CaSi2: A PROMISING MATERIAL FOR THE SYNTHESIS OF NOVEL

FUNCTIONALIZED TWO-DIMENSIONAL CRYSTALLINE SILICON

NANOSHEETS A. Nayad a,+, A. Hasnaoui a, S. K. Hnawi a,b, L. Fkhar a,c, Y. Hadouch d, D. Mezzane d, L. El firdoussi a, M. Ait Ali a

a Laboratoire de Chimie de Coordination et Catalyse, Université Cadi Ayyad, BP 2390, 40001, Marrakech, Maroc. b Laboratory of Nanomaterials, Energy and Environment (LNEE), Cadi Ayyad University, BP 2390, 40001, Marrakech, Morocco.

c Materials and Nanomaterials Center, MAScIR Foundation, Rabat, Morocco. d Laboratoire de la Matière condensée et Nanostructures, Université Cadi Ayyad, BP 2390, 40001, Marrakech, Maroc.

E-mail : abdallahnayad@gmail.com

INTRODUCTION

Among the variety of 2D materials, Silicene Nanosheets (SiNSs) having nanometers thicknesses and lateral

dimensions ranging from the submicrometer to the micrometer scale has become a great challenge. Calcium disilicide

(CaSi2) Zintl phase is by far the precursor material used for the preparation of SiNSs via topochemical deintercalation

of calcium by thermal treatment with metal chlorides [2] and by soft chemical exfoliation in the presence of K or I2 as

redox-assistants [3,4]. However, the synthesized silicene are highly unstable under ambient conditions and are prone

to random oxidation and surface attachments, which make difficult its potential utilization. Thus, the surface

functionalization of SiNSs, expected to provide stable materials in air-ambient, has been recently explored lately by

modifications of layered polysilane Si6H6 with organic compounds [5,6]. However, no crystalline phase was reported.

In this work, we introduce an easy and direct functionalization approach of novel stable crystalline SiNSs by

exfoliation of CaSi2 with benzyl halides. For the first time, the dielectric proprieties of such bi-dimensional silicon-

based materials were evaluated.

SYNTHESIS AND STRUCTURAL CHARACTERIZATION

Characterizations:

- XRD

- SEM/TEM

- IR

-TG

SCHEMATIC SYNTHESIS OF FUNCTIONALIZATIONS

X-Ray diffraction

Thermogravimetric and infrared analysis

The percentage of weight loss of organic compounds incorporated on the silicon materials were found to be equal, with 36.5%

between 400 and 550 ºC for BnSi and 36.6% between 330 and 580 ºC for FBnSi.

The infrared spectra of both samples exhibited the characteristic absorption bands of benzyl group (Ph-H: 3000-3100, 712 cm-1 ;

C=C: 1400-1600 cm-1 ; C-H: 2850-2950 cm-1. The introduction of organic group on the surface of sheets is confirmed by Si-C

bonds found at around 815-900 cm-1. The presence of Si-H (2200-2300 cm-1) and Si-OH (3500 cm-1) are due to the layered

siloxene. Si-O-Si bond is also observable at 1050 cm-1. The most important peak is Si-Si bonds (450-500 cm-1).

DIELECTRIC APPLICATION

Unlike the XRD pattern of

CaSi2,exfoliated silicon in both

synthesized materials shows a

broad peak at around 2θ = 17 -

23º which confirms the organic

material on the materials. The

major reflection in the structure is

(111) at 2θ = 28.5º ; (220), (311

are also clearly identifiable. XRD

results indicate also the presence

of layered siloxene at 2θ = 37º and

49º . The materials are probably

terminated with hydrogen and

hydroxyl groups

Dielectric constant, dielectric loss and AC conductivity

Electronic microscopy

As-prepared product After purification

The scanning electronic microscopy of the as-prepared product shows a lamellar structure characteristic to a bi-dimensional

material. After purification, lamellar sheets are well observed. It can seen on the TEM image that sheets are nanometrics.

Impedance spectroscopy (EIS)

FBnSi exhibits a higher permittivity than BnSi

probably due to the mesomeric +M effect of the

fluorine group. Dielectric constant of materials

decreases rapidly at the low frequency following

the Wagner and Maxwell effect. At 100 Hz, FBnSi

has a dielectric constant of 2407 while it reaches

42 for BnSi. These high values are explained by

the presence of π-π interactions, a charge transfer

and a hole mobility attributed to the 2D structure.

FBnSi shows a lower dielectric loss than BnSi. The

AC conductivity of both materials is stable till 10

KHz then greatly increases. FBnSi is higher

conductive than BnSi.

Temperature dependence at high frequencies

BnSi

FBnSi

For both materials, the dielectric constant increases until 52 – 53 ºC then decreases as the temperature increases. At 10

KHz, BnSi shows a maxima of 930 which decreases to 12 at 138 ºC, while its dielectric loss follows the same

tendency and becomes very low at high frequency and temperature. FBnSi exhibits a higher permittivity, a maxima

over 3500 at 10 KHz and 850 at 1 MHz. Interestingly, it tends to stabilize at around 120 ºC with a good value of 455

for 1 MHz and the dielectric loss was found to be negative.

The EIS shows first a phenomena of semi-circles due to a charge transfer and space charge capacitance, followed by a

diffusion phenomena. The variation in the impedance of two materials can be related to the bidimensional morphology of

the structure of materials. Randles circuit is chosen to model the impedance behavior of materials.

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CONCLUSION :

We described here a new synthetic strategy of thin crystalline hybrid SiNSs functionalized by benzyl groups.

The materials were synthesized by chemical exfoliation of CaSi2 with benzylhalides in nitrogen gas. The

bidimensional morphology of the new materials is well observed by SEM while thin layers were observed by

TEM analysis. The unique morphology of these materials offers a high permittivity and a good conductivity.

Moreover, good results were found at high frequency with the increase of temperatures with very low and

negative dielectric losses. Moreover, fluorine group has a great influence in the increase of dielectric constant

and the decrease of dielectric loss. These preliminary results in such silicon-based materials will certainly open

access in new dielectric materials with great potential.

The Fifth International

Symposium on Dielectric

Materials and

Applications

1 μm