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Investigation on electronic properties of functionalized arsenene nanoribbon and nanotubes: A first-principles study
Accepted Manuscript Investigation on electronic properties of functionalized arsenene nanoribbon and nanotubes: a first-principles study V. Nagarajan, R. Chandiramouli PII: DOI: Reference:
S0301-0104(17)30439-1 http://dx.doi.org/10.1016/j.chemphys.2017.08.007 CHEMPH 9839
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Chemical Physics
Received Date: Revised Date: Accepted Date:
28 May 2017 8 August 2017 20 August 2017
Please cite this article as: V. Nagarajan, R. Chandiramouli, Investigation on electronic properties of functionalized arsenene nanoribbon and nanotubes: a first-principles study, Chemical Physics (2017), doi: http://dx.doi.org/ 10.1016/j.chemphys.2017.08.007
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Investigation on electronic properties of functionalized arsenene nanoribbon and nanotubes: a first-principles study
V. Nagarajan, R. Chandiramouli* School of Electrical & Electronics Engineering SASTRA University, Tirumalaisamudram, Thanjavur -613 401, India
*Corresponding Author: Prof. R. Chandiramouli, School of Electrical & Electronics Engineering, SASTRA University Tel: +919489566466 Fax.:+91-4362-264120 E-mail: [email protected]
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ABSTRACT The electronic properties of arsenene nanotubes and nanoribbons with hydrogenation along the zigzag and armchair edges are studied using density functional theory (DFT) technique. The structural stability of hydrogenated zigzag and armchair arsenene nanostructures are confirmed with formation energy. The electronic properties of arsenene nano-conformers are described in terms of energy band structure and projected density of states spectrum. Furthermore, owing to the influence of hydrogen passivation, buckled orientation and width of arsenene nanostructures, the band gap widens in the range of 0.38 – 1.13 eV. The findings of the present work confirm that the electronic properties of arsenene nanomaterial, can be fine-tuned with the influence of passivation with hydrogen, zigzag or armchair border shapes and effect of the width of nanoribbons or nanotubes, which can be utilized as spintronic device and chemical sensor.
Keywords: nanoribbon; nanotube; arsenene; stability; band structure
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1. Introduction Like graphene, other two-dimensional materials have attracted great interest due to their chemical and physical properties, which depends on their electronic, thermal and structural properties [1-3]. In the beginning stage, graphene is found to be a good candidate for potential application in future nanoelectronic devices owing to its ultra-high charge carrier density and mobility. Even though, the lack of energy band gap obstructs its use in Boolean logic applications, which requires a high current on/off ratio. While different techniques have been reported to open the energy band gap in graphene [4], most of the works infer the complexity in the fabrication of device and opening the band gap. Owing to this reason, presently the research mainly focuses on other 2D-materials with fascinating properties. Phosphorene, a single-layer of black phosphorus (BP), have been recently initiated new possibilities. The sizable band gap of phosphorene is found to be approximately 2 eV [5] and it has high anisotropic transport [6], negative Poisson’s ratio [7], high carrier mobility [8], thermo-electric and optical responses [9]. Theoretical and experimental investigations on single-layer and thin film structures of blue and black phosphorus [8] have originated the search for analogous phosphorus nanostructures and in other group-V pnictogens. Recent computational studies revealed that phosphorus [10], nitrogen [11], bismuth [12], antimony [13] and arsenic [14] can form washboard, buckled honeycomb or single-layered structures. Preferably, arsenic, phosphorus, bismuth and antimony have stable three-dimensional layered crystallographic nanostructures, which confirm efforts to prepare very thin or mono-layer of these specified elements. These deeds prompt researchers to think whether the antimony and arsenic elements can also exhibit stable 2D nanosheets owing to their electronic configuration very similar to that of phosphorus element. The chair- and stirrup-shaped monolayer conformers of antimonene [13] and arsenene
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[14] are also found to be stable in structure. Like layered black-phosphorene, antimonene and arsenene also have unique properties, including ultra-high mechanical stretch ability, high carrier mobility, negative Poisson’s ratio and controllable topological-phase-transition [15, 16]. In contrast to high-carrier-mobility observed in 2D arsenene [15], extracting 2D antimonene and arsenene into nanoribbons would give small carrier mobilities [17]. Recently, Kamal et al. [14] reported about the first-principles study on monolayers of arsenic with buckled honeycomb structure and symmetric arsenic washboard structures, named as arsenene. They also confirmed that these single-layer conformers are more stable in structure and showed a semiconductor nature with the band gaps observed to be around 1.5-2.10 eV. At present, density functional theory (DFT) study provides a good understanding of arsenene (As) [14, 18], even then the electronic properties of arsenene are still to be explored. Arsenene a single-layer arsenic conformer was computationally reported by Kamal and Zhang et al. in the year 2015 [14, 19]. It is more stable in two kinds of hexagonal nanostructures, the so-called puckered and buckled phases. In addition, the buckled phase hexagonal structure is slightly more stable rather than puckered phase [14]. Hsu-Sheng Tsai et al [20] synthesized arsenene nanoribbons on InAs substrate using plasma assisted technique. The authors report that the thickness of layers can be controlled by plasma exposure time. A. J. Mannix et al [21] reported about the synthesis and chemistry of elemental two-dimensional materials using physical vapor deposition (PVD), micromechanical cleavage, and chemical vapor deposition (CVD). M. Pumera and Z. Sofer [22] studied two-dimensional arsenene, antimonene and bismuthene. The authors have given detailed insights on preparatory methods, size effects, doping and device development. Xiaotian Sun et al [23] studied the structures, mobility and electronic properties of arsense, antimonene and antimony arsenide alloy with point defects. M,
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Noei and A. A. Peyghan [24] have extensively studied the adsorption behavior of formaldehyde on BC2N nanotubes, synthesis of aluminium nitride nanotubes [25], adsorption of paranitrophenol on pristine and Al-doped boron nitride (BN) sheets [26] and sensing properties of 4chloroaniline on BN nanotubes [27]. These reports provide the possible application of nanosheets and nanotube for various applications in chemical sensors. Recent DFT studies confirmed its indirect-to-direct band gap transition under oxidation [28] and biaxial strain [14, 19]. On hydrogenation [29] or by tensile strain [30], it is transformed into a quantum-spin-Hall insulator [QSHI] and interstitial or substitution doping prominently change its magnetic and electronic properties [18]. Wang and co-workers [17] observed that the band gap characteristics of arsenene were highly related to directions and edge shapes of arsenene nanoribbon. Kou et al. [31] described that the layered arsenic configuration possesses stable, low-buckled 2D nanostructure. Kecik et al [32] proposed the structural stability, electronic and mechanical properties of single and multilayered arsenene nanostructures using DFT study. In the present study, the electronic properties of functionalized buckled arsenene nanoribbon with hydrogen and for the first time functionalized arsenene nanotubes are investigated and reported. 2. Computation details The first-principles calculations on bare and hydrogenated nanoribbon and nanotube form of arsenene are studied with density functional theory (DFT) method using Atomistix Toolkit (ATK) package [33], which utilizes SIESTA code [34]. For all calculations, local structural relaxation was performed with the help of DFT technique implemented by generalized gradient approximation (GGA) in combination with Perdew-Burke-Ernzerhof (PBE) exchange correlation functional in ATK package [35, 36]. This exchange correlation functional is prominent for studying arsenene, which is also validated with reported work [14]. The dispersion corrected
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density functional theory technique demonstrated by Grimme has been utilized in order to achieve the chemical functionalization on arsenene nanoribbon by hydrogen element [37]. Preferably, a plane-wave basis set along with energy cutoff of 500 eV is used and a vacuum slab of minimum ~18 Å was taken. The hydrogenated arsenene nanoribbon and nanotube were optimized until force and energy is converged to 0.01 eV/Å and 10-6eV respectively. The Brillouin zones of arsenene are sampled with 13 x 13 x 1 Monkhorst-pack k-points with 6 x 6 x 1 super cell size of 64 atoms. The electronic band structures, electron localization function (ELF) and projected density of states of hydrogenated arsenene nanoribbon and nanotubes for zigzag and armchair conformers were calculated with SIESTA code. In the present work, double zeta polarization (DZP) basis set [38] is utilized, while relaxing arsenene nanoribbon and nanotube. 3. Results and discussion 3.1. Geometric structures of hydrogenated nanoribbon and nanotube arsenene conformers The hydrogenated zigzag and armchair arsenene nanoribbon and nanotube possess low buckled conformers with high anisotropic corrugation, in which the adjacent row As atoms are buckled alternatively along zigzag and armchair directions respectively. We, therefore, passivated each terminated end of arsenene nanoribbon with hydrogen. The optimized buckled structure of pristine arsenene nanoribbon of armchair and zigzag conformers is shown in Figure 1 & 2 respectively. In the relaxed unit cell, the calculated lattice constant a = b = 3.60 Å. The AsAs-As bond angle, As-As bond length and buckling height, were found to be 92.1˚, 2.52 Å and 1.40 Å, respectively, which is in good agreement with reported theoretical works [14, 19]. The armchair buckled arsenene conformer is analogous to silicene and blue phosphorene [39-41]. Succeeding the previous agreement of nanoribbons [42], the zigzag and armchair arsenene nanoribbons are constructed by slicing the sheets along < 2110 > and < 1100 > directions. In
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Fig. 3 & 4, the corresponding terminated end atoms of armchair and zigzag arsenene nanoribbons are chemically functionalized by H atoms for removing dangling bonds and reinforce the stabilities of terminated edges [43]. In order to construct the arsenene nanotube, first the above mentioned pristine arsenene nanoribbons were constructed. Next by wrapping-up the two-dimensional nanoribbon towards a chiral vector direction, the arsenene nanotubes are formed through joining its terminating edges as shown in Fig. 5 and 6. Furthermore, like carbon nanotube (CNT), arsenene nanotube possesses two forms of edge conformers, namely armchair and zigzag edges as shown in Fig. 5 and 6 respectively. From the previously reported work of silicene nanotube, the electronic properties get modified with respect to the value of chiral vector (n, m). The main focus of the present work is to calculate the formation energy, energy band structure, projected density of states and electron density of arsenene nanoribbons and nanotubes (both zigzag and armchair) in order to study the electronic properties and structural stability of the system. 3.2. Structural stability and electronic properties of arsenene nano-conformers The structural stability of pristine arsenene nanotubes, bare and hydrogenated arsenene nanoribbons is described in terms of formation energy [44, 45] as shown in equation (1) & (2), Eform = 1/n [E (bare-As) – n E(As)]
(1) (for bare arsenene nanotubes and nanoribbons)
Eform = 1/n [E (H-As) – x E(As) – y E(H)]
(2) (for hydrogenated arsenene nanoribbons)
where E (bare-As) and E (H-As) represent the total energy of bare and hydrogenated arsenene nano-conformers respectively. E (As) and E (H) refers the energy of As and H atoms respectively. ‘n’ is the total number of atoms in arsenene conformers, including passivated hydrogen atoms. In addition, x and y infer the number of As and H atoms respectively. In the first case, bare arsenene conformers (n = x) and for the second case of hydrogenated arsenene
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nanoribbons (n = x + y). The formation energies of arsenene nanotubes and nanoribbons are tabulated in Table1. The formation energy of bare zigzag arsenene nanoribbon and nanotube is found to be -6.197 and -5.095 eV respectively. Similarly, the corresponding formation energy of bare armchair arsenene nanoribbon and nanotube are -6.16 and -5.057 eV. In the case of hydrogenated arsenene nanoribbons of zigzag and armchair conformers, the formation energy is observed to be around -5.653 and -5.734 eV respectively. It is clearly confirmed that both bare and hydrogenated arsenene nanoribbons are highly stable in structure rather than arsenene nanotubes. Owing to decrease in formation energy, the stability of As-nanotubes slightly decreases; however it exhibits stable structures. Ming-Yang Liu et al [46] studied the electronic structure of doped and alloyed arsenene nanosheets using ab initio molecular dynamics, which confirms the stability of arsenene nanosheets in terms of formation energy. It is inferred that the stability of arsenene conformers differs with respect to border termination and functionalization of arsenene nanostructures and these can be used for quantum-spin-Hall devices, chemical gas/vapor sensor and spintronics devices. The electronic properties of arsenene conformers are investigated in terms of energy band structure and projected density of states (PDOS) spectrum [47]. On the basis of density functional theory, pristine arsenene armchair nanoribbon shows a zero energy band gap, whereas the functionalization with hydrogen along the armchair edges of arsenene nanoribbons leads to open the energy band gap of about 1.0 eV as shown in Fig. (7) and (8) respectively. In order to ascertain the energy gap of arsenene nanoribbon and nanotubes conformers, different exchange correlation functional is calculated and tabulated in Table 2. Interestingly, for pristine zigzag buckled arsenene nanoribbon, the band gap is observed to be 1.13 eV and on functionalization with hydrogen along the zigzag borders, it becomes metallic for arsenene nanoribbons as shown
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in Fig. (9) and (10) respectively. In general, zigzag based nanoribbons are semiconductors in nature with spin polarized edges, since, the zigzag edges gives the edge localized state along with non-bonding molecular-orbitals closer to Fermi level energy. While armchairs nanoribbons can exist in metallic or semiconductor, based on their width. For both edge oriented nanoribbons, the band gap size increases with decreasing width of the nanoribbons [4, 48]. In addition, in the case of bare arsenene nanotubes for both conformers, band gap opens to 0.38 for armchair and 0.69 for zigzag borders as shown in Fig. (11) and (12) respectively. It is inferred that the electronic properties of arsenene nanostructure significantly changes due to armchair or zigzag borders. In general, the electronic properties of arsenene nanoribbons can be tuned with their borders and width of the conformer [43]. The electronic band gap of nanoribbons also varies with respect to passivation of edges with different functional groups [49]. In addition, it is observed that the energy band gap differs inversely with nanoribbon width [4]. Moreover, the electronic properties of arsenene nanoribbons are strongly influenced with orientation of nanoribbons [43]. In the present work, we have chosen the width of the zigzag and armchair arsenene nanoribbon as n = 8 and n = 16 respectively as shown in Fig. S1 (refer supplementary information). From the observation, it is clearly revealed that the electronic properties of arsenene nanoribbons can be fine-tuned with the width and orientation of edges in nanoribbons. In addition, the energy band gap also varies owing to the passivation of hydrogen on their respective nanoribbon edges. For arsenene nanotubes, the electronic properties get changed with respect to the chiral vector (n, m). The electronic band gap is observed along the gamma point (G) for arsenene nano-conformers, which is found to be a direct band gap. Furthermore, the channels along the conduction band minimum (CBM) and valence band maximum (VBM) are in phase along G point, which confirms the direct band gap. Besides, the obtained band gap value
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of arsenene nanoribbons is in good agreement with the proposed work [50]. The band gap opening is observed in arsenene nanostructures owing to their zigzag or armchair edges, passivation of hydrogen along the edges, the width of the nanostructure. The width of the arsenene nanoribbons is inversely proportional to the energy band gap of As-nanoribbons. Since the band gap of arsenene nanoribbon, decreases with increasing the width from n = 8 (zigzag) to n = 16 (armchair). Therefore, the electronic properties of arsenene nano-conformers can be modified by changing the orientation, width and upon passivation of As-nanoribbons and nanotubes, including the chemical functionalization with hydrogen on buckled edges, which can be used in spintronics and chemical sensors. The projected density of states (PDOS) spectrum [51-54] gives the perception on localization of charges in different energy intervals along arsenene nanoribbons and nanotubes. Fig. 13 – 18 illustrates the PDOS spectrum of arsenene nano-conformers (PDOS spectrum is drawn in multi-curve fashion, the scale along y-axis is not given, the magnitude is taken into consideration along the y-axis). The peak maxima are observed near the Fermi level energy (EF) on both zigzag and the armchair orientation of arsenene nanoribbons and nanotubes, which is one of the significant conditions for the potential application for spintronic device and chemical sensor. The peak maximum in different energy intervals arose owing to the orbital overlapping between As atoms in arsenene nano-conformers. As a result from PDOS spectrum, it is clearly confirmed that peak shift is found in the base material owing to the chemical functionalization of hydrogen along the zigzag and armchair borders of arsenene nanoribbons. Furthermore, the peak shift in the conduction band and in the valence band results in variation of electronic properties of arsenene base material, which is suitable for spintronic devices and sensor applications. Fig. S2 – S7 represent the electron density of bare and hydrogenated arsenene nanoribbons and bare
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arsenene nanotubes for both armchair and zigzag conformers. From the visualization of electron density diagram, it is confirmed that the electron density is located to be more in bare arsenene nanotube rather than pristine nanoribbons. Furthermore, the electron density increases in arsenene nanoribbons with passivation of hydrogen owing to the donation of electrons from hydrogen atoms along the As-nanoribbons. However, the density of electrons varies with respect to the width and orientation of arsenene nanoribbons and also with chirality in arsenene nanotubes. From the overall results, we conclude that the structural stability and electronic properties of arsenene nano-conformers can be tailored with different factors, namely orientation, width and passivation with hydrogen along the borders in arsenene nanoribbon and nanotubes. Therefore, arsenene nanoribbons and nanotubes can be utilized for spintronic devices; chemical sensor and quantum spin Hall devices. 4. Conclusion To sum up, using DFT technique, the structural stability, including the electronic properties of arsenene nanoribbons and nanotubes are investigated with the help of ATK package, considering orientation, width and edge passivation effects. Using energy band structure and PDOS spectrum, electronic properties of arsenene nano-conformers are studied. The structural stability of arsenene nano-conformers is confirmed from formation energy, which shows that all conformers of arsenene are stable. The findings of the present work clearly infer that the passivation of arsenene with hydrogen, variation in width of the nanostructure and the orientation of nanoribbons influence the electronic properties and structural stability. The results give the insights on the electronic properties of arsenene nanostructures, which can be modified with orientation, width and passivation along the borders that can be used as a base material for designing spintronic devices and chemical sensor.
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Acknowledgements The authors would like to acknowledge Dr. Arkaprava Bhattacharyya, SEEE, SASTRA University for providing computational facilities and suggestions.
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Figure captions Fig. 1. (a) The schematic diagram of armchair bare arsenene nanoribbon (ABAsNR), (b) zigzag bare arsenene nanoribbon (ZBAsNR), (c) armchair hydrogenated arsenene nanoribbon (AHAsNR), (d) zigzag hydrogenated arsenene nanoribbon (ZHAsNR), (e) armchair bare arsenene nanotube (ABAsNT) and (f) zigzag bare arsenene nanotube (ZBAsNT). Fig. 2. The energy band gap of (a) armchair bare arsenene nanoribbon, (b) armchair hydrogenated arsenene nanoribbon, (c) zigzag bare arsenene nanoribbon, (d) zigzag hydrogenated arsenene nanoribbon, (e) armchair bare arsenene nanotube and (f) zigzag bare arsenene nanotube Fig. 3. The projected density of states (PDOS) spectrum of (a) armchair bare arsenene nanoribbon, (b) armchair hydrogenated arsenene nanoribbon, (c) zigzag bare arsenene nanoribbon, (d) zigzag hydrogenated arsenene nanoribbon, (e) spectrum of armchair bare arsenene nanotube and (f) spectrum of zigzag bare arsenene nanotube. (black-s, red-p, blue-d and green-TDOS)
Table caption Table 1. Formation energy and energy band structure of arsenene nano-conformers Table 2. Energy gap calculation for arsenene base material with different exchange correlation functional
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Fig. 1. (a) The schematic diagram of armchair bare arsenene nanoribbon (ABAsNR), (b) zigzag bare arsenene nanoribbon (ZBAsNR), (c) armchair hydrogenated arsenene nanoribbon (AHAsNR), (d) zigzag hydrogenated arsenene nanoribbon (ZHAsNR), (e) armchair bare arsenene nanotube (ABAsNT) and (f) zigzag bare arsenene nanotube (ZBAsNT).
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Fig. 2. The energy band gap of (a) armchair bare arsenene nanoribbon, (b) armchair hydrogenated arsenene nanoribbon, (c) zigzag bare arsenene nanoribbon, (d) zigzag hydrogenated arsenene nanoribbon, (e) armchair bare arsenene nanotube and (f) zigzag bare arsenene nanotube.
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Fig. 3. The projected density of states (PDOS) spectrum of (a) armchair bare arsenene nanoribbon, (b) armchair hydrogenated arsenene nanoribbon, (c) zigzag bare arsenene nanoribbon, (d) zigzag hydrogenated arsenene nanoribbon, (e) spectrum of armchair bare arsenene nanotube and (f) spectrum of zigzag bare arsenene nanotube. (black-s, red-p, blue-d and green-TDOS)
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Table 1. Formation energy and energy band structure of arsenene nano-conformers
Nanostructures
Band gap
Formation energy
Bare Arsenene nanoribbon
0
-6.16
Hydrogenated Arsenene nanoribbon
1.00
-5.734
Arsenene nanotube
0.38
-5.057
Bare Arsenene nanoribbon
1.13
-6.197
Hydrogenated Arsenene nanoribbon
0
-5.653
Arsenene nanotube
0.69
-5.095
Armchair conformer
Zigzag conformer
Table 2. Energy gap calculation for arsenene base material with different exchange correlation functional Nanostructures
PBE
BLYP PBEsol RPBE revPBE Type
Bare Arsenene nanoribbon
0.00
0.00
0.00
0.00
0.00
Hydrogenated Arsenene nanoribbon
1.00
1.00
0.96
1.01
1.01
GGA
Arsenene nanotube
0.38
0.39
0.32
0.41
0.4
GGA
Bare Arsenene nanoribbon
1.13
1.14
1.08
1.15
1.14
GGA
Hydrogenated Arsenene nanoribbon
0.00
0.00
0.00
0.00
0.00
GGA
Arsenene nanotube
0.69
0.67
0.67
0.70
0.70
GGA
Armchair conformer GGA
Zigzag conformer
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Graphical Abstract
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Research Highlights
The electronic properties of arsenene nanoribbon and nanotubes are studied. The electronic properties are influenced by zigzag and armchair borders. The orientation, width and passivation of borders modify the electronic properties. The findings show that arsenene nanoribbon and nanotubes can be used for spintronics and chemical sensors.
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S0301-0104(17)30439-1 http://dx.doi.org/10.1016/j.chemphys.2017.08.007 CHEMPH 9839
To appear in:
Chemical Physics
Received Date: Revised Date: Accepted Date:
28 May 2017 8 August 2017 20 August 2017
Please cite this article as: V. Nagarajan, R. Chandiramouli, Investigation on electronic properties of functionalized arsenene nanoribbon and nanotubes: a first-principles study, Chemical Physics (2017), doi: http://dx.doi.org/ 10.1016/j.chemphys.2017.08.007
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Investigation on electronic properties of functionalized arsenene nanoribbon and nanotubes: a first-principles study
V. Nagarajan, R. Chandiramouli* School of Electrical & Electronics Engineering SASTRA University, Tirumalaisamudram, Thanjavur -613 401, India
*Corresponding Author: Prof. R. Chandiramouli, School of Electrical & Electronics Engineering, SASTRA University Tel: +919489566466 Fax.:+91-4362-264120 E-mail: [email protected]
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ABSTRACT The electronic properties of arsenene nanotubes and nanoribbons with hydrogenation along the zigzag and armchair edges are studied using density functional theory (DFT) technique. The structural stability of hydrogenated zigzag and armchair arsenene nanostructures are confirmed with formation energy. The electronic properties of arsenene nano-conformers are described in terms of energy band structure and projected density of states spectrum. Furthermore, owing to the influence of hydrogen passivation, buckled orientation and width of arsenene nanostructures, the band gap widens in the range of 0.38 – 1.13 eV. The findings of the present work confirm that the electronic properties of arsenene nanomaterial, can be fine-tuned with the influence of passivation with hydrogen, zigzag or armchair border shapes and effect of the width of nanoribbons or nanotubes, which can be utilized as spintronic device and chemical sensor.
Keywords: nanoribbon; nanotube; arsenene; stability; band structure
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1. Introduction Like graphene, other two-dimensional materials have attracted great interest due to their chemical and physical properties, which depends on their electronic, thermal and structural properties [1-3]. In the beginning stage, graphene is found to be a good candidate for potential application in future nanoelectronic devices owing to its ultra-high charge carrier density and mobility. Even though, the lack of energy band gap obstructs its use in Boolean logic applications, which requires a high current on/off ratio. While different techniques have been reported to open the energy band gap in graphene [4], most of the works infer the complexity in the fabrication of device and opening the band gap. Owing to this reason, presently the research mainly focuses on other 2D-materials with fascinating properties. Phosphorene, a single-layer of black phosphorus (BP), have been recently initiated new possibilities. The sizable band gap of phosphorene is found to be approximately 2 eV [5] and it has high anisotropic transport [6], negative Poisson’s ratio [7], high carrier mobility [8], thermo-electric and optical responses [9]. Theoretical and experimental investigations on single-layer and thin film structures of blue and black phosphorus [8] have originated the search for analogous phosphorus nanostructures and in other group-V pnictogens. Recent computational studies revealed that phosphorus [10], nitrogen [11], bismuth [12], antimony [13] and arsenic [14] can form washboard, buckled honeycomb or single-layered structures. Preferably, arsenic, phosphorus, bismuth and antimony have stable three-dimensional layered crystallographic nanostructures, which confirm efforts to prepare very thin or mono-layer of these specified elements. These deeds prompt researchers to think whether the antimony and arsenic elements can also exhibit stable 2D nanosheets owing to their electronic configuration very similar to that of phosphorus element. The chair- and stirrup-shaped monolayer conformers of antimonene [13] and arsenene
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[14] are also found to be stable in structure. Like layered black-phosphorene, antimonene and arsenene also have unique properties, including ultra-high mechanical stretch ability, high carrier mobility, negative Poisson’s ratio and controllable topological-phase-transition [15, 16]. In contrast to high-carrier-mobility observed in 2D arsenene [15], extracting 2D antimonene and arsenene into nanoribbons would give small carrier mobilities [17]. Recently, Kamal et al. [14] reported about the first-principles study on monolayers of arsenic with buckled honeycomb structure and symmetric arsenic washboard structures, named as arsenene. They also confirmed that these single-layer conformers are more stable in structure and showed a semiconductor nature with the band gaps observed to be around 1.5-2.10 eV. At present, density functional theory (DFT) study provides a good understanding of arsenene (As) [14, 18], even then the electronic properties of arsenene are still to be explored. Arsenene a single-layer arsenic conformer was computationally reported by Kamal and Zhang et al. in the year 2015 [14, 19]. It is more stable in two kinds of hexagonal nanostructures, the so-called puckered and buckled phases. In addition, the buckled phase hexagonal structure is slightly more stable rather than puckered phase [14]. Hsu-Sheng Tsai et al [20] synthesized arsenene nanoribbons on InAs substrate using plasma assisted technique. The authors report that the thickness of layers can be controlled by plasma exposure time. A. J. Mannix et al [21] reported about the synthesis and chemistry of elemental two-dimensional materials using physical vapor deposition (PVD), micromechanical cleavage, and chemical vapor deposition (CVD). M. Pumera and Z. Sofer [22] studied two-dimensional arsenene, antimonene and bismuthene. The authors have given detailed insights on preparatory methods, size effects, doping and device development. Xiaotian Sun et al [23] studied the structures, mobility and electronic properties of arsense, antimonene and antimony arsenide alloy with point defects. M,
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Noei and A. A. Peyghan [24] have extensively studied the adsorption behavior of formaldehyde on BC2N nanotubes, synthesis of aluminium nitride nanotubes [25], adsorption of paranitrophenol on pristine and Al-doped boron nitride (BN) sheets [26] and sensing properties of 4chloroaniline on BN nanotubes [27]. These reports provide the possible application of nanosheets and nanotube for various applications in chemical sensors. Recent DFT studies confirmed its indirect-to-direct band gap transition under oxidation [28] and biaxial strain [14, 19]. On hydrogenation [29] or by tensile strain [30], it is transformed into a quantum-spin-Hall insulator [QSHI] and interstitial or substitution doping prominently change its magnetic and electronic properties [18]. Wang and co-workers [17] observed that the band gap characteristics of arsenene were highly related to directions and edge shapes of arsenene nanoribbon. Kou et al. [31] described that the layered arsenic configuration possesses stable, low-buckled 2D nanostructure. Kecik et al [32] proposed the structural stability, electronic and mechanical properties of single and multilayered arsenene nanostructures using DFT study. In the present study, the electronic properties of functionalized buckled arsenene nanoribbon with hydrogen and for the first time functionalized arsenene nanotubes are investigated and reported. 2. Computation details The first-principles calculations on bare and hydrogenated nanoribbon and nanotube form of arsenene are studied with density functional theory (DFT) method using Atomistix Toolkit (ATK) package [33], which utilizes SIESTA code [34]. For all calculations, local structural relaxation was performed with the help of DFT technique implemented by generalized gradient approximation (GGA) in combination with Perdew-Burke-Ernzerhof (PBE) exchange correlation functional in ATK package [35, 36]. This exchange correlation functional is prominent for studying arsenene, which is also validated with reported work [14]. The dispersion corrected
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density functional theory technique demonstrated by Grimme has been utilized in order to achieve the chemical functionalization on arsenene nanoribbon by hydrogen element [37]. Preferably, a plane-wave basis set along with energy cutoff of 500 eV is used and a vacuum slab of minimum ~18 Å was taken. The hydrogenated arsenene nanoribbon and nanotube were optimized until force and energy is converged to 0.01 eV/Å and 10-6eV respectively. The Brillouin zones of arsenene are sampled with 13 x 13 x 1 Monkhorst-pack k-points with 6 x 6 x 1 super cell size of 64 atoms. The electronic band structures, electron localization function (ELF) and projected density of states of hydrogenated arsenene nanoribbon and nanotubes for zigzag and armchair conformers were calculated with SIESTA code. In the present work, double zeta polarization (DZP) basis set [38] is utilized, while relaxing arsenene nanoribbon and nanotube. 3. Results and discussion 3.1. Geometric structures of hydrogenated nanoribbon and nanotube arsenene conformers The hydrogenated zigzag and armchair arsenene nanoribbon and nanotube possess low buckled conformers with high anisotropic corrugation, in which the adjacent row As atoms are buckled alternatively along zigzag and armchair directions respectively. We, therefore, passivated each terminated end of arsenene nanoribbon with hydrogen. The optimized buckled structure of pristine arsenene nanoribbon of armchair and zigzag conformers is shown in Figure 1 & 2 respectively. In the relaxed unit cell, the calculated lattice constant a = b = 3.60 Å. The AsAs-As bond angle, As-As bond length and buckling height, were found to be 92.1˚, 2.52 Å and 1.40 Å, respectively, which is in good agreement with reported theoretical works [14, 19]. The armchair buckled arsenene conformer is analogous to silicene and blue phosphorene [39-41]. Succeeding the previous agreement of nanoribbons [42], the zigzag and armchair arsenene nanoribbons are constructed by slicing the sheets along < 2110 > and < 1100 > directions. In
6
Fig. 3 & 4, the corresponding terminated end atoms of armchair and zigzag arsenene nanoribbons are chemically functionalized by H atoms for removing dangling bonds and reinforce the stabilities of terminated edges [43]. In order to construct the arsenene nanotube, first the above mentioned pristine arsenene nanoribbons were constructed. Next by wrapping-up the two-dimensional nanoribbon towards a chiral vector direction, the arsenene nanotubes are formed through joining its terminating edges as shown in Fig. 5 and 6. Furthermore, like carbon nanotube (CNT), arsenene nanotube possesses two forms of edge conformers, namely armchair and zigzag edges as shown in Fig. 5 and 6 respectively. From the previously reported work of silicene nanotube, the electronic properties get modified with respect to the value of chiral vector (n, m). The main focus of the present work is to calculate the formation energy, energy band structure, projected density of states and electron density of arsenene nanoribbons and nanotubes (both zigzag and armchair) in order to study the electronic properties and structural stability of the system. 3.2. Structural stability and electronic properties of arsenene nano-conformers The structural stability of pristine arsenene nanotubes, bare and hydrogenated arsenene nanoribbons is described in terms of formation energy [44, 45] as shown in equation (1) & (2), Eform = 1/n [E (bare-As) – n E(As)]
(1) (for bare arsenene nanotubes and nanoribbons)
Eform = 1/n [E (H-As) – x E(As) – y E(H)]
(2) (for hydrogenated arsenene nanoribbons)
where E (bare-As) and E (H-As) represent the total energy of bare and hydrogenated arsenene nano-conformers respectively. E (As) and E (H) refers the energy of As and H atoms respectively. ‘n’ is the total number of atoms in arsenene conformers, including passivated hydrogen atoms. In addition, x and y infer the number of As and H atoms respectively. In the first case, bare arsenene conformers (n = x) and for the second case of hydrogenated arsenene
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nanoribbons (n = x + y). The formation energies of arsenene nanotubes and nanoribbons are tabulated in Table1. The formation energy of bare zigzag arsenene nanoribbon and nanotube is found to be -6.197 and -5.095 eV respectively. Similarly, the corresponding formation energy of bare armchair arsenene nanoribbon and nanotube are -6.16 and -5.057 eV. In the case of hydrogenated arsenene nanoribbons of zigzag and armchair conformers, the formation energy is observed to be around -5.653 and -5.734 eV respectively. It is clearly confirmed that both bare and hydrogenated arsenene nanoribbons are highly stable in structure rather than arsenene nanotubes. Owing to decrease in formation energy, the stability of As-nanotubes slightly decreases; however it exhibits stable structures. Ming-Yang Liu et al [46] studied the electronic structure of doped and alloyed arsenene nanosheets using ab initio molecular dynamics, which confirms the stability of arsenene nanosheets in terms of formation energy. It is inferred that the stability of arsenene conformers differs with respect to border termination and functionalization of arsenene nanostructures and these can be used for quantum-spin-Hall devices, chemical gas/vapor sensor and spintronics devices. The electronic properties of arsenene conformers are investigated in terms of energy band structure and projected density of states (PDOS) spectrum [47]. On the basis of density functional theory, pristine arsenene armchair nanoribbon shows a zero energy band gap, whereas the functionalization with hydrogen along the armchair edges of arsenene nanoribbons leads to open the energy band gap of about 1.0 eV as shown in Fig. (7) and (8) respectively. In order to ascertain the energy gap of arsenene nanoribbon and nanotubes conformers, different exchange correlation functional is calculated and tabulated in Table 2. Interestingly, for pristine zigzag buckled arsenene nanoribbon, the band gap is observed to be 1.13 eV and on functionalization with hydrogen along the zigzag borders, it becomes metallic for arsenene nanoribbons as shown
8
in Fig. (9) and (10) respectively. In general, zigzag based nanoribbons are semiconductors in nature with spin polarized edges, since, the zigzag edges gives the edge localized state along with non-bonding molecular-orbitals closer to Fermi level energy. While armchairs nanoribbons can exist in metallic or semiconductor, based on their width. For both edge oriented nanoribbons, the band gap size increases with decreasing width of the nanoribbons [4, 48]. In addition, in the case of bare arsenene nanotubes for both conformers, band gap opens to 0.38 for armchair and 0.69 for zigzag borders as shown in Fig. (11) and (12) respectively. It is inferred that the electronic properties of arsenene nanostructure significantly changes due to armchair or zigzag borders. In general, the electronic properties of arsenene nanoribbons can be tuned with their borders and width of the conformer [43]. The electronic band gap of nanoribbons also varies with respect to passivation of edges with different functional groups [49]. In addition, it is observed that the energy band gap differs inversely with nanoribbon width [4]. Moreover, the electronic properties of arsenene nanoribbons are strongly influenced with orientation of nanoribbons [43]. In the present work, we have chosen the width of the zigzag and armchair arsenene nanoribbon as n = 8 and n = 16 respectively as shown in Fig. S1 (refer supplementary information). From the observation, it is clearly revealed that the electronic properties of arsenene nanoribbons can be fine-tuned with the width and orientation of edges in nanoribbons. In addition, the energy band gap also varies owing to the passivation of hydrogen on their respective nanoribbon edges. For arsenene nanotubes, the electronic properties get changed with respect to the chiral vector (n, m). The electronic band gap is observed along the gamma point (G) for arsenene nano-conformers, which is found to be a direct band gap. Furthermore, the channels along the conduction band minimum (CBM) and valence band maximum (VBM) are in phase along G point, which confirms the direct band gap. Besides, the obtained band gap value
9
of arsenene nanoribbons is in good agreement with the proposed work [50]. The band gap opening is observed in arsenene nanostructures owing to their zigzag or armchair edges, passivation of hydrogen along the edges, the width of the nanostructure. The width of the arsenene nanoribbons is inversely proportional to the energy band gap of As-nanoribbons. Since the band gap of arsenene nanoribbon, decreases with increasing the width from n = 8 (zigzag) to n = 16 (armchair). Therefore, the electronic properties of arsenene nano-conformers can be modified by changing the orientation, width and upon passivation of As-nanoribbons and nanotubes, including the chemical functionalization with hydrogen on buckled edges, which can be used in spintronics and chemical sensors. The projected density of states (PDOS) spectrum [51-54] gives the perception on localization of charges in different energy intervals along arsenene nanoribbons and nanotubes. Fig. 13 – 18 illustrates the PDOS spectrum of arsenene nano-conformers (PDOS spectrum is drawn in multi-curve fashion, the scale along y-axis is not given, the magnitude is taken into consideration along the y-axis). The peak maxima are observed near the Fermi level energy (EF) on both zigzag and the armchair orientation of arsenene nanoribbons and nanotubes, which is one of the significant conditions for the potential application for spintronic device and chemical sensor. The peak maximum in different energy intervals arose owing to the orbital overlapping between As atoms in arsenene nano-conformers. As a result from PDOS spectrum, it is clearly confirmed that peak shift is found in the base material owing to the chemical functionalization of hydrogen along the zigzag and armchair borders of arsenene nanoribbons. Furthermore, the peak shift in the conduction band and in the valence band results in variation of electronic properties of arsenene base material, which is suitable for spintronic devices and sensor applications. Fig. S2 – S7 represent the electron density of bare and hydrogenated arsenene nanoribbons and bare
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arsenene nanotubes for both armchair and zigzag conformers. From the visualization of electron density diagram, it is confirmed that the electron density is located to be more in bare arsenene nanotube rather than pristine nanoribbons. Furthermore, the electron density increases in arsenene nanoribbons with passivation of hydrogen owing to the donation of electrons from hydrogen atoms along the As-nanoribbons. However, the density of electrons varies with respect to the width and orientation of arsenene nanoribbons and also with chirality in arsenene nanotubes. From the overall results, we conclude that the structural stability and electronic properties of arsenene nano-conformers can be tailored with different factors, namely orientation, width and passivation with hydrogen along the borders in arsenene nanoribbon and nanotubes. Therefore, arsenene nanoribbons and nanotubes can be utilized for spintronic devices; chemical sensor and quantum spin Hall devices. 4. Conclusion To sum up, using DFT technique, the structural stability, including the electronic properties of arsenene nanoribbons and nanotubes are investigated with the help of ATK package, considering orientation, width and edge passivation effects. Using energy band structure and PDOS spectrum, electronic properties of arsenene nano-conformers are studied. The structural stability of arsenene nano-conformers is confirmed from formation energy, which shows that all conformers of arsenene are stable. The findings of the present work clearly infer that the passivation of arsenene with hydrogen, variation in width of the nanostructure and the orientation of nanoribbons influence the electronic properties and structural stability. The results give the insights on the electronic properties of arsenene nanostructures, which can be modified with orientation, width and passivation along the borders that can be used as a base material for designing spintronic devices and chemical sensor.
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Acknowledgements The authors would like to acknowledge Dr. Arkaprava Bhattacharyya, SEEE, SASTRA University for providing computational facilities and suggestions.
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Figure captions Fig. 1. (a) The schematic diagram of armchair bare arsenene nanoribbon (ABAsNR), (b) zigzag bare arsenene nanoribbon (ZBAsNR), (c) armchair hydrogenated arsenene nanoribbon (AHAsNR), (d) zigzag hydrogenated arsenene nanoribbon (ZHAsNR), (e) armchair bare arsenene nanotube (ABAsNT) and (f) zigzag bare arsenene nanotube (ZBAsNT). Fig. 2. The energy band gap of (a) armchair bare arsenene nanoribbon, (b) armchair hydrogenated arsenene nanoribbon, (c) zigzag bare arsenene nanoribbon, (d) zigzag hydrogenated arsenene nanoribbon, (e) armchair bare arsenene nanotube and (f) zigzag bare arsenene nanotube Fig. 3. The projected density of states (PDOS) spectrum of (a) armchair bare arsenene nanoribbon, (b) armchair hydrogenated arsenene nanoribbon, (c) zigzag bare arsenene nanoribbon, (d) zigzag hydrogenated arsenene nanoribbon, (e) spectrum of armchair bare arsenene nanotube and (f) spectrum of zigzag bare arsenene nanotube. (black-s, red-p, blue-d and green-TDOS)
Table caption Table 1. Formation energy and energy band structure of arsenene nano-conformers Table 2. Energy gap calculation for arsenene base material with different exchange correlation functional
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Fig. 1. (a) The schematic diagram of armchair bare arsenene nanoribbon (ABAsNR), (b) zigzag bare arsenene nanoribbon (ZBAsNR), (c) armchair hydrogenated arsenene nanoribbon (AHAsNR), (d) zigzag hydrogenated arsenene nanoribbon (ZHAsNR), (e) armchair bare arsenene nanotube (ABAsNT) and (f) zigzag bare arsenene nanotube (ZBAsNT).
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Fig. 2. The energy band gap of (a) armchair bare arsenene nanoribbon, (b) armchair hydrogenated arsenene nanoribbon, (c) zigzag bare arsenene nanoribbon, (d) zigzag hydrogenated arsenene nanoribbon, (e) armchair bare arsenene nanotube and (f) zigzag bare arsenene nanotube.
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Fig. 3. The projected density of states (PDOS) spectrum of (a) armchair bare arsenene nanoribbon, (b) armchair hydrogenated arsenene nanoribbon, (c) zigzag bare arsenene nanoribbon, (d) zigzag hydrogenated arsenene nanoribbon, (e) spectrum of armchair bare arsenene nanotube and (f) spectrum of zigzag bare arsenene nanotube. (black-s, red-p, blue-d and green-TDOS)
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Table 1. Formation energy and energy band structure of arsenene nano-conformers
Nanostructures
Band gap
Formation energy
Bare Arsenene nanoribbon
0
-6.16
Hydrogenated Arsenene nanoribbon
1.00
-5.734
Arsenene nanotube
0.38
-5.057
Bare Arsenene nanoribbon
1.13
-6.197
Hydrogenated Arsenene nanoribbon
0
-5.653
Arsenene nanotube
0.69
-5.095
Armchair conformer
Zigzag conformer
Table 2. Energy gap calculation for arsenene base material with different exchange correlation functional Nanostructures
PBE
BLYP PBEsol RPBE revPBE Type
Bare Arsenene nanoribbon
0.00
0.00
0.00
0.00
0.00
Hydrogenated Arsenene nanoribbon
1.00
1.00
0.96
1.01
1.01
GGA
Arsenene nanotube
0.38
0.39
0.32
0.41
0.4
GGA
Bare Arsenene nanoribbon
1.13
1.14
1.08
1.15
1.14
GGA
Hydrogenated Arsenene nanoribbon
0.00
0.00
0.00
0.00
0.00
GGA
Arsenene nanotube
0.69
0.67
0.67
0.70
0.70
GGA
Armchair conformer GGA
Zigzag conformer
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Graphical Abstract
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Research Highlights
The electronic properties of arsenene nanoribbon and nanotubes are studied. The electronic properties are influenced by zigzag and armchair borders. The orientation, width and passivation of borders modify the electronic properties. The findings show that arsenene nanoribbon and nanotubes can be used for spintronics and chemical sensors.
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