Since transition metal dichalcogenides (TMdDs) had been intensively interest during my Ph.D. period due to its sizable band gap which is absent in graphene and fascinating properties with two dimensional nature, I had mainly focused on synthesis and modification of TMD material by using chemical vapor deposition (CVD). Here it is, I presents two representative research works. The first is synthesis of monolayer tungsten disulfide (WS2) on gold foil by CVD and the second is telluriding monolayer MoS2 and WS2 via alkali metal scooter. 1) Synthesis of centimeter-scale monolayer WS2 on gold foil by CVD.Since the advent of two-dimensional (2D) metallic graphene and insulating h-BN, monolayer semiconducting transition metal dichalcogenides (s-TMdCs) has attracted attention for a possibility to be used as a building block for realization of transparent and flexible nano-devices. Among s-TMdCs, tungsten disulfide (WS2) has drawn great attention due to its high PL quantum efficiency and mobility.Recently, a lot of researches have been conducted to synthesize WS2 by chemical vapor deposition (CVD). However, barriers still exists in terms of scalability, uniformity, and quality in CVD synthetic process. A new strategy is required to overcome these problems for applying WS2 to real industry. We introduce gold foil as a high catalytic reactive substrate with precursors for synthesizing monolayer WS2. The synthetic scale of WS2 film was simply proportional to an area of gold foil and CVD chamber size. Currently we obtained maximum 6 cm2 WS2 film. Monolayer WS2 film grown on Au foil is highly uniform and predominantly monolayer around 99% over the whole area. For economical and convenient transfer process, we adopt bubbling transfer which is non-destructive and fast way to transfer 2D material to an arbitrary substrate. After transfer process, we can repeatedly use Au foil. We confirmed the crystallinity of WS2 film by observing average size of WS2 flakes of over 100 μm and fabricating FET device which show high on-off ratio (108) and high mobility around 20 cm2 V-1 s-1 (highest value in CVD grown sample). This suggests that reasonable quality of our grown WS2 film. We believe that our finding allows not only to realize the high performance of optoelectronic devices including solar cell, photodetector, and light emitting device, but also to pave a pathway for the synthesis of other TMdCs. 2) Telluriding monolayer MoS2 and WS2 vial alkali metal scooter.Monolayer transition metal dichalcogenides (TMdCs) has been realized by many approaches but chemical vapor deposition (CVD) is mostly adopted because of its scalability and high crystallinity.The conversion of chalcogen atoms into other type of atoms in monolayer TMdCs can also give us a way to synthesize various TMdCs by thermal treatment under target chalcogen rich conditions (ex: MoS2 in Se vapor with heatMoSe2). Since its ease of use for alloying and providing facile synthetic way from host material without structural deformation, conversion process has been utilized to modified TMdCs properties such as band gap, hydrogen evolution reaction rate and spin orbit coupling strength via alloying and also fabricating 2-dimensional (2D) lateral heterojunction ,which has been only realized by CVD growth process, by selective area conversion. Even though sulfur, selenium and tellurium can be converted each other in TMdCs, Te-conversion has been hindered only in sulfur and selenium but no tellurium case due to low reactivity of Te atom than S and Se and low thermal stability of transition metal telluride. Therefore, it is worthy to develop a way to enlarge conversion window up to tellurium because recently tellurides has acquired a great importance due to its intriguing physical phenomena such as giant magneto resistance, Weyl semimetal feature and unique phase engineering. Moreover, Te-alloy with tailored spin-orbit coupling will further elucidate such exotic physical phenomena.We report on efficient tellurization process for MoS2 and WS2 via Na-scooter (Na2Te). Reduced activation barrier height of tellurization by Na-scooter leads us to convert of MS2 to MTe2 with retaining stable products of tellurides. Tellurization behavior depending on Na-scooter content and temperature are further studied and characterized thoughtfully. 1) We dramatically decreased activation barrier height (0.73 eV or ~300 ⁰C temperature reduction) for tellurization of monolayer MS2 via introducing Na-scooter. This temperature reduction leads to lower the reaction temperature as low as 530 ⁰C below dissociation temperature (700 ⁰C) of MoTe2, which is crucial to stabilize converted tellurides. (No tellurization nor etching occurred without Na-scooter).2) Edge selective tellurization is realized, which leads us to make 2D lateral heterojunction (MoS2/MoTe2 or WS2-xTex/WTe2).3) The band gap tuning window is enlarged from 2.1 eV (MoS2) to 1.1 eV (MoTe2). 4) We investigate Raman spectra and band gap shrinking of Janus phase MoS1Te1 (where Te atoms are located only in the top layer but S atoms still remain in the bottom layer) 5) We construct a new tellurization phase diagram of MoS2 with temperature and Na-Scooter concentration. MoS2-xTex Alloy, 2H and 1T’-MoTe2 phase can be obtained by adjusting tellurization parameters. 6) Spin-orbit coupling is efficiently engineered by Te-alloying in WS2-xTex which demonstrates enhanced valley polarization up to ~37% (pure WS2 shows ~3%) at room temperature. In one sentence summary, by introducing a sodium scooter as a shuttle to supply Te atoms and as a catalyst to lower the conversion barrier height, we were able to lower the reaction temperature below dissociation temperature of the product, which is the key step to retain the chemical stability of the product during conversion process.