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Nanoscale interfaces made easily

Methods for making interfaces between atomically thin sheets of materials might open the way to a range of nanotechnologies. A practically simple method has been reported, based on the cyclical switching of gaseous reagents.
Weijie Zhao is in the Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.

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Qihua Xiong is in the Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore.
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Atomically thin sheets of semiconducting materials, known as two-dimensional semiconductors, have outstanding potential for making low-power, high-speed electronic and optoelectronic devices13, including flexible electronics. Such applications often require heterostructures: interfaces formed between two or more 2D semiconductors, which can either stack on top of each other (vertical heterostructures) or be joined at their edges (lateral heterostructures). Versatile and scalable techniques for the mass production of heterostructures are therefore required. On page 63, Sahoo et al.4 report a substantial advance that allows the controllable growth of seamless, high-quality lateral heterostructures made from widely studied 2D semiconductors known as transition-metal dichalcogenides (TMDs).

Transition-metal dichalcogenides have the general formula MX2, in which M is molybdenum (Mo) or tungsten (W) and X can be sulfur (S) or selenium (Se). Lateral TMD heterostructures can be constructed by ‘stitching’ the edges of two TMD sheets together using covalent bonds. In the past few years, there has been a flurry of papers59 reporting methods for synthesizing TMD lateral heterostructures using edge epitaxial growth, a method that allows a second TMD to grow at the edge of another, pre-grown TMD crystal. These heterostructures can be fabricated into p–n junctions, which conduct currents in only one direction (a property known as rectification), and constitute one of the building blocks of modern electronic and optoelectronic devices. Two-dimensional p–n junctions hold great promise for the development of atomically thin devices such as light-emitting diodes, solar cells and integrated circuits (chips).

Lateral TMD heterostructures have previously been made in one-step procedures5,6 that lacked the flexibility to make multi-junction heterostructures or more than one type of heterostructure, or in two-step or multi-step processes that involve many changes of TMD precursors and reaction chambers79. Sahoo and colleagues’ method overcomes those constraints in a ‘one-pot’ procedure — a process that allows several steps to be performed in one reaction chamber. One of the many advantages of their strategy is the operational simplicity with which different TMDs can be selectively grown.

The authors’ approach builds on a method known as chemical-vapour deposition (CVD), in which a substrate is exposed to gaseous precursor compounds (sometimes mixed with carrier gases) that react or decompose on the substrate to deposit the targeted solid product at an optimal temperature and pressure. The researchers found that 2D MoX2 and WX2 can be grown sequentially from a mixture of powders of the two compounds, thus forming lateral heterostructures, simply by switching the carrier gases in the CVD growth chamber (Fig. 1).

Figure 1 | A strategy for growing lateral multi-junction heterostructures. Interfaces between the edges of atomically thin sheets of different semiconductors are called lateral heterostructures, and have potential technological applications. Sahoo et al.4 report a method for making lateral heterostructures from compounds known as transition-metal dichalcogenides (TMDs), which include molybdenum disulfide (MoS2) and tungsten disulfide (WS2). The authors heat a mixture of two powdered TMDs in a furnace, and pass carrier gases over them (coloured arrows). The carrier gases react with the TMDs to produce gaseous intermediates (not shown), which then react on the surface of a substrate to deposit sheets of the TMDs. When a mixture of nitrogen and water vapour is used as the carrier gas, only MoS2 forms. When the carrier gas is switched to a mixture of hydrogen and argon, the growth of MoS2 is terminated and WS2 grows at the edge of the pre-grown MoS2. By switching cyclically between the carrier gases, 2D multi-junction heterostructures are produced.

The secret to success lies in the intriguing and complicated chemical reactions that occur between the carrier gases and the powdered TMD solids. The reactions produce highly volatile species such as hydroxides and oxides, which undergo redox reactions at distinct rates to deposit MoX2 or WX2 selectively, depending on the carrier gases used. When the carrier is a mixture of nitrogen and water vapour, the growth of only MoX2 is promoted. But when the carrier is switched to a mixture of hydrogen and argon, the volatile molybdenum compounds are quickly depleted by reactions with the hydrogen, so that only WX2 grows. By switching carrier gases multiple times, as many alternating domains of MoX2 and WX2 as desired can be prepared — corresponding to a sequence of lateral heterostructures.

Sahoo and co-workers used high-resolution transmission electron microscopy to show that some types of junction in their heterostructures were seamless and atomically sharp. They also used spectroscopic techniques to confirm the alternating pattern of TMD domains, to verify that each domain contains just one type of TMD, and to show that the junctions in the heterostructures are made reproducibly.

The authors went on to demonstrate that their technique could be used to make multi-junction lateral heterostructures for compounds known as TMD ternary alloys (which contain one type of metal, but a mixture of sulfur and selenium atoms). To do this, the authors used a powdered mixture of MoSe2 and WS2, or of MoS2 and WSe2 (rather than a mixture of MoS2 and WS2, or of MoSe2 and WSe2, as in their first experiments). This produced high-quality, 2D lateral heterostructures consisting of domains containing the alloys MoS2(1−x)Se2x or WS2(1−x)Se2x (where x is a number less than 1). The optical and electrical properties of such heterostructures could now be fine-tuned by altering the alloy composition10.

The authors conducted preliminary electrical characterizations of single-junction heterostructures produced using their method. They observed that planar p–n junctions that formed at the boundaries of electron-doped MoX2 (made by adding a small amount of electrons to MoX2) and hole-doped WX2 (formed by removing a few electrons from WX2) show good rectification behaviour, which is a further indication of the high quality of the heterostructures. They also observed photodiode behaviour — the generation of a substantial current when the junction area was illuminated by light. Having the ability to build such tiny p–n diodes and photodiodes holds great potential for future efforts to miniaturize electronic and optoelectronic devices.

Sahoo and co-authors’ method opens up a promising route for the synthesis of high-quality lateral heterostructures. Insights into the thermodynamics and chemistry operating at the atomic scale in this process are now needed to develop the ability to prepare heterostructures involving any desired combination of TMDs. Moreover, research must be performed to work out why interfaces that switch from MoX2 to WX2 are not as sharp as those in which WX2 switches to MoX2, and to optimize the production of sharper MoX2–WX2 interfaces.

It will also be important to explore variations of the technique that might allow the growth of lateral heterostructures between MX2 and other exotic 2D materials, including those that have metallic, semi-metallic or superconducting properties1,2, to make new types of device. The availability of complex TMD heterostructures — including those that have several junctions in series — should also allow the exploration of fundamental physics, such as the mechanism by which charge transfer occurs at interfaces. Lastly, Sahoo and co-workers’ technique will enable the development of proof-of-concept prototype devices, to advance our knowledge of the viability and scope of 2D technologies.

Nature 553, 32-34 (2018)

doi: 10.1038/d41586-017-08755-8
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