Materials Today
Volume 39, October 2020, Pages 110-117
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Dynamically reconfigurable electronic and phononic properties in intercalated HfS2

https://doi.org/10.1016/j.mattod.2020.04.030Get rights and content

Abstract

Dynamic reconfigurability of material properties is essential to enabling innovative neuromorphic- and quantum-computing paradigms. The unique structure of van der Waals layers can facilitate a robust mechanism for this desired reconfigurability. Here, we present a highly versatile and effective approach, based on electrochemical intercalation of organometallics, to control the electron and phonon behavior in hafnium disulfide. Computational and experimental exploration of the physical properties in the intercalated material indicates a significant and measured change. Furthermore, the weak chemical interactions between the organometallics and hafnium disulfide enable an electric-field mediated intercalant drift and charge–discharge process. The control of organometallic concentration in this way provides a dynamic 400-fold control of cross-plane electrical conductivity (1.8 μS/cm–741 μS/cm) and a corresponding 4-fold control of cross-plane thermal conductivity in hafnium disulfide (0.35 Wm−1 K−1–1.45 Wm−1 K−1). Our findings unveil a broad approach to dynamically design layered-material properties for high-performance electronic and phononic applications.

Introduction

The successful isolation of graphene [1] and the subsequent discovery of other stable atomically-thin two-dimensional materials (2DMs) [2] have revolutionized materials research. The family of 2DMs offer unparalleled structural and electronic diversity that have made them breeding grounds for exploring new concepts and phenomena that until recently were unattainable. These novel properties have been intensively explored for applications in areas of flexible optoelectronics [3], [4], valley- and spin-tronics [5], [6], [7], energy-efficient field-effect transistors (FETs) [8], [9], and next-generation nanoelectronics [10]. While the most unique properties of 2DMs relate to the quantum confinement effects, the task of tuning these properties is essential to device integration. As a result, the development of versatile and effective approaches for modification and control of 2D device properties is key to advances in this field.

To devise material design strategies in 2DMs, it is important to understand and control their most unique physical and structural characteristics. It is known that these layered materials are made of strong intralayer covalent bonds, and the interlayer interactions are mediated by weak van der Waals (vdW) forces consisting of highly tunable vdW gaps [11], [12], [13]. This provides the opportunity for an unprecedented level of control through intercalation and stabilization of guest species in the vdW gaps. The intercalants often induce changes in the magnetic [11], electronic [14], [15], optical [16], and thermal properties [12], [17] of the 2D system and may lead to both compositional and structural disorder in the host material [11], [13], [14], [16]. Most important interactions arise from the redox processes, whereby electrons are exchanged between the guest and host materials. In addition, the complex nature of electron and phonon properties at the interfaces of 2D materials and the intercalants lead to unique collective and tunable properties. As a result, layered material intercalation opens up an extraordinary opportunity to broaden the versatility and the potential to develop superior devices.

Various intercalants, including alkali metals such as Li and Na, pseudo-alkali and monovalent metals such as Ag and Cu, post-transition elements such as Bi and P, and organic Lewis bases of various sizes and shapes, have been successfully intercalated into layered materials [18], [19], [20], [21], [22]. However, the chemistry and functionality of the organometallic intercalants make them an excellent candidate to harness and tailor the novel properties of 2D materials [15], [17], [23], [24]. This is because metal or carbon ring substitution is very effective in altering the steric and electronic properties of organometallics. In this work, we present a broad approach to modify 2D material properties using metallocene intercalation in hafnium disulfide (HfS2). Through first-principles calculations and experiments, we examine the underlying mechanisms of interactions in the intercalated materials. The changes in the structure, electronic, and phonon properties of the hybrid systems are examined. The control of anisotropic electrical and thermal properties demonstrates a dynamic approach to reconfiguring the physical properties of HfS2 devices.

Section snippets

Results and Discussions

Atomically thin HfS2 is a member of group IVB transition metal dichalcogenides (TMDs) with unique electronic and phononic characteristics that can enable distinct device properties [25], [26], [27]. Also, the surface chemistry of HfS2 makes it a favorable candidate for intercalation of metallocenes, cobaltocene Co(Cp)2 and chromocene Cr(Cp)2 (supplementary information). We use a previously verified recipe to intercalate HfS2 (details in methods section) [18], [19]. To examine the yield and

Conclusions

In summary, we demonstrate a highly versatile and wide-ranging approach for control of 2D HfS2 properties based on metallocene intercalation. The approach provides a dynamically controlled process for stimulating substantial change in both the electronic and phononic properties of HfS2. Computational and experimental examination of the interactions between the organic–inorganic species unveils the origins of the physical property changes. We conclude that the large charge transfer between the

Materials

We purchase the hafnium disulfide bulk crystals from 2D semiconductors USA with confirmed purity of 99.9999%. The material is grown by an ultra-pure flux vapor technique (∼3 months of growth time). This procedure produces highly crystalline, impurity-free, and low defect density (as low as (10−9–10−10 cm−2) materials. We exfoliate the HfS2 crystals either with ultra-sonication in liquid or mechanical scotch tape method on Si/SiO2 substrates. These host HfS2 materials are exposed to cobaltocene

CRediT authorship contribution statement

Sina Najmaei: Conceptualization, Methodology, Investigation, Writing - review & editing. Chinedu E. Ekuma: Formal analysis, Writing - review & editing. Adam A. Wilson: Investigation. Asher C. Leff: Investigation. Madan Dubey: Supervision.

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.

Acknowledgements

The research was sponsored by the Army Research Laboratory (ARL). C.E.E. acknowledges a start-up grant from Lehigh University. Supercomputer support is provided by the DOD High-Performance Computing Modernization Program at the Army Engineer Research and Development Center, Vicksburg, MS.

Author contribution

S.N and M. D conceived the idea. S.N designed and performed the material preparation and characterization experiments. S.N also planned and performed all the electrical and Raman measurements. C.E.E designed and performed the simulations. A.C.L performed the TEM imaging and measurements. A. A. W performed the TDTR measurements. All the authors contributed to the analysis of the results. All the authors discussed the results and prepared the paper.

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