Ab initio study on the electromechanical response of Janus transition metal dihalide nanotubes Arpit Bhardwaj and Phanish Suryanarayana

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Ab initio study on the electromechanical response of Janus

transition metal dihalide nanotubes

Arpit Bhardwaj and Phanish Suryanarayana∗

College of Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

Abstract

We study the electronic response of Janus transition metal dihalide (TMH) nanotubes to me-

chanical deformations using Kohn-Sham density functional theory. Speciﬁcally, considering twelve

armchair and zigzag Janus TMH nanotubes that are expected to be stable from the phonon analysis

of ﬂat monolayer counterparts, we ﬁrst compute their equilibrium diameters and then determine

the variation in bandgap and eﬀective mass of charge carriers with the application of tensile and

torsional deformations. We ﬁnd that the nanotubes undergo a linear and quadratic decrease in

bandgap with tensile and shear strain, respectively. In addition, there is a continual increase and

decrease in the eﬀective mass of electrons and holes, respectively. We show that for a given strain,

the change in bandgap for the armchair nanotubes can be correlated with the transition metal’s

in-plane dorbital’s contribution to the projected density of states at the bottom of the conduction

band.

arXiv:2210.12747v1 [cond-mat.mtrl-sci] 23 Oct 2022

I. INTRODUCTION

Nanotubes are hollow cylindrical structures whose diameters are the nanometer scale,

with lengths that are orders of magnitude larger. These quasi one-dimensional materi-

als, more than thirty of which have been synthesized since the pioneering work for carbon

nanotubes1, are known to display enhanced and exotic mechanical, electronic, optical, and

thermal properties/behavior relative to their bulk counterparts2–4. Moreover, these prop-

erties diﬀer based on the diameter/chirality of the nanotubes5–14 and can be further con-

trolled/tuned through external stimuli, such as addition of defects15–17, electric ﬁeld18,19,

magnetic ﬁeld20, temperature21–23, and mechanical deformations24–33, making them particu-

larly suited for technological applications.

Among the nanotubes that have been synthesized, a large fraction are from the transition

metal dichalcogenide (TMD) group2–4, which consists of materials of the form MX2, where

M and X denote the transition metal and chalcogen, respectively. Given the large number of

transition metal (34 in number) and chalcogen (4 in number) combinations (136 in number)

that are possible, it is likely that even more TMD nanotubes will be synthesized in the

future. However, such nanotubes are generally multiwalled and have large diameters that

are typically in the range of 10–40 nm2–4, a consequence of the relatively large bending moduli

of TMD monolayers34. This limits the possibility of fascinating and novel properties that

are associated with quantum conﬁnement eﬀects. Also, the large variations in the diameters

makes systematic theoretical/computational studies extremely challenging.

The Janus TMD nanotube group35, which consists of materials of the form MXY, where

X and Y are diﬀerent chalcogens, overcome many of the limitations of the TMD group.

In particular, given the asymmetry in the system, single-walled Janus nanotubes become

energetically more favorable than their ﬂat counterparts36,37, which signiﬁcantly increases

the likelihood of them being synthesized. Indeed, the WSSe nanotube has been recently

synthesized38. Moreover, they have an energy minimizing diameter, typically in the range

of 3–16 nm37, which is much smaller than the corresponding values for TMD nanotubes,

signiﬁcantly increasing the likelihood of novel and exotic properties. In particular, recent

work on the electromechanical response of Janus TMD nanotubes has shown that they have

potential applications in mechanical sensors and semiconductor switches39, similar to the

case of TMD nanotubes40. It is therefore to be expected that at the very least, Janus TMD

nanotubes also inherit the other applications of their non-Janus counterparts, including

nano-electromechanical (NEMS) devices41–43, biosensors44, superconductive materials22,23,

and photodetectors45–47.

The Janus transition metal dihalide (TMH) nanotube group48, which consists of materials

of the form MXY, where X and Y are now diﬀerent halogens, are likely to possess fascinating

and exciting properties similar to those displayed in ﬂat TMH monolayers and their Janus

variants, e.g., FeCl2is piezoelectric ferromagnetic with the Curie temperature around room

temperature49, and FeClBr and FeClF are ferrovalley materials based on their magnetic

anisotropy50,51. Simultaneously, the nanotubes inherit the aforementioned advantageous fea-

tures of being a Janus structure. However, the properties and behavior of these nanotubes

has not been studied till now, providing the motivation for the current work. In particular,

we study the electronic response of Janus TMH nanotubes to mechanical deformations using

ab initio Kohn-Sham density functional theory (DFT) calculations. Speciﬁcally, consider-

ing twelve armchair and zigzag Janus TMH nanotubes that are expected to be stable, we

ﬁrst compute their equilibrium diameters and then determine the variation in bandgap and

eﬀective mass of charge carriers with the application of tensile and torsional deformations.

Overall, we ﬁnd that mechanical deformations represent a powerful tool for controlling the

electronic properties of Janus TMH nanotubes.

The remainder of this article is organized as follows. In Section II, we list the Janus

TMH nanotubes selected and detail the calculation of their electronic response to mechanical

deformations. The results so obtained are presented and discussed in Section III. Finally,

we provide concluding remarks in Section IV.

II. SYSTEMS AND METHODS

We consider zigzag and armchair variants of the following Janus TMH nanotubes: (i)

M={Ti, Zr, Hf}and X,Y={Cl, Br, I}, with 2H-t symmetry52,53; and (ii) M={Fe}and

X,Y={Cl, Br, I}, with 1T-o symmetry52,53, all having the lighter halogen on the inner

side. These represent the set of all Janus TMH nanotubes that have been predicted to be

thermodynamically stable from ﬁrst principles investigations48.

We perform all nanotube simulations using the Cyclix-DFT54 feature — well tested in

various physical applications34,37,39,40,54–58 — in the state-of-the-art real-space DFT code

SPARC59–61. In this formalism, as illustrated in ﬁgure 1, the cyclic and/or helical symmetry

of the system is exploited to reduce all computations to a unit cell that contains only a small

fraction of the atoms in the traditional periodic unit cell5,54,62, e.g., the periodic unit cell for

a (45,45) HfClBr nanotube with diameter ∼9 nm and an external twist of 6 ×10-4 rad/Bohr

has 169,155 atoms, whereas the cyclic+helical symmetry-adapted unit cell has only 3 atoms

(one of each chemical element), a number that remains unchanged by axial and/or torsional

deformations. This symmetry-adaption provides tremendous computational savings, given

that Kohn-Sham DFT computations scale cubically with system size.

Axial deformation Torsional deformation

Figure 1: Illustration generated using VESTA63 that depicts the inherent cyclic and helical

symmetry of an axially and torsionally deformed (10,10) 1T-o Janus TMH nanotube. The

entire nanotube can be generated using 3 atoms, e.g., metal and halogens colored red and

blue/yellow, respectively, that lie within the cyclic+helical symmetry-adapted unit cell.

This symmetry is exploited while performing Kohn-Sham DFT calculations using the

electronic structure code SPARC’s Cyclix-DFT feature.

In all simulations, we employ the Perdew–Burke–Ernzerhof (PBE)64 exchange-correlation

functional, and scalar relativistic optimized norm-conserving Vanderbilt (ONCV)65 pseu-

dopotentials with nonlinear core correction from the SPMS collection66. The equilibrium

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AbinitiostudyontheelectromechanicalresponseofJanustransitionmetaldihalidenanotubesArpitBhardwajandPhanishSuryanarayanaCollegeofEngineering,GeorgiaInstituteofTechnology,Atlanta,GA30332,USAAbstractWestudytheelectronicresponseofJanustransitionmetaldihalide(TMH)nanotubestome-chanicaldeformationsusingKo...

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Ab initio study on the electromechanical response of Janus transition metal dihalide nanotubes Arpit Bhardwaj and Phanish Suryanarayana

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