Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin–orbit coupling and favourable electronic and mechanical properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examined and their properties are discussed, with particular attention to their charge density wave, superconductive and topological phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties. Two-dimensional transition metal dichalcogenides (TMDCs) exhibit attractive electronic and mechanical properties. In this Review, the charge density wave, superconductive and topological phases of TMCDs are discussed, along with their synthesis and applications in devices with enhanced mobility and with the use of strain engineering to improve their properties.

2D transition metal dichalcogenides

Two-dimensional layered materials (2DLMs) have been a central focus of materials research since the discovery of graphene just over a decade ago. Each layer in 2DLMs consists of a covalently bonded, dangling-bond-free lattice and is weakly bound to neighbouring layers by van der Waals interactions. This makes it feasible to isolate, mix and match highly disparate atomic layers to create a wide range of van der Waals heterostructures (vdWHs) without the constraints of lattice matching and processing compatibility. Exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors or insulators, new designs of electronic devices emerge, including tunnelling transistors, barristors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities. We review the recent progress and challenges, and offer our perspective on the exploration of 2DLM-based vdWHs for future application in electronics and optoelectronics. With a dangling-bond-free surface, two dimensional layered materials (2DLMs) can enable the creation of diverse van der Waals heterostructures (vdWHs) without the conventional constraint of lattice matching or process compatibility. This Review discusses the recent advances in exploring 2DLM vdWHs for future electronics and optoelectronics.

Van der Waals heterostructures and devices

The performance of electronic and optoelectronic devices based on two-dimensional layered crystals, including graphene, semiconductors of the transition metal dichalcogenide family such as molybdenum disulphide (MoS2) and tungsten diselenide (WSe2), as well as other emerging two-dimensional semiconductors such as atomically thin black phosphorus, is significantly affected by the electrical contacts that connect these materials with external circuitry. Here, we present a comprehensive treatment of the physics of such interfaces at the contact region and discuss recent progress towards realizing optimal contacts for two-dimensional materials. We also discuss the requirements that must be fulfilled to realize efficient spin injection in transition metal dichalcogenides.

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Electrical contacts to two-dimensional semiconductors

The discovery of graphene and other two-dimensional (2D) materials together with recent advances in exfoliation techniques have set the foundations for the manufacturing of single layered sheets from any layered 3D material. The family of 2D materials encompasses a wide selection of compositions including almost all the elements of the periodic table. This derives into a rich variety of electronic properties including metals, semimetals, insulators and semiconductors with direct and indirect band gaps ranging from ultraviolet to infrared throughout the visible range. Thus, they have the potential to play a fundamental role in the future of nanoelectronics, optoelectronics and the assembly of novel ultrathin and flexible devices. We categorize the 2D materials according to their structure, composition and electronic properties. In this review we distinguish atomically thin materials (graphene, silicene, germanene, and their saturated forms; hexagonal boron nitride; silicon carbide), rare earth, semimetals, transition metal chalcogenides and halides, and finally synthetic organic 2D materials, exemplified by 2D covalent organic frameworks. Our exhaustive data collection presented in this Atlas demonstrates the large diversity of electronic properties, including band gaps and electron mobilities. The key points of modern computational approaches applied to 2D materials are presented with special emphasis to cover their range of application, peculiarities and pitfalls.

https://www.rsc.org/suppdata/cs/c4/c4cs00102h/c4cs00102h1.pdf

An atlas of two-dimensional materials

High charge-carrier mobility that enables the observation of quantum oscillation is reported in mono- and few-layer MoS2 encapsulated and contacted by other two-dimensional materials.

/pdf/multi-terminal-transport-measurements-of-mos2-using-a-van-guhknqbhgy.pdf

Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform.

A highly flexible and transparent transistor is developed based on an exfoliated MoS2 channel and CVD-grown graphene source/drain electrodes. Introducing the 2D nanomaterials provides a high mechanical flexibility, optical transmittance (∼74%), and current on/off ratio (>10(4)) with an average field effect mobility of ∼4.7 cm(2) V(-1) s(-1), all of which cannot be achieved by other transistors consisting of a MoS2 active channel/metal electrodes or graphene channel/graphene electrodes. In particular, a low Schottky barrier (∼22 meV) forms at the MoS2 /graphene interface, which is comparable to the MoS2 /metal interface. The high stability in electronic performance of the devices upon bending up to ±2.2 mm in compressive and tensile modes, and the ability to recover electrical properties after degradation upon annealing, reveal the efficacy of using 2D materials for creating highly flexible and transparent devices.

Highly flexible and transparent multilayer MoS2 transistors with graphene electrodes.

Objectives This study aims to investigate the learning of university students who had Education for Social Responsibility practice experiences participating in Urban Innovation School in Busan project through a descriptive case study. The learning of research participants who practiced ESR was analyzed by a qualitative method to explore implications for ESR research, education, and practice. Finally, this study aims to identify the meaning and value of ESR. 
Methods In order to achieve the research purpose, Urban Innovation School in Busan project was selected as a case to be analyzed. In this study, a descriptive case study was employed. The results of this study were derived based on the qualitative analysis of the in-depth interview data, and additionally, the research team performed participant observation and analysis of document data on the research participants. 
Results In order to achieve research objectives and derive research results on research problems, this study analyzed in detail the learning that research participants experienced in the process of practicing ESR based on a descriptive case study. The results of this study were as follows. First, the participants experienced independent learning and growth through the application of knowledge and skills by practicing ESR. Second, the participants experienced a change of consciousness about social issues by practicing ESR. Third, it was found that the unstructured problem-solving ability of the participants was increased by practicing ESR. Fourth, the perception of influence of individuals on society was improved. 
Conclusions This study confirms that the experience of ESR practice and participation can promote the continuous growth of members of society and the realization of self-actualization. It was confirmed that the university's ESR practice project is an opportunity to seek solutions to various and complex social problems that cannot be predicted in modern and future society.

A Case Study on ESR Project Participation Experience of University Students: Focused on ‘Urban Innovation School in Busan’

Objectives The purpose of this study is to develop and validate a Process-based Assessement Scale for meddle school teachers' expertise to enhance the assessment competencey. 
Methods The preliminary version of the scale which was developed based on a theoretically grounded definition of teacher’s evaluation expertise and Process-based Assessment was refined and revised through focus group interview with middle school teachers and experts in the field of educational measurement and evaluation. A pilot test was conducted on samples of 315 middle school teachers to perform Rasch item fit, category probability curve, item difficulty, exploratory factor analysis, and confirmatory factor analysis. For statistical programs Jmetrik, AMOS 22.0, SPSS 26.0 were used. 
Results The result supporting the model fit, construct, discriminant validity of the 5 factors. And the item fit, categorical probability curve, and item difficulty on the five-point scale were also acceptable. Consequently, 16 items were derived with 5 factors: ‘understanding Process-based Assessment’, ‘establishment of Process-based Assessment plan’, ‘development of Process-based Assessment tool’, ‘execution of Process-based Assessment’, and ‘sharing and utilization of Process-based Assessment results’. 
Conclusions We discussed the academic and practical significance of this study in the aspect of the diffusion and substantialization of the policy, especially, as a tool for measuring educational performance outcomes following the operation of the Process-based Assessment. It is expected that the developed scale will be used to supplement the shortcomings of middle school teachers by identifying the elements necessary for them to have Process-based Assessment expertise. 
 


A Validation of Process-based Assessement Scale for Middle School Teachers' Expertise

Objectives This study classified youths living in Busan into different latent groups depending on career development competencies and performed an analysis to identify what factors can be used to predict related patterns including a pattern of transition between latent groups. 
Methods In this study, we used the first wave (2016, first grade middle school students) and fifth wave (2020, second grade high school students) of data from Busan Education Longitudinal Study (BELS). The subjects are 1,915 youths, who responded to all of the 16 questionnaire items concerning career development competencies. We performed latent transition analysis to identify the latent profile types and transition between classes across two-time points. Subsequently, multinomial logistic analysis was applied to explore variables that can be used to predict a transition pattern. 
Results First, the first grade middle school students and second grade high school students were classified into two groups: active and passive career planning groups. Second, the patterns of profile transition between the two points in time were classified into two types: maintenance and change types. Generally, latent groups of career development competencies in middle school transitioned into similar latent groups. The probability of maintenance was higher in an active career planning group than in other types. Of the subjects, 29.4% showed negative change, while 36.4% showed positive change. Third, the analysis of factors affecting career development competencies of middle schoolers found that the first grade middle school students were more likely to belong to the active career planning group if they had visited an upper school, attended a school admission briefing session or collected information from media and online communities. For the second grade high school students, a consultation with the homeroom teacher was not a statistically significant variable. They were more likely to belong to the active career planning group if they had a consultation with the career teacher at school. 
Conclusions The variables having a significant effect on latent groups were different between middle and high schoolers, and there were no variables affecting the transition between the two points in time. What is emphasized in career experience needs to be different in each grade. 


Ga Yeong Bae

Papers

Highly flexible and transparent multilayer MoS2 transistors with graphene electrodes.

A Case Study on ESR Project Participation Experience of University Students: Focused on ‘Urban Innovation School in Busan’

A Validation of Process-based Assessement Scale for Middle School Teachers' Expertise

A Study on Types of Latent Transition of Youth’s Career Development Competencies and Related Variables: Focused on Busan Education Longitudinal Study