Letter pubs.acs.org/NanoLett
A Silicon-Based Two-Dimensional Chalcogenide: Growth of Si2Te3 Nanoribbons and Nanoplates Sean Keuleyan, Mengjing Wang, Frank R. Chung, Jeffrey Commons, and Kristie J. Koski* Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *
ABSTRACT: We report the synthesis of high-quality singlecrystal two-dimensional, layered nanostructures of silicon telluride, Si2Te3, in multiple morphologies controlled by substrate temperature and Te seeding. Morphologies include nanoribbons formed by VLS growth from Te droplets, vertical hexagonal nanoplates through vapor−solid crystallographically oriented growth on amorphous oxide substrates, and flat hexagonal nanoplates formed through large-area VLS growth in liquid Te pools. We show the potential for doping through the choice of substrate and growth conditions. Vertical nanoplates grown on sapphire substrates, for example, can incorporate a uniform density of Al atoms from the substrate. We also show that the material may be modified after synthesis, including both mechanical exfoliation (reducing the thickness to as few as five layers) and intercalation of metal ions including Li+ and Mg2+, which suggests applications in energy storage materials. The material exhibits an intense red color corresponding to its strong and broad interband absorption extending from the red into the infrared. Si2Te3 enjoys chemical and processing compatibility with other silicon-based material including amorphous SiO2 but is very chemically sensitive to its environment, which suggests applications in silicon-based devices ranging from fully integrated thermoelectrics to optoelectronics to chemical sensors. KEYWORDS: Si2Te3, silicon telluride, 2D materials, layered chalcogenide, exfoliation
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growth either standing out of the substrate, where the orientation is controlled by the initial interaction of the precursor vapors with the substrate, or flat on the substrate through the large-area VLS mechanism. Control of crystal orientation relative to the substrate opens access to a whole host of potential device geometries.9 Substrates can be used to dope Si2Te3 nanoplates through growth, possibly as a means to increase conductivity. Exfoliation of this material can be readily achieved to yield five layers thick or less of Si2Te3. Given the growing interest in silicon-based systems for alternative energy storage technologies,10 and the unique layered structure of Si2Te3 nanomaterials, we show that this material can be chemically intercalated with lithium and magnesium. Si2Te3 exhibits a remarkable layered, trigonal crystal structure (space group P3̅1c; Figure 1a). Si atom pairs are distributed among four equivalent orientations inside octahedral vacancies in a hexagonal close-packed Te lattice. Silicon occupancy varies throughout the lattice with 28 possible positions for eight silicon atoms. Silicon occupancies11 are represented in Figure 1 by the transparency of Si atoms. The conventional unit cell spans two layers with 20 atoms per unit cell, with lattice constants a = 7.429 Å and c = 13.471 Å.11 Layer (0001) planes of −Te−Si−Te− are shifted relative to each other and are held
ilicon dominates semiconductor technology not simply because of its properties as a semiconductor, which are actually quite ordinary, but because of its compounds. Silicon processing technology is as much about silicon oxide, silicides, and silicon nitride as it is about silicon itself. Silicon forms the base of a large class of materials with extraordinary properties and an amazing degree of mutual processing compatibility. These compounds enable silicon’s high performance and allow practical processing, which provide the backbone of much of modern technology. While significant recent research efforts have focused on new two-dimensional (2D) materials ranging from graphene1,2 to layered chalcogenides such as MoS23,4 or Bi2Se3,5 Si2Te3 seems an obvious choice for bringing unique 2D material properties into the realm of integrated electronic and optical applications. Si2Te3 was previously identified as the only binary phase in the Si−Te system.6 Bulk crystals, up to 1 cm across and 500 μm thick, were previously grown by vapor and iodine transport. Bulk silicon telluride is a layered p-type semiconductor, with an indirect gap near 1 eV and a direct gap near 2 eV.7 The typical pure material has a low conductivity with an activation energy much smaller than the gap, which suggests a large defect density, estimated in one report to be ∼1017 cm−3.8 We present a method to prepare Si2Te3 nanoplates of 50− 1000 nm thickness and ribbons roughly 300 nm wide and ∼10 μm long. Nanoribbons grow in the presence of liquid Te droplets, which suggests the vapor−liquid−solid (VLS) growth mechanism. Nanoplates grow via layer-by-layer vapor−solid © XXXX American Chemical Society
Received: November 11, 2014 Revised: March 2, 2015
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DOI: 10.1021/nl504330g Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. (a) Crystal structure of Si2Te3 showing a single (0001) layer. Silicon occupies interstitial octahedral sites with occupancy shown through Si transparency. Layers are shifted relative to each other as shown in the lower perspective. (b) At ∼680 °C, substrate temperature in oxygen-free conditions, nanoribbons grow by VLS growth from μm-scale or smaller Te droplets (shown in the right-hand column). (c) At ∼650 °C, freestanding Si2Te3 vertical nanoplates grow from nuclei chemically bound to the oxide substrates. (d) Under conditions of a large excess of Te, flat Si2Te3 nanoplates grow through large-area VLS growth from pools of Te that initially cover much of the substrate (right-hand column). (e) Without excess Te, even at much lower temperatures of ∼425 °C, some flat plates grow from screw dislocations and tend toward macroscale crystal growth.
together by van der Waals interactions. Layers can be mechanically exfoliated using the Scotch tape method.1,2 We find that silicon telluride nanocrystal morphology can be varied from nanoribbons to hexagonal nanoplates (Figure 1b− e) and is controlled through the temperature, pressure, and precursor growth configurations. Si2Te3 was grown in a 12 in tube furnace (Lindberg Blue/M) with a 1 in diameter quartz tube. Te (30 mesh, 99.997%, 100 mg per growth) and Si powders (325 mesh, 99%, 100 mg per growth) from SigmaAldrich were placed in a ceramic crucible at specific temperature regions of the furnace (Figure 2a,b). Substrates of silicon ⟨100⟩, fused quartz, or sapphire were placed downstream in the furnace at a position chosen to reach about 400−680 °C. The temperature profile of the furnace was found to be similar (±10 °C) to that reported in ref 12 for the same model. In a typical growth, the tube was evacuated to 15 cm
substrate temperature ∼680
