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Growth and characterization of two-dimensional crystals for communication and energy applications

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Abstract

This review article covers the growth and characterization of two-dimensional (2D) crystals of transition metal chalcogenides, h-BN, graphene, etc. The chemical vapor transport method for bulk single crystal growth is discussed in detail. Top-down methods like mechanical and liquid exfoliation and bottom-up methods like chemical vapor deposition and molecular beam epitaxy for mono/few-layer growth are described. The optimal characterization techniques such as optical, atomic force, scanning electron, and Raman spectroscopy for identification of mono/few-layer(s) of the 2D crystals are discussed. In addition, a survey was done for the application of 2D crystals for both creation and deterministic transfer of single-photon sources and photovoltaic systems. Finally, the application of plasmonic nanoantenna was proposed for enhanced solar-to-electrical energy conversion and faster/brighter quantum communication devices.

Introduction

An entire family of one-atom-thick two-dimensional (2D) materials with novel properties were inspired by isolation of graphene by Andre Geim and Kostya Novoselov from the University of Manchester in 2004 [1]. Since then, graphene which is a zero bandgap material, a property that is used in optoelectronic devices including transistors, has been grown using several Physico-chemical methods. Recently, semiconducting 2D materials including transition metal dichalcogenides (TMDCs) such as MoS2, MoSe2, WS2, WSe2, TiS2, etc., have been of the utmost interest because of their finite bandgap. For example, the bandgap of the MoS2 monolayer is 1.8 eV and also has extraordinary properties such as enhanced carrier mobility when integrated with high dielectric materials [2]. Two-dimensional materials are flexible and easy to transfer to a device structure. These are suitable for optoelectronic devices because of high electron mobility, tunable bandgaps, and easy formation of hetero-structures with semi-metals such as graphene and high bandgap semiconductors (6 eV) such as hexagonal boron nitride (h-BN) [3]. Semiconducting 2D crystals such as MoS2 and WSe2 have numerous applications in energy devices such as photovoltaics [4], [5], second harmonic generation (SHG) because of broken inversion symmetry [6], and quantum communication devices [7], [8]. TMDCs are compounds of a transition metal and a chalcogenide (S, Se, Te). Most of the compounds in the TMDC family crystallize in a layered structure [9] and possess strong covalent bonding within a layer and a weak van der Waal bond between adjacent planes in the crystal, which provides them with a strong 2D character. Single crystals of these materials can be easily cleaved along the planes of these layers, hence top-down techniques such as mechanical/liquid exfoliation are successful in obtaining nanoflakes or even monolayers of these materials. The layered TMDCs often show polytypism, a phenomenon in which the same compound crystallizes in more than one crystalline structure, with widely differing physical properties [9]. For example, MoTe2 crystallizes in two crystal structures, 2H and 1T, with the 2H structure showing semiconducting properties and the 1T structure showing semi-metallic properties [10], [11], [12]. In addition, the compounds in the TMDC family show a wide variety of electronic properties ranging from metallic to semi-metallic to semiconducting [9]. However, a scalable and deterministic method of growth and efficient characterization remain a challenge and an open research problem.

In this review article, we enlist and compare up-to-date methods and challenges in the growth of these 2D crystals [13]. Optimal methods of growth and characterization, with an emphasis on mechanical exfoliation and chemical vapor deposition (CVD) for efficient, large-scale growth and deterministic transfer of these 2D crystals on desired substrates, are presented. Characterization of the mono/few-layer(s) of the 2D crystal is suggested and described by state-of-the-art optical microscopic/phase-contrast imaging and Raman characterization. For the application part, hybrid 2D crystals and plasmonic waveguides for the transport of quantum information are discussed. The use of these 2D crystals in photovoltaic devices also is also discussed. For better light-matter interaction in a pristine 2D crystal, we discuss the application of plasmonic nano-antennas [14], [15], [16], [17]. Plasmonic nano-antennas not only enhance light absorption in these 2D crystals but also aid in the generation of numerous faster single photons. These 2D crystals then act as quantum emitters. In the end, efficient methods/strategies and challenges in growth, characterization, photovoltaic outlook, and quantum communication device applications are summarized.

Section snippets

Crystalline structures of 2D materials

2D monolayers show unique electrical and optical properties because of quantum confinement and surface states. This dramatic change in physical properties arises because of the transition from indirect bandgap to direct bandgap material while scaling down to a monolayer from bulk. This tunable bandgap is accompanied by strong photoluminescence (PL) and large exciton binding energy, for example, MoS2 has a bandgap of 1.8 eV, high mobility of 700 cm 2 V1 s1, high current ON/OFF ratio (107–108),

Physical properties resulting from structural change, strain, number of layers, and composition

As previously discussed, the layered 2D materials have weak inter-planar bonding and hence are referred to as vdW materials [31]. In the 2D crystals, the interlayer coupling in heterostructures results in different vibrational properties compared to those of pristine crystals which are easily studied using Raman spectroscopy [32], [33]. Two Raman modes, such as E2g1 because of in-plane vibration and A1g because of out-of-plane vibration of atoms as shown in Fig. 3(a), are characteristics of a

Light–matter management

Controlling how light interacts with 2D materials is important for various applications such as photovoltaics [45], [46], [47], [48], [49], [50], [51]. Flexible 2D materials are easily integrated on a silicon chip by forming silicon 2D material hetero-junction solar cells [52]. The built-in electric field at the interface of Si-MoS2 aids photo-generation of the electrons [52]. The intrinsic properties such as absorption of photons and thereby generation of excitons (electron-hole pairs [53])

Bulk single crystals of transition metal dichalcogenides

The top-down approach of mechanical/liquid exfoliation techniques requires bulk single crystals of TMDCs. The single crystals of these materials can be grown using a technique known as the chemical vapor transport (CVT) method. In this method, single crystals of a compound are grown by transport using a suitable transport agent of either the pre-reacted polycrystalline material of the same compound [90] or a mixture of the unreacted constituent elements of the compound taken in a stoichiometric

Growth of 2D crystals

Scalable, efficient, and large-scale growth and transfer of 2D layers are extremely important for applications in optoelectronic devices. The growth methods of 2D crystals can be broadly divided into two categories: top - down methods and bottom-up methods.

Optical/phase contrast

After mechanical exfoliation, identifying the number of layers can be completed by first using an optical microscope; for example, graphite crystallites using a white light could be identified via optical contrast (OC). The OC of a 2D crystal on a dielectric substrate, mostly a few 100 nm of SiO2 on Si(100), is obtained by measuring the relative intensities of reflected light intensity without a 2D crystal, I (n = 1) and, reflected intensity with the 2D crystal, I (n = n1). The OC is defined by

Communication and energy applications

As previously discussed, the binary TMDCs are excellent sources of single photons and these sources can be deterministically transferred on the desired waveguide, for example, a silver nanowire [164]. Thus using TMDCs for sub-wavelength guiding in integrated nanophotonic circuits can be achieved for higher processing-speed and lower-operating-power devices [164].

Conclusion and outlook

A comprehensive literature survey of the growth of TMDC monolayers using top-down and bottom-up approaches was undertaken. In the top-down method, we discussed in detail the growth of high-quality bulk single crystal using the CVT method. Thus, the grown high purity crystal could be mechanically exfoliated and transferred onto a desired substrate or structure for optoelectronic applications. We believe that most of the literature uses a mechanical exfoliation first-hand method for obtaining a

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the Birla Institute of Technology, Mesra, Ranchi, India, for providing research facilities. Laxmi Narayan Tripathi and Sourabh Barua acknowledge the Ministry of Human Resource Development (MHRD), Government of India for support through the TEQIP-III and Collaborative research project numbers: CRS-ID: 1-5736483014 and CRS-ID: 1-5743255881 respectively. We acknowledge Akriti Raj for a useful discussion regarding growth methods of 2D crystals and Lakshmi Narayan Sharma for

Laxmi Narayan Tripathi is an assistant professor (TEQIP-III) at Birla Institute of Technology, Mesra, India. His current research interests include plasmonic-nanoantennas for enhanced light matter interaction in two-dimensional materials for enhanced photovoltaic efficiency and quantum communication. He studied exciton-plasmon coupling both experimentally and using finite-difference time-domain simulations for Ph.D. at Indian Institute of Science, Bangalore, India with Prof. Jaydeep Basu. In

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    Laxmi Narayan Tripathi is an assistant professor (TEQIP-III) at Birla Institute of Technology, Mesra, India. His current research interests include plasmonic-nanoantennas for enhanced light matter interaction in two-dimensional materials for enhanced photovoltaic efficiency and quantum communication. He studied exciton-plasmon coupling both experimentally and using finite-difference time-domain simulations for Ph.D. at Indian Institute of Science, Bangalore, India with Prof. Jaydeep Basu. In his post-doctoral studies at Seoul National University with Prof. Dai-Sik Kim, he fabricated metal nanogaps and measured terahertz field enhancement for biosensing applications. At University of Würzburg, Würzburg, Germany, working with Dr. Christian Schneider and Prof. Sven Höfling, he reported plasmon-enhanced single-photon emission from two-dimensional materials.

    Sourabh Barua is an Assistant Professor at Birla Institute of Technology, Mesra, India under the TEQIP-III programme. His research interest is in low-temperature electrical and magnetotransport studies of topological insulators and transition metal dichalcogenides. In his Ph.D. at Indian Institute of Technology Kanpur, India, he has studied along with Prof. K. P. Rajeev, topological insulator materials like Bi2Te3, using the Shubnikov-de Haas oscillations in the magnetoresistance. In the Superconductivity and Magnetism group at Warwick University, he has worked with Prof. Geetha Balakrishnan and Dr. Martin R. Lees, on growing crystals of various transition metal dichalcogenides and studied these materials using various experimental methods like electrical transport, magnetoresistance, magnetization and specific heat measurements. One notable result among these was the report of Kondo like effect in the 2-dimensional material VSe2.

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