Electronics properties of ZnSe nanotube with substitutional impurity atoms - A first-principles investigation

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ScienceDirect Materials Today: Proceedings 5 (2018) 14405–14415

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ICAFM_2017

Electronics properties of ZnSe nanotube with substitutional impurity atoms - A first-principles investigation Dr. PA Gowri Sankara* a

Assistant Professor, Jeppiaar Engineering College, Anna University, Chennai and 600119, India

Abstract

The band structure of pristine zinc selenide (ZnSe) nanotube, gallium, chlorine, nitrogen and arsenic substituted ZnSe nanotube are studied using density functional theory (DFT) with GGA/PBE exchange correlation functional. The state of the art of this work is to study the electronic properties of ZnSe nanotubes with substitution impurities. The ZnSe nanotube electronic properties are studied in terms of band structures and density of states spectrum. The band structure of pure ZnSe nanotube shows a semiconducting nature. The gallium, chlorine, nitrogen and arsenic substituted ZnSe structured results in metallic behavior. The density of states provides the insight for the localization of charges in the valence band and conduction band. The substitution of chlorine and nitrogen enhance the density of charges in valence band. We found that nitrogen is the most efficient acceptor impurity for p-type doping, while chlorine is the most suitable impurity for n-type doping. The results of the present work provide a clear vision to tailor the band structure of ZnSe nanotube with substitution impurity. This study is useful to researchers investigating p- and n-type doping as well as optoelectronic device manufacturers. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of ICAFM’17. Keywords:Density functional theory; Density of state; Electron density; Molecular device; Nanotube; Zinc selenide;

* Corresponding author. Tel.: +09894569383. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of ICAFM’17.

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1. Introduction Semiconductor nanostructure materials have attracted great interest during the past two decades. The discovery and consequent development of semiconductor nanostructures such as nanoribbon, nanowire and nanotube with exceptional properties plays a vital role in the development of future nanoelectronics devices for technological applications. To promote device applications, electronic, magnetic, optical, and optoelectronic properties of the nanostructure materials to be rationally tune is essential. The doping technique is one of the most efficient method could be remarkably enhance and finely tune by introducing appropriate foreign elements into the nanostructure materials. The materials properties drastically change from bulk to their low-dimensional counterparts. The low dimensionality and modified properties of binary compound semiconductor nanostructures such as nanotube, nanowire and nanoribbon is in focus among the scientific community due to their main applications in nano electronics, optoelectronic devices, heterojunction photovoltaic cells, and self-assembled quantum dots [1–3]. Zinc selenide (ZnSe) is one of the binary compound II–VI group semiconductors. The successful realization of complementary doping, i.e. both n- and p-type doping, in IV and III–V group nanostructures has led to the actualization of a variety of novel nanodevices, such as complementary metal-oxide-semiconductor transistors (CMOS) [4], light emitting diodes (LEDs) [5], laser diodes (LDs) [6], photovoltaic devices [7] and so on. Despite this progress, studies on the controlled doping of the semiconductor nanostructures, particularly in II–VI group nanostructures are critical and still at their very initial stages. In this paper, ZnSe nanotube is our choice of interested nanostructure material for the future nanoelectronics and optoelectronics devices application. Recently, ZnSe nanostructures, with various geometrical morphologies, such as nanocrystals [8–10], quantum dots [11–13], nanorods [14], nanowires [15–17], thin films [18], nanobelts [19,20], nanoribbons [21–23] and nanotubes [24-28] have been fabricated by a variety of methods. With this as objective, literature survey was conducted CrossRef metadata search for binary semiconductor nanotube materials. Analogous to carbon nanotubes and graphene, many researchers have reported structural studies, electronic properties of various binary compound nanotube materials such as InP nanotube [29], GaN nanotube [30], CdTe nanotube [31], MgS nanotube [32], ZrN nanotube [33], MoS2 nanotube [34], TiO2 nanotube [35], BN nanotube [36, 56], SiC nanotube [37, 38], ZnO nanotube [39, 40], zinc-blende or wurtzite (w) type structures ZnX(X=S/Se/Te) [41–48], CdO nanotube [49]. From the literature survey, it is inferred that most of the reported works are in the study of electronic or mechanical property, synthesis and characterization of ZnSe nano crystalline and wurtzite structure and their counter parts of group III–V semiconductors nanotubes. To the best of my knowledge, the electronic band structure engineering of ZnSe nanotube with gallium (Ga), chlorine (Cl), nitrogen (N) and arsenic (As) substitution impurities have not been investigated using density functional theory (DFT). The systematic computational study could be a pioneer step for the experimental characterization of nanostructures. The novel aspect of the present work is to study the band structure of ZnSe nanotube with substitution of impurity elements like gallium (Ga), chlorine (Cl), nitrogen (N) and arsenic (As). 2. Computational details Computation study on ZnSe nanotube is successfully carried out through an ab initio first-principles density functional theory (DFT) [50, 51] method employing TranSIESTA module in SIESTA package [52]. DFT method is an efficient method to investigate the band structure, density-of-states and transmission coefficient of the ZnSe nanotube. The generalized gradient approximation (GGA) is employed with the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional [53, 54]. For DFT calculations, an optimization of the single wall (SW) ZnSe nanotube molecular geometry unit cell with periodic boundary conditions is taken into count by reducing the atomic forces to be smaller than 0.05 eV/Å. A Monkhorst–Packgrid of 1×1×100 are used for structure optimization and electronic structure calculations. In the present model, the Brillouin zones are sampled with 1x1x100 k-points. The real space grid for ZnSe nanotube is calculated with mesh cut off energy of 150 Ry and a double-zeta polarization (DZP) basis set for all of the atoms to achieve the balance between the calculation efficiency and accuracy.

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3. Result and Discussion 3.1. Structures of ZnSe Nanotube ZnSe nanotubes (NTs) is constructed from ZnSe nanosheet with twenty-eight zinc (Zn) atoms and twenty-eight selenium (Se) atoms to form a hexagonal structured pristine zinc-selenium nanotube (ZnSe NT) consists of total fifty-two atoms. In the present work, ZnSe nanotube resembles chiral type which has different values of n and m. The chirality dimension of ZnSe nanotube are n = 5 and m = 2 with the repetition along c-axis as one. The length of ZnSe nanotube is around 12.97Å and the tube diameter is 6.5 Å. The edges of the ZnSe nanotubes are chemically more active and it can accommodate appropriate dopants to obtain different electronic properties having the same geometrical structure of the nanotube. ZnSe nanotube doped with gallium, chlorine, nitrogen and arsenic impurity atoms are remained in symmetrical structure. Looking at the structure of gallium (Ga) substituted ZnSe nanotube, it has twenty-four zinc atoms with twenty-eight selenium atoms and four gallium atoms substituted in place of zinc atoms in the left-hand side.

Fig. 1a. Schematic diagram of pristine ZnSe nanotube.

Fig. 1b. Schematic diagram of gallium substituted ZnSe nanotube.

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Fig. 1c. Schematic diagram of chlorine substituted ZnSe nanotube.

Fig. 1d. Schematic diagram of nitrogen substituted ZnSe nanotube.

Fig. 1e. Schematic diagram of arsenic substituted ZnSe nanotube. Likewise, the nitrogen (N) and arsenic (As) substituted ZnSe nanotube is designed by replacing four zinc atoms in the left-hand side with four nitrogen and arsenic atoms. In the case of chlorine (Cl) substituted ZnSe nanotube comprises of twenty-eight zinc atoms with twenty-three selenium atoms, three selenium atoms being substituted with three chlorine atoms in the left-hand side. A doping process is used to realize ZnSe nanotube for various novel nano-electronic and optoelectronic applications. The ZnSe nanotube material can be doped n-type and p-type with specified group-III, V, VII element. The reason behind the selection of substitution elements like gallium, nitrogen, arsenic and chlorine may change the band structure and electronic properties of ZnSe nanotube. Fig. 1a–1e represents the schematic diagram of pristine ZnSe nanotube, gallium substituted ZnSe nanotube, chlorine substituted ZnSe nanotube, nitrogen substituted ZnSe nanotube and arsenic substituted ZnSe nanotube respectively.

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3.2. Band structure of ZnSe nanotube Band structure provides the insight for the materials properties involved in band structure engineering. The electronic band structure analysis of ZnSe nanotube helps to understand the material property comparable with different nanotubes. The band structures can be explained based on the number of conducting channels (lines) crossing the Fermi energy level (EF) between the valence band and conduction band to decide whether the nanostructure has metallic or semiconducting properties. If lines cross the Fermi level, it infers the metallic nature of the material. Moreover, the band gap can be estimated from the gap across the point in the band structure diagram. The Fermi level is set to zero in all other graphics of band structure and density of state. Fig. 2a represents the band structure of pristine ZnSe nanotube. Our calculations show that chiral type pristine ZnSe single wall nanotubes (SWNT) are semiconductors with direct band gaps at the point. The band gaps of ZnSe SWNT are found to be around 1.85 eV. The values of the band gap of ZnSe nanotubes are quite lesser than those of bulk ZnSe (2.7 eV). This behavior of ZnSe SWNT is quite different from the gallium nitride (GaN) [55], boron nitride (BN) [56] singlewalled nanotubes, but is like ZnO nanotubes [57, 58].Fascinatingly, there is a great change in the band structure of the gallium substituted ZnSe nanotube. As in fig. 2b, the Fermi level shift of the gallium doped nanotube is clear indication that the number of free electrons of gallium doped semiconducting nanotubes decreases. As a result, these nanotubes are a finite conductance and consequently lending a metallic property to the nanotubes. 7 6 5 4

Energy (eV)

3 2 1

εF

0 -1 -2 -3 -4 -5 -6 -7

Γ

Ζ

Fig. 2a. Band structure of pristine ZnSe nanotube. 7 6 5 4

Energy (eV)

3 2 1

εF

0 -1 -2 -3 -4 -5 -6 -7

Γ

Ζ

Fig. 2b. Band structure of gallium substituted ZnSe nanotube. Likewise, the band structure of chlorine substituted ZnSe nanotube shift the Fermi level downward into the valence band as in fig. 2c. Thus, the number of free electrons of these nanotubes decreases and conductance become weak. This also clearly shows a metal like structure. Gallium and chlorine substituted impurity form a n-type in ZnSe nanotube.Interestingly, fig. 2d represents the band structure of the nitrogen substituted ZnSe nanotube. nitrogen

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doping leads to lowering of the Fermi level into the conduction band according to the doping rate. Due to the shift of the Fermi level the nitrogen levels hybridize with the selenium level forming highly dispersive acceptor-like bands and consequently lending a metallic property to the nanotube. 7 6 5 4

Energy (eV)

3 2 1

εF

0 -1 -2 -3 -4 -5 -6 -7

Ζ

Γ

Fig. 2c. Band structure of chlorine substituted ZnSe nanotube. 7 6 5 4

Energy (eV)

3 2 1

εF

0 -1 -2 -3 -4 -5 -6 -7

Γ

Ζ

Fig. 2d. Band structure of nitrogen substituted ZnSe nanotube. 7 6 5 4

Energy (eV)

3 2 1

εF

0 -1 -2 -3 -4 -5 -6 -7

Γ

Ζ

Fig. 2e. Band structure of arsenic substituted ZnSe nanotube. Likewise, fig. 2e depicts the arsenic substituted ZnSe nanotube; in this case, the band gap of the arsenicsubstituted ZnSe nanotube also exhibits metallic nature. Nitrogen and arsenic substituted impurity form a ptype in ZnSe nanotube. The conducting channels have never crossed one over the other channels. Due to the Fermi shift, the number of free electrons in a conduction band of arsenic doped nanotubes increases, when further increase of doping rate. Therefore, conductivity of these nanotube increases as well. 3.3. Density of states of ZnSe nanotube The density of states (DOS) spectrum enables perception of visualization of charges at different energy intervals in valence band and conduction band of ZnSe nanotube [59, 60]. Zinc and selenium atoms orbital are overlapping

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each other in the nanostructure lead to accumulation of charges in a energy interval [60]. The overlapping of s, p, and d orbitals and substitution impurity atoms results to density of states (DOS) spectrum. The projected density of states (PDOS) arises due to individual orbitals of atoms in ZnSe nanotube. It gives a clear insight to the localized electronic structure of ZnSe nanotube. Fig. 3a illustrates the PDOS spectrum of pristine ZnSe nanotube. Looking at the spectrum, it is clearly observed that major influence in PDOS spectrum comes from p orbital of zinc with selenium atom. The peak maximum is observed near -0.9 eV in the valence band due to the overlapping of Zn is [Ar] 3d10 4s2 with selenium [Ar] 3d10 4s2 4p4. d p s

PD O S(/ev)

εF

-5

-4

-3

-2

-1

0

1

2

3

4

5

Energy (ev)

Fig. 3a. PDOS spectrum of pristine ZnSe nanotube. εF

PD O S(/ev)

d p s

-5

-4

-3

-2

-1

0

1

2

3

4

5

Energy (ev)

Fig. 3b. PDOS spectrum of gallium substituted ZnSe nanotube. εF

PD O S(/ev)

d p s

-5

-4

-3

-2

-1

0

1

2

3

4

5

Energy (ev)

Fig. 3c. PDOS spectrum of chlorine substituted ZnSe nanotube.

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PD O S(/ev)

d p s

-5

-4

-3

-2

-1

0

1

2

3

4

5

Energy (ev)

Fig. 3d. PDOS spectrum of nitrogen substituted ZnSe nanotube. εF

PD O S(/ev)

d p s

-5

-4

-3

-2

-1

0

1

2

3

4

5

Energy (ev)

Fig. 3e. PDOS spectrum of arsenic substituted ZnSe nanotube. εF

PD O S(/ev)

As doped N doped C ldoped G a doped pure

-5

-4

-3

-2

-1

0

1

2

3

4

5

Energy (ev)

Fig. 4. Density of states spectrum of ZnSe nanotube. Looking at the PDOS spectrum of gallium-substituted ZnSe nanotube as in fig. 3b, the electronic configuration of gallium plays a major role [Ar] 3d10 4s2 4p1orbitals leading to peak maximum in valence band and conduction band. Moreover, the peak maximum is seen in the valence band below EF, due to excess of electron in gallium atom. Fig. 3c represents the PDOS spectrum of chlorine substituted ZnSe nanotube, comparing the PDOS spectrum of pure ZnSe, the effect of chlorine substitution only increases the peak maximum in the valence band. The excess of electrons in chlorine atoms instead of selenium atoms modifies the peak amplitude in the valence band. Interestingly, fig. 3d illustrates the PDOS spectrum of nitrogen substituted ZnSe nanotube, zinc atoms has the electronic configuration of [Ar] 3d10 4s2 and nitrogen has 1s2 2s2 2p3, this clearly transforms the p orbital of

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nitrogen substituted ZnSe nanotube. In the p orbital, near the Fermi level some of the soft peak amplitude are noticed due to the excess electron in group V nitrogen element. Likewise, fig. 3e represents PDOS spectrum of arsenic substituted ZnSe nanotube. In this case, there is no drastic variation when comparing the PDOS spectrum of nitrogen doped ZnSe nanotube. The trend remains same in the case of nitrogen-substituted ZnSe nanotube as in fig. 3d, due to the substitution of same group V arsenic element only modify the peak amplitudes. Fig. 4 illustrates the density of states (DOS) spectrum of pristine, gallium, chlorine, nitrogen and arsenic substituted ZnSe nanotube. It is clearly inferred from DOS spectrum, except for group III gallium-substituted element, the peak maximum is observed in valence band for nitrogen and chlorine-substituted ZnSe nanotube. In contrast, there is not much variation in DOS with the substitution of arsenic and gallium elements. It is evident that the density of states can be fine-tuned with substitution of impurity in ZnSe nanostructure. 4. ConclusionThe band structure of pristine ZnSe, gallium, chlorine, nitrogen and arsenic substituted ZnSe nanotubes are successfully investigated using density functional theory utilizing GGA/PBE exchange–correlation functional. The results of electronic band structure show that the pristine ZnSe nanotube is direct wide band gap semiconductor by theoretical prediction. The band gap is around 1.85 eV. However, the band gap of impurities substituted ZnSe nanotube structure is drastically changes, which shows metallic nature of behavior. Moreover, the band structure can be fine-tuned with substitution of gallium, chlorine and nitrogen with ZnSe nanotube. The density of states spectrum (DOS) provides the information regarding the localization of charges in the valence band and conduction band of ZnSe nanotube. 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