Ferroelectric switching at symmetry -broken interface s by local control of dislocation networks Laurent Molino1 Leena Aggarwal1 Vladimir Enaldiev3 Ryan Plumadore1 Vladimir Falko234

2025-04-27 0 0 1.77MB 17 页 10玖币

侵权投诉

Ferroelectric switching at symmetry-broken interfaces by local control of dislocation

networks

Laurent Molino1, Leena Aggarwal1, Vladimir Enaldiev3, Ryan Plumadore1, Vladimir Falko2,3,4*,

Adina Luican-Mayer1**

1 Department of Physics, University of Ottawa, Ottawa, Canada

2 National Graphene Institute, University of Manchester, Manchester, UK

3 Department of Physics and Astronomy, University of Manchester, Manchester, UK

4 Henry Royce Institute for Advanced Materials, University of Manchester, Manchester, UK

* vladimir.falko@manchester.ac.uk

** luican-mayer@uottawa.ca

Abstract

Semiconducting ferroelectric materials with low energy polarisation switching offer a platform for

next-generation electronics such as ferroelectric field-effect transistors. Ferroelectric domains at

symmetry-broken interfaces of transition metal dichalcogenide films provide an opportunity to

combine the potential of semiconducting ferroelectrics with the design flexibility of two-

dimensional material devices. Here, local control of ferroelectric domains in a marginally twisted

WS2 bilayer is demonstrated with a scanning tunneling microscope at room temperature, and their

observed reversible evolution understood using a string-like model of the domain wall network.

We identify two characteristic regimes of domain evolution: (i) elastic bending of partial screw

dislocations separating smaller domains with twin stacking and (ii) formation of perfect screw

dislocations by merging pairs of primary domain walls. We also show that the latter act as the

seeds for the reversible restoration of the inverted polarisation. These results open the possibility

to achieve full control over atomically thin semiconducting ferroelectric domains using local

electric fields, which is a critical step towards their technological use.

Main

Overcoming challenges in modern computer engineering and telecommunication technologies,

requires compact multi-functional components. Atomically thin films of semiconducting transition

metal dichalcogenides (TMDs) offer multiple advantages for such a development. They retain

robust semiconducting properties down to sub-nanometer thickness, which allows for an efficient

transistor operation, and they exhibit strong light-matter coupling. Moreover, TMDs have recently

emerged as ferroelectric two-dimensional (2D) materials, in which switching between two twin

stacking orders produces room-temperature stable out-of-plane polarisation with low switching

barriers1–5. Semiconducting ferroelectrics with low energy barrier polarisation switching are of

interest, as they enable ferroelectric field-effect transistors, devices that combine memory storage

and logic processing into a single device6. The possibility of assembling vertical stacks of designer

sequences from atomically thin planes of layered van der Waal materials allows for unprecedented

flexibility in engineering novel electronic devices7,8. In particular, previously inexistent electronic

properties can emerge in these systems through relative rotation of the atomically thin layers,

which leads to the formation of moiré patterns and reconstruction domains9–11.

Lack of inversion symmetry among individual monolayers of TMDs permits control over the

symmetry of interfaces formed in layer-by-layer assembled heterostructures. For example, when

assembling two layers with inverted orientation of unit cells, one obtains bilayers with a local

inversion-symmetric structure, which reconstructs10,12 (at small twist angles) into a honeycomb set

of 2H stacking domains12–14, separated by domain walls which have the form of intra-layer shear

solitons15. Moreover, the assembly of layers with parallel orientation of unit cells (P-bilayers)

generates structures with non-symmetric interfaces which allow for a spontaneous interlayer

charge transfer and, therefore, out-of-plane ferroelectric polarisation1–4,16. The bilayers with

parallel orientation of monolayer unit cells exhibit two energetically equivalent favourable

stacking orders, which are mirror images of each other (illustrated in Fig. 1a). In one type of

stacking (XM’), the metal site M’ of the bottom layer appears under the chalcogen site X of the

Figure 1: a) Top and side views of XM’ and MX’ stackings, with arrows indicating the direction

of electric polarisation. b)

Schematic of the STM experiment, indicating the electric field

produced by applying a bias voltage between the tip and the sample. c)

Large STM topographic

map (VB = -1.3 V, IT = 50pA), showing two distinct regions in the sample indicated by A and B.

In area B, the triangular moiré superstructure is homogeneous, with equilateral triangles, and

has periodicity given by the translation vectors 



. In area A, the moiré pattern is skewed due

to a small strain in one of the layers (whose effect is magnified by the moiré superlattice effect20

Spatial displacement field map for the inter-layer deformations, obtained using Eq. (1) from

the variation of moiré superlattice pattern in c).

top layer, whereas the chalcogen/metal sites (X’/M) in the bottom/top layers sit under/over the

middle of the honeycomb of the other layer’s lattice. In the second type of stacking (MX’), the

metal site M of the top layer appears over the chalcogen site X’ of the bottom layer, whereas the

chalcogen/metal sites (X/M’) in the top/bottom layers sit over/under the middle of the honeycomb

of the other layer’s lattice. For small-angle twisted P-bilayers, in-plane relaxation10,12–14 leads to

the alternating twinning of XM’ and MX’ domains carrying opposite electric polarisation12–14,16,17.

These domains are separated by domain walls which are similar to partial dislocations in the 3R

polytype of bulk TMDs.

Here, two P-aligned WS2 samples were assembled as schematically indicated in Fig. 1b (optical

micrographs and fabrication details are available in Supplementary Figure 1 in SI). A thin graphite

film was placed under the TMDs to improve electrical contact. Scanning tunneling microscopy

and spectroscopy (STM/STS) was used to image and control the reconstruction domains (XM’ and

MX’) formed in the twisted WS2 samples. As indicated in Fig. 1b, such an experiment implies the

presence of a perpendicular electric field between the tip and the sample, which can couple to the

local polarisation of the XM’ and MX’ stacking areas. A typical scanning tunneling topograph for

our samples (Fig. 1c) shows large triangular domains of XM’ and MX’ stacking (Fig. 1a),

separated by a network of domain walls with clearly identifiable nodes (to which one can

attribute12 energetically unfavourable XX’ stacking characterised by vertical alignment of

chalcogens in the top and bottom layers). The formation of these domains is understood by the fact

that in twisted P-aligned bilayers of hexagonal TMDs the relaxation of a moiré pattern into

domains occurs for the twist angles below 2.5° 13,14,16.

Within the triangular domains of the network in Fig. 1c, we first focus on the area B containing a

network of approximately equilateral triangular domains. These equilateral domains have

periodicity of approximately 78 nm corresponding13,14,16,18 to a twist angle 0.23°. This area will be

used as a reference for the analysis of a larger-scale aperiodic domain pattern (domains of various

sizes and with pronounced anisotropy). Such a distortion of the domain network is caused by a

small accidental strain in either of the assembled WS2 monolayers, inflicted in the fabrication

process19. Theoretically, it has been demonstrated20 that a moiré pattern acts as a magnifying glass

for small deformations in the constituent monolayers of the bilayer. Strain-induced shifts of the

atomic registry between the layer – even as small as a fraction of the lattice constant – lead to a

shift of the equivalent stacking areas in the moiré pattern by distances comparable to the moiré

superlattice period. Here, we use this generic property of moiré patterns to map the displacement

field distribution (due to the accidental strain) and analyse the position of domain wall network

nodes, which feature XX’ stacking. For this, we successively enumerate all nodes by integers

, starting from the referenced point O inside area B in Fig. 1c with approximately

equilateral domains. For this area, we determine a twist angle   









  (≈0.23°) from

the moiré period, 

 in area B (

 nm and    nm). Then, we take into

account that, without strain, the positions, , of the XX’ network nodes in the rest of the sample

would correspond to the mutual shift of atomic registry such that ,

whereas the difference between the actual, 





, and expected, , positions of the network

sites enables us to determine the strain-induced interlayer shift (superimposed onto the local lattice

reconstruction into domains) of the monolayer WS2 lattices as20,













 





. (1)

The resulting deformations map is shown in Fig. 1d, indicating a shear strain vortex in top left

corner and discernible deformations from the sides of the analyzed area.

The effect of the reconstruction domains on the local electronic properties, as measured using

STM/STS, is discussed in Fig. 2. The STM topographic map in Fig. 2a shows a typical region with

triangular domains. By observing the domain wall bending (domain expansion/shrinking) in

response to the applied biased voltage, the bright and dark domains were identified from the

ferroelectric polarisation as MX’ and XM’ stacking regions, respectively21. The contrast in the

STM topography of the two stackings is consistent with previously reported STM measurements

of twisted TMD homobilayers with similar scanning parameters22: likely, due to the prominence

of the metal atom in this stacking. The tunneling spectrum in Fig. 2b, averaged over all domains,

shows the conduction and valence band edges at approximately 0.6 eV and -1.5 eV respectively,

indicating an electronic band gap consistent with previous measurements of this system23. The

differential tunneling conductance (dI/dV) maps acquired at the indicated voltages in Fig. 2c,

demonstrate the dependence of the local density of states on the stacking order inside the domains

and at the domain walls.

Although a scanning tunneling microscope uses an atomically sharp tip end as a local probe, the

overall shape of the tip and its holder make the electric field between the tip and the sample

approximately uniform over a typical domain size (Fig. 1b). To verify this, we compared forward

Figure 2: a) STM topographic map of a typical region with equilateral triangular stacking

order domains (It= 100 pA, VB = -1.3V). b)

STS averaged over bright and dark domains (set

point: IT = 60 pA and VB = -1.9 V). c) dI/dV maps at the indicated bias voltages (It= 100pA).

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

10 玖币 0人已下载

立即下载

摘要：

Ferroelectricswitchingatsymmetry-brokeninterfacesbylocalcontrolofdislocationnetworksLaurentMolino1,LeenaAggarwal1,VladimirEnaldiev3,RyanPlumadore1,VladimirFalko2,3,4*,AdinaLuican-Mayer1**1DepartmentofPhysics,UniversityofOttawa,Ottawa,Canada2NationalGrapheneInstitute,UniversityofManchester,Manchester...

展开>> 收起<<

Ferroelectric switching at symmetry -broken interface s by local control of dislocation networks Laurent Molino1 Leena Aggarwal1 Vladimir Enaldiev3 Ryan Plumadore1 Vladimir Falko234.pdf

共17页,预览4页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

Ferroelectric switching at symmetry -broken interface s by local control of dislocation networks Laurent Molino1 Leena Aggarwal1 Vladimir Enaldiev3 Ryan Plumadore1 Vladimir Falko234

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: