Vibrational and electronic structures of tin selenide nanowires confined inside carbon nanotubes
Introduction
Inorganic and organic nanowires and nanostructured materials like nanorods or nanoplates are a topic of utmost importance in many studies of physics, chemistry, biology, and medicine. The mechanical, electrical, electronic, magnetic, and chemical properties of these nano-objects can differ considerably from those of the parent bulk materials leading to applications for storage devices, electronic chips, photovoltaics, batteries, and therapeutic processes [1], [2], [3], [4], [5], [6], [7].
In particular, tin selenide, SnSe, is an important binary chalcogenide material which displays remarkable electronic, thermoelectric, and optoelectronic properties depending on its morphology as bulk crystal, thin films, or low-dimensional nanostructures [8]. This material can be applied notably for photovoltaics, anodes in rechargeable batteries, supercapacitors, phase-change memory devices, and topological insulators. Bulk tin(II) selenide is a narrow band-gap (IV-VI) semiconductor with both indirect and direct band gaps of 0.9 eV and 1.3 eV, respectively. The bulk chalcogenide crystal has an accordion-like stacked two-dimensional (2D) structure that gives it low anharmonicity and low lattice thermal conductivity making it a poor thermal conductor while its ZT factor of merit has a high temperature value of 2.6 [9],[10].
Single-walled carbon nanotubes (SWCNT) are low-dimensional allotropes of carbon which have unique mechanical, electronic and optical properties depending on their morphology [11]. As expected for one-dimensional (1D) systems, they present van Hove singularities in the density of states whose energies vary with the diameter and length of the nanotube. Depending on their chirality SWCNTs are either metallic or semi-conductors allowing their application in flexible electronic devices, lithium battery anodes, flat panel displays, sensors, supercapacitors, drug delivery, and STM/AFM tips [12], [13]. Therefore, binary chalcogenides combined with SWCNTs can give rise to very unusual physical properties in the as-grown composites, but also this can foster drastic changes in the electronic properties of either component. Recently, several binary chalcogenides have been encapsulated in both single-walled and double-walled carbon nanotubes (DWCNT) resulting in unique 1D chalcogenide crystalline wires with diameter comparable to a unit cell of the 3D crystal [14], [15], [16], [17]. Due to their extreme low lateral size and specific atomic arrangement, the physical properties of these confined and atomically constrained nanowires are expected to be significantly different from those of the corresponding parent bulk materials.
Here we present a detailed comparative study of the structural, electronic, and vibrational properties of two original nanostructures of atom-sized SnSe nanowires encapsulated in SWCNTs. Previous high-resolution transmission electron microscopy (HRTEM) investigations have shown that SnSe crystallizes into (2 × 2)-atom per cross section structure inside SWCNTs, adopting the same atomic arrangement as earlier described 1D (2 × 2) KI nanocrystals, hereafter referred to as Sn4Se4, embedded in double-walled or single-walled carbon nanotubes [14]. Recently, a novel 1D hexagonal SnSe crystalline form was discovered in SWCNTs [18] with an Mo(S,Se)-type of coordination environment [19], [20], hereafter referred to as Sn6Se6, comprising three Sn atoms and three Se atoms per constituent layer, which is not observed in any of the other low-dimensional forms of the selenium chalcogenides. Interestingly, Nagata et al. [17] reported also similar MoTe nanowires structures in SWNTs. Previous DFT simulations predict a metallic nature of hexagonal MX nanowires with M=Mo,W and X=S,Se [21], [22] but semiconducting properties for the isostructural Cr6Te6 compound [21]. Both, theoretical simulations and experiments, confirm the structural stability of the stand-alone Mo(S,Se) nanowires, which were found as effective nanoscale electrical connectors between Mo(S,Se)2 monolayers [19], [20]. In this aspect the physical properties of the isostructural Sn-based nanowires are far from being understood, and our work has been motivated by the following questions:
- (i)
Are the Sn4Se4 and Sn6Se6 nanowires structurally stable, or can they exist only encapsulated in SWCNTs?
- (ii)
Are the two types of nanowires semiconducting or metallic? While Carter et al. previously predicted an expanded band gap structure for the (2 × 2) Sn4Se4 nanowire using different functional and pseudopotentials [16], electronic properties of the then unobserved Sn6Se6 1D crystal were not determined.
- (iii)
What are the vibrational modes of both Sn4Se4 and Sn6Se6 nanowires which could give signatures in Raman spectra of SnSe@SWNT composites? This question was not treated up to now.
- (iv)
What is the impact of encapsulation of the SnSe material on the physical properties of the surrounding nanotubes with matching diameter, e. g. metallic or semiconducting character ?
We addressed the first problems theoretically by means of extensive DFT calculations of the equilibrium structure, electronic band structure, and vibrational eigenmodes for the two forms of SnSe nanowires. In order to assess the last two questions, we studied experimentally the Raman spectra of SnSe@SWCNT composites. Raman spectroscopy has proven to be a very efficient tool for characterizing SWCNTs embedding 1D atom-sized crystalline HgTe and KI nanowires [23], [24]. In both cases it has been demonstrated that it is possible to identify several intrinsic vibrational modes of the nanowires superimposed with the Raman spectrum of SWCNTs. By following the same methodology here we study the in-situ dynamics of the embedded SnSe nanocrystals and their interactions with the nanotube walls. The radial breathing mode (RBM) and longitudinal vibrational modes (G-band) of the surrounding nanotubes are found to be strongly affected by the presence of the encapsulated material, which leads us to important conclusions concerning the electronic properties of the SnSe@SWCNT composites corroborating the DFT predictions.
Section snippets
Experimental methods and calculation details
The SnSe@SWCNT samples were fabricated using infiltration protocols described previously [16], [18]. In this set of experiments, commercial SWCNTs from NanoIntegris PureTubes (99% pure, 1.2–1.7 nm diameter, 300 nm to 5 µm length) were used to encapsulate tin selenide. Here, the SWCNTs were washed, dissolved in N- Methyl-2-pyrrolidone (NMP), dried, pre-heated to 1173 K to remove solvent and to open the tubes. The nanotubes were filled by the modified high yield molten-phase capillarity technique
Results and discussion
Experimental Raman spectra are discussed after the subsection of SnSe nanowires calculations. The spectra were recorded on several pristine and composite SWCNT samples. We sorted among these spectra those of three SnSe@SWCNT composites referred to as # 1, # 2, and # 3. They mainly differ from each other in the low frequency region characteristic of the RBM modes of the SWCNT molecular containers and of possible lines stemming from the SnSe encapsulated crystals. Spectra labelled # 0 are from
Conclusion
We have performed a combined experimental and theoretical study of SnSe@SWCNT composites where periodic DFT calculations support the Raman spectra recorded on several samples. At the HSE level of theory, the Sn4Se4 1D crystal exhibits a direct gap larger than that of the 3D crystal allowing direct optical excitations in the near infrared. In contrast, the band structure of the 1D hexagonal crystal is typical to that of a semimetal due to the overlap at the Fermi level of the upper valence band
Author statement
EF, NK, and VG have realized Raman experiments. EF and VG managed project administration with funding and performed DFT calculations. JS, AS, and CS performed HRTEM measurements and found the SnSe nanowires structures. All authors contributed to the writing and the review editing of the paper.
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
We acknowledge Dr. C. Ewels for helpful discussions. This work was supported by the French-Bulgarian bilateral exchange program through Campus France Partenariat Hubert Curien RILA Grants No. 38661ZF and No. DHTC France 01/10/09.05.2017, France, and by Contract No. KP-06-N38/10/6.12.2019 of the Bulgarian National Science Fund, Bulgaria; and by the NATO SPS MYP program N∘ 985481 “Quantum sensor” between Ukraine and France. JS is indebted to the EPSRC for additional support from EPSRC Grants
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