Effects of Different Surface Functionalization and Doping on the

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C: Physical Processes in Nanomaterials and Nanostructures

Effects of Different Surface Functionalization and Doping on the Electronic Transport Properties of M2CTx-M2CO2 Heterojunction Devices Yuhong Zhou, Guangmei Zhai, Tao Yan, Joseph S. Francisco, Hui Tian, Qing Huang, and Shiyu Du J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02026 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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The Journal of Physical Chemistry

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Effects of Different Surface Functionalization and Doping on the

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Electronic Transport Properties of M2CTx-M2CO2 Heterojunction

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Devices Yuhong Zhou1, Guangmei Zhai2, Tao Yan1, 3, Joseph S. Francisco4, Hui Tian1, Qing Huang1 and

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Shiyu Du1*

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6

1

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Engineering, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo 315201, China

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2

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University of Technology, Taiyuan 030024, China.

Speciality Fibers and Nuclear Materials Engineering Laboratory, Ningbo Institute of Materials Technology and

Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan

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3

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4

Department of Physics, Faculty of Science, Ningbo University, Ningbo 315201, China

Departments of Chemistry, Purdue University, West Lafayette, IN 47906, USA.

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*Corresponding author: E-mail:[email protected] 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

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Employing nonequilibrium Greeen’s functions in combination with the density-functional theory, we

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have examined the electronic and transport properties of p-type doped, undoped and n-type doped

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MXene heterojunctions [M2CTx-M2CO2 (M = Ti, Zr, or Hf; T=F, OH; x= 0 or 2)]. The geometries and

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electronic band structures are all obtained and the current-voltage characteristics are predicted. We find

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that M2CF2-M2CO2 (M=Ti, Zr) heterojunctions have better electrical conductivity than M2C-M2CO2

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and M2C(OH)2-M2CO2, and Hf2C(OH)2-Hf2CO2 shows the best conductivity than the cases with other

27

terminations studied hereby. Rectification behaviors are observed as important characteristics from

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some of these devices. Moderate n-type doping is found effective to enhance rectification for

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Hf2C(OH)2-Hf2CO2, and the currents at the intermediate positive bias show excellent rectification ratio.

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Moreover, high n-type doping may generate negative differential resistance (NDR) effect in the

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Hf2C(OH)2-Hf2CO2 heterojunction at high voltage with a wide bias range, and the high doping

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concentration of both n- and p- types are found to generate high electrical conductivity. The mechanism

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of rectification and NDR effects is elaborated from the electronic structure level in detail. These

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findings not only help make appropriate choices in surface groups, doped carrier types and

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concentration to improve the performance of MXene heterojunction, but also provide new insight for

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guiding the design of novel MXene nanoelectronics devices.

37 38 39 40 41

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1.

Introduction

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The low-dimensional materials have nowadays become a general concern due to their intriguing

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physical properties and potential applications in future nanoelectronics.1-3 Some well-known

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two-dimensional (2D) materials such as graphene, black phosphorus and MoS2 have shown excellent

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electronic and transport characteristics.4-6 Recently, a new series of 2D layered materials, termed as

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MXenes with a general formula of Mn+1XnTx, which have been synthesized from the layered metallic

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ceramics Mn+1AXn (n=1, 2, and 3) phases, are also rising and become a focus of research.7-9 Here M

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represents an early transition metal, A is mainly a group IIIA or IVA element, X denotes carbon or

50

nitrogen, and T stands for the surface functional groups (OH, O or F). At present, MXenes have

51

promising applications in lithium-ion batteries,10 Lithium–Sulfur batteries11, supercapacitors,12

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selective adsorption of heavy ions,13 gas sensors,14 biosensors, transparent conductive electrodes, 15and

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water treatment.16 Besides, it is also demonstrated that MXenes show excellent charge transport

54

characteristics and extraordinary motilities, which renders MXenes as promising electronic device

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materials.

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with remarkable possibilities for composition variations and property tuning, which may help extend

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the Moore’s law.

17,18

Nowadays, MXenes have rapidly become established as a novel class of 2D materials

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Generally, MXenes are produced by selective extraction synthesis from their layered precursors.2,9

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Different from most studied MXene Ti3C2Tx and Ti2CTX, it is not easy to use Zr- and Hf-MAX phases

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to synthesize the corresponding MXene.9 Recently, it was shown that the M3C2 MXene could be

61

synthesized from the parent M3A3C5(M=Ti, Zr, Hf), while the M2C MXenes are expected to be

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producible by the parent M2AC.9,19 Recent progress has been reported on MAX phases in the Zr-Al-C

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and Hf-Al-C systems.3,20 For example, new MAX phase of Zr2AlC and Hf2AlC were synthesized by

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reactive hot pressing and pressureless sintering through the use of Zr-and Hf-hydride raw power.

65

How to obtain pure phase MXene remains a challenge because the MAX phase is hard to be

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completely exfoliated from the mixture. Lapauw et al. synthesized the Hf−Al−C phase, but the mixed

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211 , 312 and 523 stacking were observed inside the same grains; the 523 is a transient state from the

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211 to 312.23

21,22

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It is well known that the performance of materials applied in nanodevice is mainly determined by

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its electron transport behaviors under a specific structural design. Employing designed defects,

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impurity doping, adsorption, chemical functionalization and heterojunction, the performance of 2D 3



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24,25

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materials such as graphene is tunable for the future electronic devices.

Not surprisingly, similar

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schemes should also be applicable to MXenes.26,27 For example, some of metallic MXenes could be

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converted to semiconductors with a small energy band gap after surface passivation.7 The electronic

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transport properties of MXene compound Ti3C2 were found strongly influenced by surface

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terminations.28 Therefore, to extend the applications of MXene materials in nanoelectronics, it is

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necessary to find the dependence of the transport properties on designed structures of MXenes and

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reveal the underlying mechanisms. The well-developed first-principles electronic structure calculation

79

methods and the nonequilibrium Green’s function (NEGF) technique could help us achieve such a

80

destination.29,30

81

In this work, the computational investigations on the designed heterojunction structures

82

M2CTx-M2CO2 (M = Ti, Zr, or Hf; T=F, OH; x= 0 or 2) with and without doping are performed

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concentrating on the corresponding electronic and transport properties. In principle, these devices are

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experimentally achievable by controlled functionalization of pristine MXenes. The results show that

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excellent rectifying and NDR behaviors can be achieved in some heterojunction devices. Meanwhile,

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the surface group and doping concentrations have a significant impact on the transport properties of the

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systems. The current results will not only help unveil unusual transport characters of MXenes and its

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heterojunctions, but also favor to the designs and applications of nano-/molecular-sized MXene

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devices.

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2.

Methods

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The first-principle computations of electronic structures and transport properties are employed

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using the Atomistix Toolkit (ATK) package based on NEGF in combination with density functional

93

theory (DFT). The monolayer of Ti2C with different surface functionalizations is firstly optimized

94

using

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Perdew-Burke-Ernzerhof (PBE) to represent the exchange-correlation energy. For M2CTx-M2CO2 (M =

96

Ti, Zr, or Hf) heterojunction devices, i.e. M2CF2-M2CO2, M2C(OH)2-M2CO2, and M2C-M2CO2, the

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computational models with left electrode, the central region, and the right electrode are shown in

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Figure 1. Doping is the intentional introduction of impurities into an intrinsic semiconductor for the

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purpose of modulating its electrical properties. The dopants can change concentration of carriers and

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have important effects on shifting the energy bands relative to the Fermi level. In this work, doping

101

type (n or p) and doping concentration to the selected atoms are controlled by the atomic compensation

DFT

calculations

with

the

generalized

gradient

4


approximation

(GGA)

of

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charge method, which models doping effect in an atomistic semiconductor device without explicitly

103

introducing dopant atoms. Doping is the intentional introduction of impurities into an intrinsic

104

semiconductor for the purpose of modulating its electrical properties. The dopants can change

105

concentration of carriers and have important effect on shifting the energy bands relative to the Fermi

106

level. In this work, doping type (n or p) and doping concentrations are controlled by the atomic

107

compensation charge method,31,32 which models doping effect in an atomistic semiconductor device

108

without explicitly introducing dopant atoms. As for both types, the doping concentration of 2×1012cm-2

109

is adopted in the semiconductor M2CO2 if not specified. The left and right electrodes are semi-infinite

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periodic in the negative/positive Z direction. The planes of MXene materials are parallel to the XZ

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plane. Each lead is described by a super cell with two MXene unit cells along the transport direction. A

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vacuum region of at least 20Å is set in the supercell by the Y direction so that the electron density and

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electrostatic potential decay correctly and erroneous interactions with images of the system are avoided.

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In the transport calculation, the PBE functional is utilized as in the geometry optimization. The

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convergence for total energy and self-consistency are controlled by the numerical tolerance of 10-5eV.

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The fineness of the real space grid is determined by an equivalent plane wave cutoff 400 Ry. The

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k-point sampling for the semi-infinite leads are performed with 16×1×100 Monkhorst-Pack k-grid.

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Hartwigsen-Goedeker-Hutter (HGH) norm-conserving pseudopotentials and the basis set of tier 0 are

119

used. The current I(V) can be calculated by the Landauer-Buttiker formula given by Eq. (1).

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I (V )=

121

Here e is the electron charge, f is the Fermi function, h is Planck’s constant, T(E, V) is the transmission

122

function of the system, and UL and UR are the electrochemical potential of the left and right leads,

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respectively. Under the external bias V, the value of UL and UR, will be shifted downward (or upward)

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by V/2 relative to the original electrochemical potentials, namely UL=EF+eV/2and UR=EF–eV/2, where

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EF is the average Fermi level. Thus, the region of the bias window is [–V/2, +V/2]. T(E, V) is the

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bias-dependent transmission function of the system, which is the sum of the transmission probabilities

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of all channels available at energy E under the external bias voltage V, and can be calculated by the

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following formula.

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T ( E,V ) = Tr  ΓL ( E) GR ( E,V ) ΓR ( E) GA ( E,V ) 

2e +∞ ∫ [ f L ( E − U L ) − f R ( E − U R )]T ( E , V ) dE h −∞

(1)

(2) 5



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Here Γ L ( E ) and Γ R ( E ) are the broadening functions derived from left and right electrode

131

self-energies, respectively.

132

respectively.

G R ( E , V ) and G A ( E , V ) are the retarded and advanced Green’s functions,

X Y Z Y ZZZ X

Z

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Figure 1. (a) and (b) represent the monolayered M2CF2-M2CO2 (M=Ti, Hf or Zr) devices in XZ plane

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and YZ plane, respectively; (c) and (d) represent the monolayered M2C(OH)2-M2CO2 (M=Ti, Hf or Zr)

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devices in XZ plane and YZ plane, respectively. (e) and (f) represent the monolayered M2C-M2CO2

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(M=Ti, Hf or Zr) devices in XZ plane and YZ plane, respectively. The red balls present the oxygen

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atom, the gray balls present the carbon atom, the green balls present the fluorine atom and the white

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balls present the titanium atom.

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3.

Results and discussions

3.1 Optimized geometries and electronic structures

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For MXenes with oxygen (O), fluorine (F) and hydroxyl (OH) termination groups, the band gaps,

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optimized lattice constants and ground-state structural parameters are listed in Table 1. Ti2CO2, Zr2CO2

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and Hf2CO2, are shown to be semiconducting with band gaps of 0.22 (0.18) eV, 0.94 (0.60) eV and

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1.17(0.74) eV with the basis set Tier4 (Tier0), the others are found to be conducting. The band gaps by

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the smaller Tier0 basis sets are relatively lower than those by Tier4. These calculated band gaps are in

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good agreement with previous studies.33,34 The lattice constants are predicted to be higher in Zr2CO2

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than Ti2CO2 and Hf2CO2 (3.053 Å for Ti2CO2, 3.295Å for Zr2CO2 and 3.266Å for Hf2CO2,

149

respectively), which is consistent with other works. 33,34 The values for MXenes with OH and F groups 6


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are coincident to those with oxygen surface terminations except that Zr2CF2 exhibit slightly higher

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lattice parameter.

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Table 1. Optimized lattice constants (a, in Å) along with the lowest-energy structural models, the band

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gaps (Eg, in eV), the Ti (or Zr, Hf)-O (or F, OH) bond length (L1) and the Ti (or Zr, Hf)-C bond length

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(L2). O termination MXene

a

Eg(Tier4/Tier0)

F termination L1

L2

a

Eg

OH termination

L1

L2

a

Eg

L1

L2

Ti2C

3.053

0.22/0.18

1.99

2.21

3.048

＼

2.19

2.13

3.053

＼

1.99

2.21

Zr2C

3.295

0.94/0.60

2.13

2.39

3.380

＼

2.36

2.31

3.295

＼

2.37

2.27

Hf2C

3.266

1.17/0.74

2.10

2.35

3.273

＼

2.31

2.25

3.266

＼

2.32

2.26

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As reported,35 M2C (M=Ti, Hf or Zr) with non-oxygen surface termination are metals, and M2CO2

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have semiconductive property with indirect band gaps. Hereby, we calculate the band structure for the

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electrode of p-type doped, undoped and n-type doped M2CO2 in the orthogonal supercell as plotted in

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Figure 2. From this work, electron (hole) injection shifts the Fermi level towards the conduction

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(valance) band, leading consequently to n (p)-type semiconductors for all cases. Obviously, the shifting

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of Fermi level may be responsible to the evolution in transport characteristics of the MXene systems as

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discussed in the following sections.

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Figure 2. The calculated band structures of p-type doped, undoped, n-type doped M2CO2 (M=Ti, Hf or

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Zr) leads. (a), (b) and (c) are band structures of p-type doped, undoped and n-type doped Ti2CO2 leads,

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respectively; (d), (e) and (f) are band structures of p-type doped, undoped and n-type doped Zr2CO2

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leads, respectively; (g), (h) and (i) are band structures of p-type doped, undoped and n-type doped

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Hf2CO2 leads, respectively.

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3.2 Transport properties

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The I-V curves of corresponding devices with different surface functional groups and different

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dopant concentrations (n-type doping, p-type doping or undoing) are modeled under the bias range

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from -1.2 V to 1.2 V as shown in Figure 3. As a general trend, M2CF2-M2CO2 (M=Ti or Zr) devices

177

with different doping type and concentrations have the better electrical conductivity than the devices

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characterized by left electrodes with hydroxyl (OH) groups or without surface termination i.e.

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M2C(OH)2-M2CO2 and M2C-M2CO2. However, Hf2C(OH)2-Hf2CO2 show better conductivity than the

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cases with –F or no terminations. Moreover, one can easily identify the major feature that the forward

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currents of Ti2CT2-Ti2CO2 devices are larger than the reverse currents, but for most M2CT2-M2CO2

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(M=Zr or Hf) devices, the orders are reversed. As a result, the rectification behaviors are observed as

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important characteristics from some of these devices. For the Ti2CT2-Ti2CO2 devices, the stronger 8


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rectification effect can be seen at large voltage and Ti2CF2-Ti2CO2 is better than the other two no matter

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whether the systems are doped. As to Zr2CT2-Zr2CO2 devices, the current rectification is relatively low

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for Zr2CF2-Zr2CO2. When OH and O groups are in the left electrode, the current is low but the

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rectification also exist. For both Ti2CT2-Ti2CO2 and Zr2CT2-Zr2CO2 devices, doping types does not

188

have significant impact on rectification. Differently, in the n-type doped Hf2CTx-Hf2CO2 devices, the

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currents at the intermediate positive bias are much lower than that at the same reverse bias. For

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example, the current of n-type doped Hf2C(OH)2- Hf2CO2 device is -5.14 (0.30) µA at the bias of -0.8

191

V (0.8V) in Figure 2(h). Here, the rectification ratio is defined as the current ratio under reverse and

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forward voltages for the same voltage magnitude, i.e. R(V)= |I(-V)/I(+V)|. The rectification ratio of

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n-type doping Hf2C(OH)2-Hf2CO2 at the bias voltage of 0.8 V reaches 17.14. For n-type doping

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Hf2CF2-Hf2CO2 and Hf2C-Hf2CO2 devices, the calculated rectifications at the bias voltage of 0.8 V

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reach 6.2 and 20.8, respectively. Despite of the slightly higher rectification ratio, the electrical

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conductivity of the Hf2C-Hf2CO2 device only approximately one third of that in Hf2C(OH)2-Hf2CO2

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over the bias range. For p-type doped and undoped Hf2C(OH)2-Hf2CO2 device, the calculated

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rectifications at the bias voltage of 0.8 V reach 1.2 and 4.2, respectively, much lower than the n-type

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doping cases. These suggest that the n-type doping may be a satisfactory method for achieving the

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rectification effect for Hf2CT2-Hf2CO2 devices.

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Figure 3 gives the I-V curves of different MXene structures labeled as M2CF2-M2CO2,

207

M2C(OH)2-M2CO2 and M2C-M2CO2 (M=Ti, Hf or Zr). (a), (b) and (c) are I-V curves of undoped,

208

n-type doped and p-type doped Ti2CTx-Ti2CO2 devices, respectively; (d), (e) and (f) are I-V curves of

209

undoped, n-type doped and p-type doped Zr2CTx-Zr2CO2 devices, respectively; (g), (h) and (i) are I-V

210

curves of undoped, n-type doped and p-type doped Hf2CTx-Hf2CO2 devices, respectively.

211 212

In order to provide a better description on the effect of different surface functional groups and

213

dopant types in MXene heterojunction devices, the total transmission spectra of all systems are

214

calculated and those at bias voltage of V = ±1.0V for all p-type doped, undoped and n-type doped

215

devices are exhibited in the Figure 4. According to Landauer–Büttiker formula, the current is related to

216

the integral area of the transmissions in the bias window. When the bias voltage is ±1.0V, the current

217

rises only by the transmission in the energy window of ±0.5 eV. The transmission peak areas of

218

Ti2CTx-Ti2CO2 devices by the positive bias voltage are larger than those by the same negative bias,

219

which implies the positive currents are higher. These results are consistent with the current-voltage 10


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220

curves. For Zr2CF2-Zr2CO2 devices, the total transmissions show little dependence on the current

221

direction, reflected by the close transmission at positive and negative bias voltages. In Hf2CF2-Hf2CO2

222

devices, the discrepancy caused by the voltage direction (±1.0 V) is found significantly influenced by

223

doping. Especially, the n-type doping possess the largest and p-type doping causes the smallest

224

difference in transmission. As to other devices of M2CTx-M2CO2 (M=Zr or Hf) with T = OH or no

225

termination, the transmission is found dominant from the negative direction. When the bias voltage is

226

1.0V, the transmission coefficients of p(n)-type doped and undoped M2C(OH)2-M2CO2 and

227

M2C-M2CO2 devices in the bias window are either zero or very small, which indicates the current is

228

low. Whereas, the transmission coefficients of these devices turn to be significant at the voltage of

229

-1.0V, where the currents of devices are clearly generated in the bias window. It should be mentioned

230

that the areas at the reverse (forward) bias voltage are slightly increased (decrease) from the p-type

231

doping to n-type doping in the Hf2C(OH)2-Hf2CO2 device, which implies the n-type doping can

232

improve the rectification. With consideration of electrical conductivity, rectification as well as potential

233

accessibility for fabrication, we continue the investigation focused on the n-type doped

234

Hf2C(OH)2-Hf2CO2 device in the following sections. It should be mentioned that the theoretical results

235

of electron transport for the MXene heterojunction devices are generally corresponding to its behaviors

236

under ideal conditions. This means the computational results can be in good agreement with

237

experimental data quantitatively when the experimental conditions are well controlled. In a practical

238

case that the interface of heterogeneous junction is difficult to control, one may expect that the

239

theoretical predictions are rational qualitatively.

240

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242 243 244

Figure 4. The transmission spectra for the p-type doped, undoped and n-type doped M2CT2-M2CO2

245

(M=Ti, Hf or Zr; T=F, OH or none) devices at different bias voltages, respectively. The blue or red

246

curve denotes the transmission of positive or negative bias voltages, respectively. The Fermi energy

247

deﬁnes the zero of energy. (a), (b) and (c) are the transmission spectra for the p-type doped, undoped

248

and n-type doped Ti2CF2-Ti2CO2, Ti2C(OH)2-Ti2CO2 and Ti2C-Ti2CO2 devices at the bias voltages of

249

±1.0V, respectively; (d), (e) and (f) are the transmission spectra for the p-type doped, undoped and

250

n-type doped Zr2CF2-Zr2CO2, Zr2C(OH)2-Zr2CO2 and Zr2C-Zr2CO2 devices at the bias voltages of

251

±1.0V, respectively; (g), (h) and (i) are the transmission spectra for the p-type doped, undoped and

252

n-type doped Hf2CF2-Hf2CO2, Hf2C(OH)2-Hf2CO2 and Hf2C-Hf2CO2 devices at the bias voltages of

253

±1.0V, respectively;

254 255

3.3 Transmission spectra

256

To gain better understanding on the mechanism of rectification behavior in I-V curves, further

257

investigations are performed on the bias-dependent transmission spectra and energy band coupling of

258

left and right leads for the n-type doped Hf2C(OH)2-Hf2CO2 device which shows remarkable

259

rectification effect. The results are illustrated in Figure 5. When the bias is -1.0 V as in Figure 5(a), the

260

energy bands are shifted up (down) by 0.5eV for the left (right) electrode. No gap in the left lead of

261

Hf2C(OH)2 is present in the bias window where only part of the gap in the right lead remains. As a 12


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262

result, the bands of the left lead in the energy range [-0.10 eV, 0.50 eV] can couple with the conduction

263

band of the right lead, therefore, the transmission spectra can be observed in the bias window. When

264

the bias is 1.0V as in Figure 5(b), the gap in Hf2CO2 happens to be nearly coincident with the bias

265

window, which makes band coupling with the left lead vanish in the bias window as shown by the

266

bisque horizontal dash lines. Consequently, only little transmission peaks appear in the whole energy

267

range [-0.50 eV, 0.50 eV], which causes that the current is almost depleted at the bias of 1.0V. Further,

268

the transmission pathway is an analysis option which splits the transmission coefficient into local bond

269

contribution as shown in Figure 5(c) and (d). The volume of each arrow indicates the magnitude of the

270

local transmission between each pair of atoms, while the arrow and color indicate the direction of the

271

electron flow. From Figure 5(c), the transmission pathway can be seen turned on for the whole channel,

272

so the electron transport from the Hf2C(OH)2 to Hf2CO2 can occur smoothly at the bias of -1.0V at the

273

energy of 0.4 eV, though one may notice that the scattering region has better delocalization than the

274

leads. But in the forward bias voltage in Figure 5(d), there are no transmission pathway in the right

275

lead of Hf2CO2, although the transmission in the left lead and scattering region appear similar with the

276

case of -1.0V. This means the semiconducting Hf2CO2 acts as a hurdle that prevents electrons from

277

flowing through. Namely, the rectifying behavior can be observed originated from the directional

278

feature in electron transport bypassing the semiconducting lead.

279 280

13



281 (c) Vbias= -1.0V, E= 0.4eV

282

283 (d) Vbias= 1.0V, E= 0.4eV

284 285

Figure 5. (a), (b) Combination plots for Band structure of the left lead (left panels), transmission

286

spectra (middle panels), and band structure for the right lead (right panels) for n-type doped

287

Hf2C(OH)2-Hf2CO2 devices at the bias voltages of -1.0 V and 1.0 V; (c), (d) Electron transmission

288

pathway of n-type doped Hf2C(OH)2-Hf2CO2 with the energy of 0.4 eV at the bias voltages of +1.0 V

289

and -1.0V.

290 291

3.4 p- and n-type doping effect

292

The last issue studied here, is the comparison of I-V curves for n(p)-type doped

293

Hf2C(OH)2-Hf2CO2 with different doping concentration (2×1012cm-2, 2×1013cm-2, 2×1014cm-2 and

294

2×1015cm-2), as can be seen in Figure 6(a) and (b). When p-type doping is adopted, the current can be

295

seen to increase with the rising doping concentration. For example, the current increased by 90% when

296

doping concentration is raised from 2×1012 to 2×1013 cm-2 at the bias of 1.0 V. The forward current

297

seems to become dominant at doping concentration over 2×1013 cm-2, by which rectification behavior is

298

also observable at high voltage. When we turn to n-type doping, the rectification behaviors occur with

299

all the doping concentrations studied. At the bias of 0.8V voltage, the rectification ratio is 17.0, 1911.6,

300

4.2 and 4.2 for dopant concentration of 2×1012, 2×1013, 2×1014, and 2×1015cm-2. Similar to the case of

301

2×1012cm-2, the current under negative voltages are higher than that positive voltages. Especially with

302

the moderate doping concentration of 2×1013cm-2, one can find that the systems show the best

303

rectifying behavior at the bias of 0.5 V (rectification ratio = 2794). When the concentrations are up to

304

2×1014cm-2 or 2×1015cm-2, the currents increase sharply whether in n-type doping or in p-type doping.

305

Moreover, besides the high conductivity under negative voltage, negative differential resistance (NDR) 14


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306

effect can be identified for the devices with high n-type doping concentration in a wide bias range from

307

0.7 V to 1.2 V (the maximum voltage studied here). From Figure 6(b), when the dopant concentration

308

is 2×1014 and 2×1015 cm-2, and the currents are -25.7 (-24.4) µA and -14.3(-13.2) µA at the bias of -0.8

309

V and -1.2 V, respectively. To understand it, we also plot the transmission spectra at the voltage of -0.8

310

V and -1.2 V corresponding to the bias window from -0.4 (-0.6) eV to 0.4 (0.6) eV in Figure 6(c) for

311

the device with doping concentration of 2×1014 cm-2. Accordingly, some eigenstates of the

312

Hf2C(OH)2-Hf2CO2 device with the typical eigenenergy at the bias of -0.8V (or -1.2V) can be found in

313

Figure 6(d). Firstly, the amplitude of transmission spectra is higher under the voltage of -0.8 V than

314

-1.2 V in the energy range of [-0.4 eV, 0.4 eV]. For example, as seen in Figure 6(d), the eigenstate

315

shows much less delocalization through the lead of Hf2CO2 (thus the hurdle rises) with eigenenergy of

316

-0.2eV at the bias of -1.2V than that at -0.8V, which induces the lower transmission spectra at -1.2 V.

317

Secondly, the transmission spectra almost disappear from -0.6 eV to -0.4 eV at the bias of -1.2 V and

318

are low from 0.4 eV to 0.6 eV as shown in Figure 6 (c). An exemplary plot in Figure 6(d) for the

319

eigenstate of the device at eigenenergy of 0.5 eV shows the rise of the hurdle from Hf2CO2. Combining

320

(1) and (2), the blue area indicating the transmission under the bias of -1.2 V is smaller than the red

321

area representing that at -0.8 V and the NDR behavior is observed. It is worth mentioning that the

322

current NDR effect is more practical for devices in real-life than the two-end MXene devices reported

323

previously since the reported NDR up to now only appear at some discrete voltages and the present

324

device shows smooth NDR in a continuous bias range. Hence, findings from this work might promise

325

application of n-type doped Hf2C(OH)2-Hf2CO2 in novel nanoelectronics including nanomemristive

326

devices that need further investigation. As to p-type doping, the NDR effect is noticeable at the doping

327

concentration of 2×1014 cm-2, though it only appears at reverse bias and is not as significant as that for

328

n-type doping. Altogether, the n-type doping may be an option since it can not only enhance

329

rectification effect but also generate NDR effect. Moreover, the device may also be beneficial from the

330

high conductivity.

15



Page 16 of 20

331

(d)

V=-0.8V

E=-0.2eV

V=-1.2V

E=-0.2eV

V=-1.2V

E=0.5eV

332 333

Figure 6. (a) and (b) plot the current-voltage characteristics of Hf2C(OH)2-Hf2CO2 devices with

334

different concentrations; (c) plots the transmission spectra n-type doped Hf2C(OH)2-Hf2CO2 devices

335

with doping concentration of 2×1014cm-2; (d) The eigenstate of the Hf2C(OH)2-Hf2CO2 device with

336

doping concentration of 2×1014cm-2 with an isosurface criterion of 0.15.

337 338

4.

Conclusion

339

The electronic, structural, and transport properties of p-type doped, undoped and n-type doped

340

M2CTx-M2CO2 (M=Ti, Zr, and Hf) devices have been studied using first principles calculations. The

341

geometries are consistent with previous reports. The band structure calculation indicates that electron

342

(hole) doping shifts the Fermi level to the conduction (valance) band, which induces the change in

343

electron transport of the MXene systems. Rectification effect can be found to arise from some of these

344

devices, which is strongly influenced by surface functional groups and doping types. The

345

Ti2CF2-Ti2CO2 and Zr2CF2-Zr2CO2 devices with different type concentrations show relatively high

346

electrical conductivity relative to those with OH or no surface termination in the left leads;

347

Hf2C(OH)2-Hf2CO2 show the better conductivity than the cases with other left lead terminations. The

348

rectification behaviors are identified as important characteristics from some of these devices.

349

As to the doping effect, the n-type doping may be a satisfactory method for achieving the 16


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350

appreciable rectification and NDR effect for Hf2CT2-Hf2CO2 devices. In order to understand the

351

mechanism behind the transport properties of the heterojunctions, the bias-dependent transmission

352

spectra, energy band coupling, and electron transmission pathway calculations are carried out and the

353

mechanisms are elucidated from the results. The semiconducting Hf2CO2 is found to act as a hurdle

354

that originates the rectifying behavior. Then the doping concentration as another factor that alters the

355

I-V characteristics is explored for the Hf2C(OH)2-Hf2CO2 device. By the current work, currents are

356

found enhanced with increasing n- or p- doping concentrations. Interestingly, low and moderate n-type

357

doping exhibits rectification feature and high n-type doping may generate NDR effect in a wide voltage

358

range, which is found in MXene devices for the first time. This may suggest novel potential application

359

of n-type doped Hf2C(OH)2-Hf2CO2 in electronics such as nanomemristive devices. The mechanism of

360

NDR is then explained in detail by examining transmission spectra and some typical eigenstates. As a

361

comparison, the device with p-doped Hf2C(OH)2-Hf2CO2 does not show as strong NDR effect.

362

Therefore, the n-type doping may be an option worth more consideration for the Hf2C(OH)2-Hf2CO2

363

device. With approaches for optimizing device performance considered, this work may provide new

364

clues for the development of devices by MXenes.

365

Acknowledgments

366

The authors acknowledge the financial support of the National Key Research and Development

367

Program of China (No. 2016YFB0700100), the open-ended fund of Key laboratory of interface science

368

and engineering in advanced in advanced materials, Ministry of Education, Taiyuan University of

369

Technology ,Taiyuan, China (Grant No. KLISeAM201602), the Foundation of State Key Laboratory of

370

Coal Conversion (Grant J15-16-301), Key Research Program of Frontier Sciences, Chinese Academy

371

of Sciences (No. QYZDB-SSW-JSC037), and K. C. Wong Education Foundation (rczx0800). We also

372

acknowledge the National Thousand Young Talents Program of China, Hundred-Talent Program of

373

Chinese Academy of Sciences, Shanxi HundredTalent Program, ITaP at Purdue University and Special

374

Program for Applied Research on Super Computation of the NSFC Guangdong Joint Fund (second

375

phase) (U1501501 to Juan Li and Aiguo Wu) for computing resources and the key technology of

376

nuclear energy, 2014, CAS Interdisciplinary Innovation Team.

377 378 379

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Effects of Different Surface Functionalization and Doping on the

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