Two-dimensional (2D) layered materials have recently attracted much attention for both fundamental studies and future technological applications. To use thin 2D layered materials as next generation devices, well-controlled fabrication and sophisticated measurement are indispensable. In this issue, we present recent situation of growth and in-situ observations of two-dimensional layered materials.
Conventional semiconductor heterojunctions with two-dimensional (2D) interfaces have been an important topic, both in solid state physics and in electronics and optoelectronics applications. Recently, the in-plane heterostructures based on 2D materials are expected to provide a novel one-dimensional (1D) interface with unique physical properties and applications. Even though there have been many reports on the growth studies of such heterostructures, it is still an important challenge to develop a sophisticated growth process of novel heterostructures/superlattices and high-quality samples without interface degradation, contamination and/or alloying. In this article, the author introduces our recent progresses of thermal chemical vapor deposition (CVD) growth of 2D materials including graphene, hexagonal boron nitride, and transition metal dichalcogenide (TMDC), and their heterostructures.
We have developed a new method based on novel plasma catalytic reaction for directly fabricating narrow graphene nanoribbon (GNR) devices. Since the establishment of our novel GNR fabrication method, GNRs can be grown at any desired position on an insulating substrate without any post-growth treatment, and the wafer-scale synthesis of suspended GNR arrays with a very high yield (over 98%) is realized. The growth dynamics of suspended GNR is also investigated through the systematic experimental study combined with molecular dynamics simulation and theoretical calculations for phase diagram analysis. Furthermore, unique optoelectrical property, known as persistent photoconductivity (PPC), is also observed in our suspended GNR devices. By using the PPC, GNR-based non-volatile memory operation is demonstrated. We believe that our results can contribute to pushing the study of GNRs from basic science into a new stage related to the optoelectrical applications in industrial scale.
Two-dimensional (2D) materials, including graphene, hexagonal boron nitrides, transition metal dichalcogenides (TMDs), and their heterostructures, have recently attracted a great deal of attention. Various 2D materials can respond to external stimuli differently, which leads to novel physical properties and possible high-performance device applications. To push forward the 2D-material-based science and technology, application of advanced thin-film growth techniques, such as molecular beam epitaxy, is indispensable. In this paper, I focus on the present status and future prospects of MBE growth of TMDs.
Transition metal dichalcogenides (TMDs), a group of 2-dimensional layered materials, show excellent physical properties. In spite of its high expectations, the synthesis of TMDs is not yet well-established. Here, we show studies of sputtering deposition for fabrication of TMDs. Sputtering shows high uniformity, high controllability of the thickness, and suppression of impurities. Application of DC bias was employed to prevent sulfur loss during deposition. Each sputtering condition was also rigorously examined, and as a result, high quality films with S/Mo satisfying 2.0 and Hall mobility of approximately 300 cm2 V－1 s－1 were fabricated. The sputtering parameters were expressed in terms of “ion bombardment parameter” to reveal the relation between the deposition parameters and the film quality. Alloys of TMD were also fabricated with well-controlled composition. The relation between alloy formations and the deposition or thermal treatment condition was revealed which is crucial in order to produce films without phase segregation.
We report the growth of MoS2 nanowires by CVD using FeO nanoparticle catalysts. It was found that the shapes and sizes of the FeO nanoparticles drastically changes the product, i.e., switching between amorphous SiO2 nanowires and MoS2 multiwall nanotubes. The mechanism of the growth is discussed based on the experimental results. The effect of other transition metal oxides as catalysts was examined. The electronic structure of the MoS2 nanotubes was calculated and their distinctive nature was revealed.
Two-dimensional (2D) materials are attracting intense attention due to their intriguing physical properties and a wide range of potential applications. Cost-efficient methods of growing large-scale, high-quality 2D crystals are requisites for device applications of 2D materials. To establish the growth techniques, understanding of the growth mechanism is important. For this purpose, we have investigated growth processes of 2D materials using in-situ low-energy electron microscopy (LEEM). In this paper, we report LEEM observations of graphene segregation on a polycrystalline Ni foil and silicene growth on a Ag(111) substrate.
Graphene, an atomically thin sheet composed of sp2 carbon atoms, has been the most attractive material in this decade. The fascinating properties of graphene are exhibited when it is monolayer. Chemical vapor deposition (CVD) is widely used to produce monolayer graphene selectively in large-area. Here we introduce “radiation-mode optical microscopy” which we have developed in order to realize the in-situ observation of the CVD growth of graphene. We show the method to observe graphene as bright contrast on Cu substrates in thermal radiation images. The growth mechanism, the nucleation site and rate limiting process, revealed by the in-situ observation is presented. Finally, we show the CVD growth of graphene on Au substrates, resulting in the tuning of the emissivity of graphene by the pre-treatment procedures. Our method is not only a way to observe the graphene growth but also shed light on the thermal radiation property of graphene.
We have observed permeated deuterium on stainless steel surface by electron stimulated desorption (ESD) method with scanning electron microscope (SEM). The deuterium distribution was obtained by exposing deuterium from backside of sample and ionizing permeated deuterium by scanning electron beam. The deuterium was exposed from backside at 0.1 MPa. The sample is SUS304, which contains martensite dislocations in austenite structure. Thickness and grain size are both about 100 µm. It is found that deuterium concentration on stainless steel surface depends on the grains by comparing the SEM image and the ESD ion map at 473 K.