed to the suppression of the amount of Se vacancies by introducing S atoms, as it is reported in recent studies on other layered semiconducting dichalcogenide alloys of selenium and sulfur.[35-37] In contrast, the value of the average phonon energy 〈ℏ〉 increases as S composition increases, which is originated from the larger relevant interatomic force constant of In-S bonds than that of In-Se bonds. It is noted that the value of average phonon energy 〈ℏ〉 for InSxSe1-x alloys is larger than that of the transition metal dichalcogenides, which implies the electron-phonon interaction is much stronger in III-VI layered semiconductors.[38-40] Figure 4(c) plots the linewidth of the PL peak as a function of temperature for InS0.1Se0.9 alloy. The linewidth is estimated by Lorentz fitting of the measured PL spectra shown in Figure 4(a). Below 100 K, the FWHM is almost a constant. From 100 K to 300 K, it increases monotonically with increasing temperature from about 40 meV to about 70 meV following a phonon-induced broadening relation (see solid line in Fig. 4(c))[41,42] given by: () = + ! +

"

〈ℏ'〉 +,#$%& () *

where is temperature independent homogenous broadening, ! is the coupling coefficient between an exciton and an acoustic phonon, . is the coupling coefficient between an exciton and a longitudinal optical (LO) phonon, 〈ℏ〉 is average phonon energy, and kB is the Boltzmann’s constant. The solid red line in Figure 4(c) shows the fitting result for InS0.1Se0.9 alloy with = 41 meV, ! = 18 µeV/K, . = 305 meV and 〈ℏ〉 is set as 86 meV for consistency. The same fitting process is also carried out on 9


(2)


InSxSe1-x alloys and the yielded fitting parameters are listed in Table 1. It can be seen that the interaction between exciton and acoustic phonon is roughly the same for InSxSe1-x alloys, around 10~20 µeV/K. However, the interaction between the exciton and the longitudinal optical (LO) phonon decreases with S composition increases.

Conclusion In conclusion, we have synthesized bulk single crystal of InSxSe1-x (x=0 - 0.3) alloys by chemical vapor transport (CVT) method. With increasing S composition, the bandgap energy for InSxSe1-x alloys blue shifts systematically. When S composition increases to 0.3, a maximum blue shift of about 150 meV is obtained. Moreover, the introduction of S atoms decreases the strength of the electron-phonon interaction but increases the average phonon energy in InSxSe1-x alloys. The bandgap engineering of InSxSe1-x alloys demonstrated in this work allow us to gain some insight into the fundamental physical properties such as the variation of the electron-phonon interaction strength in the III-VI layered semiconductor alloys. In addition, it would inspire a wide range of potential applications in optoelectronics and microelectronics using the III-VI layered semiconductor alloys and other 2D materials based Van der Waals heterostructures including the near-infrared photodetectors and solar cells, etc..

Methods Growth. The single crystal InSxSe1-x (x=0, 0.1, 0.2 and 0.3) alloys were grown by the chemical vapor transport (CVT) technique. In the process, In powder, Se powder and S powder were mixed at a certain molar ratio and sealed in a quartz tube under a vacuum (10-3 torr). The precursor powder was gradually heated up to 685 °C in 70 minutes, and maintained at that temperature for 180 minutes, then the temperature was further 10


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increased to 700 °C and kept for 180 minutes. Afterwards, the quartz tube was cooled down to 500 °C in 1200 minutes and followed by cooling to room temperature naturally. Characterization. Structural investigations and stoichiometry analyses of single crystal InSxSe1-x (x=0 - 0.3) alloys were performed on X-ray diffraction (X'pert Pro, PANalytical) and field-emission scanning electron microscope (JEOL-7100F) at room temperature. The growth direction of InSxSe1-x (x=0 - 0.3) alloys was confirmed by TEM (JEM 2010, JEOL) investigations. The Raman and photoluminescence spectra were collected on micro Raman/Photoluminescence system (InVia, Renishaw) using an argon green laser at 514 nm. DFT calculation. We used the plane-wave pseudopotential method, implemented in the Quantum Espresso package (v 5.3.0),[43] to calculate the electronic structures of InSxSe1x.,

The exchange-correlation functional was approximated by the Perdew-Zunger local

density approximation.[44] We used the ultrasoft pseudopotentials with the plane-wave cutoff of 80 Ry and the electron density cutoff of 480 Ry.[45] We switched to the full relativistic pseudopotentials in the spin-orbital coupling calculations. The unit cell and atom positions were fully relaxed at different doping levels. The convergence tolerances for energy, force and stress were 10-5 Ry, 10-4 Ry/Bohr and 50 MPa, respectively. The Brillouin zone was sampled by the Monkhorst-Pack mesh[46] of 11×11×3 for InSe and InS0.25Se0.75, 11×7×3 for InS0.125Se0.875.

Supporting Information Stoichiometry of InSxSe1-x alloys, Calculated lattice constant of InSxSe1-x alloys, XRD pattern of InS0.4Se0.6 alloy, Experimental details and TEM images of InSxSe1-x alloys, Photoluminescence spectra of InSxSe1-x alloys and references. These are available free of charge on the ACS Publications website at DOI:

.

Author Information Corresponding Author *E-mail: [email protected]. 11



ORCID Jiannong Wang: 0000-0002-6133-6371 Notes The authors declare no competing financial interest.

Acknowledgements This work was supported in part by the Research Grants Council of the Hong Kong SAR under Grant Nos. 16302717, HKU9/CRF/13G-1 and ECS-26305017, and in part by the National Natural Science Foundation of China under Grant Nos. 11574129 and 11774072, the Natural Science Foundation of Guangdong Province under Grant No. 2015A030313840.

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Figure captions

Table 1. The values of Eg(0), A, 〈ℏ〉, , σ and γ for InSxSe1-x alloys obtained in fittings.

Figure 1. (a) Schematic crystal structure of hexagonal InSxSe1-x alloys in 3D form (top panel) and 2D form (bottom panel), where Se atoms are randomly replaced by S atoms with smaller radius. (b) Normalized XRD spectra of as-grown InSxSe1-x alloys, inset shows the zoom in view of the (004) diffraction peak for InSxSe1-x alloys. (c) The lattice constant c as a function of S composition. The dashed line is guide to the eyes. (d) The TEM image of a thin InSe flake. (e) The SAED pattern and (f) HR-TEM image of InSe flake collected in the marked area in (d)

Figure 2. (a) Micro-Raman spectra of InSxSe1-x alloys of about 80 nm in thickness at 10 K. (b) The sketch of the Raman active optical modes in single crystal InSe. (c) The relative Raman frequency shift and (d) the FWHM of the measured Raman modes as a function of S composition. The dashed lines are guides to the eyes.

Figure 3. (a) Normalized photoluminescence spectra of InSxSe1-x alloys at 10 K. (b) The relative peak position change of InSxSe1-x alloys as a function of S composition. The dashed line is a guide to the eyes. (c) Calculated band structures of InSe (left) and 19



InS0.25Se0.75 (right) by density functional theory. The energy reference point (0 eV) is set at the middle of the band gaps. (d) Calculated S composition-dependent relative bandgap shifts for InSxSe1-x alloys by density functional theory. The black dots and red dots represent the calculated results excluding and including the spin-orbit coupling interaction, respectively. The dashed lines are guides to the eyes. Figure 4. (a) Photoluminescence (PL) spectra of InS0.1Se0.9 alloy measured at different temperatures indicated. (b) Temperature-dependent peak positions of InSxSe1-x alloys. The solid red lines are fitted results using Equation (1). (c) The FWHM of PL spectra of InS0.1Se0.9 alloy as a function of temperature. The solid red line is fitting curve using Equation (2).

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Eg(0) (eV)

A

〈ℏ/〉 (meV)

01 (meV)

2 (µ µeV/K)

3 (meV)

InSe

1.278

5.602

84.92

45.0

10.31

308.9

InS0.1Se0.9

1.333

5.605

85.94

41.4

17.78

305.0

InS0.2Se0.8

1.388

5.571

86.17

47.1

10.82

285.2

InS0.3Se0.7

1.429

5.053

87.49

47.8

21.23

125.2

Table 1

21



Figure 1

22


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Figure 2

23



Figure 3

24


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Figure 4

25


Bandgap Engineering of InSe Single Crystals through S Substitution

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