Large-area niobium disulfide thin films as transparent electrodes for devices based on two-dimensional materials

Bark, Hunyoung,Choi, Yongsuk,Jung, Jaehyuck,Kim, Jung Hwa,Kwon, Hyukjoon,Lee, Jinhwan,Lee, Zonghoon,Cho, Jeong Ho,Lee, Changgu Royal Society of Chemistry 2018 Nanoscale Vol.10 No.3

<P>Direct contacts of a metal with atomically thin two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have been found to suppress device performance by producing a high contact resistance. NbS2 is a 2D TMDC and a conductor. It is expected to form ohmic contacts with 2D semiconductors because of its high work function and the van der Waals interface it forms with the semiconductor, with such an interface resulting in weak Fermi level pinning. Despite the usefulness of NbS2 as an electrode, previous synthesis methods could not control the thickness, uniformity, and shape of the NbS2 film and hence could not find practical applications in electronics. Here, we report a patternable method for carrying out the synthesis of NbS2 films in which the number of NbS2 layers formed over a large area was successfully controlled, which is necessary for the production of customized electrodes. The synthesized NbS2 films were shown to be highly transparent and uniform in thickness and conductivity over the large area. Furthermore, the synthesized NbS2 showed half the contact resistance than did the molybdenum metal in MoS2 field effect transistors (FETs) on a large transparent quartz substrate. The MoS2 device with NbS2 showed an electron mobility as high as 12.7 cm<SUP>2</SUP> V<SUP>−1</SUP> s<SUP>−1</SUP>, which was three times higher than that found for the corresponding molybdenum-contacted MoS2 device. This result showed the high potential of the NbS2 thin film as a transparent electrode for 2D transition metal dichalcogenide (TMDC) semiconductors with low contact resistance.</P>

Bias-assisted atomic force microscope nanolithography on NbS₂ thin films grown by chemical vapor deposition

Bark, Hunyoung,Kwon, Sanghyuk,Lee, Changgu IOP 2016 Journal of physics. D, applied physics Vol.49 No.48

<P>Niobium disulfide, one of the metallic transition metal dichalcogenides, has a high potential as an electrode material for electronic devices made of 2D materials. Here, we investigated the bias-assisted atomic force microscope nanolithography of NbS<SUB>2</SUB> thin films synthesized by chemical vapor deposition. We analyzed the lithographed pattern using Raman spectroscopy, transmission electron microscopy and friction force microscopy. These analyses showed that lines having various widths and thicknesses could be generated using the lithography technique by simply varying the scan speed and applied voltage. These analyses also revealed that the NbS<SUB>2</SUB> film transformed from a layered crystalline structure into an amorphous structure upon being lithographed. By generating four line segments forming a square and measuring <I>I/V</I> curves inside and outside of the square, the electrical properties of the lithographed material were characterized. These analyses indicate that NbS<SUB>2</SUB> became hydrogenated and an insulator upon being lithographed.</P>

Wafer-scale and patternable synthesis of NbS2 for electrodes of organic transistors and logic gates

Choi, Yongsuk,Bark, Hunyoung,Kang, Boseok,Lee, Myeongjae,Kim, BongSoo,Lee, Sungjoo,Lee, Changgu,Cho, Jeong Ho The Royal Society of Chemistry 2019 Journal of Materials Chemistry C Vol.7 No.28

<P>We developed a patternable synthesis method of wafer-scale NbS2, which can be applied for the fabrication of source and drain electrodes of p- and n-type organic field-effect transistors (OFETs) and logic gates. NbS2 film with high uniformity was synthesized directly on a 2-in Si wafer. NbS2 patterns of various sizes and shapes were readily synthesized onto an entire wafer. OFETs with NbS2 electrodes exhibited superior performances to those with conventional metal electrodes. The superior performance of the former OFETs was primarily a result of the enhanced crystallinity of the organic semiconductor layer deposited onto the NbS2 electrode surface. Furthermore, organic complementary circuits such as NOT, NAND, and NOR gates were successfully assembled using the resulting OFETs as a proof of applicability of these devices to complex logic circuits.</P>

Wafer-Scale Substitutional Doping of Monolayer MoS₂ Films for High-Performance Optoelectronic Devices

Kim, Youngchan,Bark, Hunyoung,Kang, Byunggil,Lee, Changgu American Chemical Society 2019 ACS APPLIED MATERIALS & INTERFACES Vol.11 No.13

<P>The substitutional doping method is ideally suited to generating doped two-dimensional (2D) materials for practical device applications as it does not damage or destabilize such materials. However, recently reported substitutional doping techniques for 2D materials have given rise to discontinuities and low uniformities, which hamper the extension of such techniques to large-scale production. In the current work, we demonstrated uniform substitutional doping of monolayer MoS<SUB>2</SUB> in a 2 in. wafer of area >13 cm<SUP>2</SUP>. The devices based on doped MoS<SUB>2</SUB> showed extremely high uniformity and stability in electrical properties in ambient conditions for 30 days. The photodetectors based on the doped MoS<SUB>2</SUB> samples showed an ultrahigh photoresponsivity of 5 × 10<SUP>5</SUP> A/W, a detectivity of 5 × 10<SUP>12</SUP> Jones, and a fast response rate of 5 ms than did those based on undoped MoS<SUB>2</SUB>. This work showed the feasibility of real-life applications based on functionalized 2D semiconductors for next-generation electronic and optoelectronic devices.</P> [FIG OMISSION]</BR>

Synthesis of wafer-scale uniform molybdenum disulfide films with control over the layer number using a gas phase sulfur precursor

Lee, Youngbin,Lee, Jinhwan,Bark, Hunyoung,Oh, Il-Kwon,Ryu, Gyeong Hee,Lee, Zonghoon,Kim, Hyungjun,Cho, Jeong Ho,Ahn, Jong-Hyun,Lee, Changgu The Royal Society of Chemistry 2014 Nanoscale Vol.6 No.5

We describe a method for synthesizing large-area and uniform molybdenum disulfide films, with control over the layer number, on insulating substrates using a gas phase sulfuric precursor (H2S) and a molybdenum metal source. The metal layer thickness was varied to effectively control the number of layers (2 to 12) present in the synthesized film. The films were grown on wafer-scale Si/SiO2 or quartz substrates and displayed excellent uniformity and a high crystallinity over the entire area. Thin film transistors were prepared using these materials, and the performances of the devices were tested. The devices displayed an on/off current ratio of 10(5), a mobility of 0.12 cm(2) V (1) s (1) (mean mobility value of 0.07 cm(2) V-1 s(-1)), and reliable operation.

Ultrafast and low-temperature synthesis of patternable MoS₂ using laser irradiation

Jung, Jaehyuck,Lee, Jinhwan,Kim, Youngchan,Bark, Hunyoung,Lee, Changgu IOP 2019 Journal of Physics. D, Applied Physics Vol.52 No.18

<P>The fascinating properties of transition metal dichalcogenides have been studied for a decade to broaden their promising applications as flexible electronics. However, conventional mass production of 2D materials is carried out at relatively high temperatures over 600 °C, which typical flexible substrates cannot withstand. For this reason, proper low-temperature synthesis methods must be developed. Here, we propose a fast and low-temperature synthesis method for metallic MoS<SUB>2</SUB>, which can be achieved on a flexible polyethylene terephthalate (PET) substrate. A precursor solution was prepared by a mixture of (NH<SUB>4</SUB>)<SUB>2</SUB>MoS<SUB>4</SUB> powder and dimethylformamide. This solution was then coated on the silicon or PET substrate and an MoS<SUB>2</SUB> film can be fabricated in unconstrained patterns such as lines or a continuous film at micro scale. The necessary laser power and exposure time were confirmed from systematic Raman spectroscopy measurements. The metallic behavior of the synthesized film was demonstrated by the electrical device fabrication and these excellent metallic properties show the feasibility of using MoS<SUB>2</SUB> as a patternable electrode material on flexible substrates.</P>

Structural and Optical Properties of Single- and Few-Layer Magnetic Semiconductor CrPS₄

Lee, Jinhwan,Ko, Taeg Yeoung,Kim, Jung Hwa,Bark, Hunyoung,Kang, Byunggil,Jung, Soon-Gil,Park, Tuson,Lee, Zonghoon,Ryu, Sunmin,Lee, Changgu American Chemical Society 2017 ACS NANO Vol.11 No.11

<P>Atomically thin binary two-dimensional (2D) semiconductors exhibit diverse physical properties depending on their composition, structure, and thickness. By adding another element in these materials, which will lead to formation of ternary 2D materials, the property and structure would greatly change and significantly expanded applications could be explored. In this work, we report structural and optical properties of atomically thin chromium thiophosphate (CrPS<SUB>4</SUB>), a ternary antiferromagnetic semiconductor. Its structural details were revealed by X-ray and electron diffraction. Transmission electron microscopy showed that preferentially cleaved edges are parallel to diagonal Cr atom rows, which readily identified their crystallographic orientations. Strong in-plane optical anisotropy induced birefringence that also enabled efficient determination of crystallographic orientation using polarized microscopy. The lattice vibrations were probed by Raman spectroscopy and exhibited significant dependence on thickness of crystals exfoliated down to a single layer. Optical absorption determined by reflectance contrast was dominated by d–d-type transitions localized at Cr<SUP>3+</SUP> ions, which was also responsible for the major photoluminescence peak at 1.31 eV. The spectral features in the absorption and emission spectra exhibited noticeable thickness dependence and hinted at a high photochemical activity for single-layer CrPS<SUB>4</SUB>. The current structural and optical investigation will provide a firm basis for future study and application of this kind of atomically thin magnetic semiconductors.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2017/ancac3.2017.11.issue-11/acsnano.7b04679/production/images/medium/nn-2017-04679m_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nn7b04679'>ACS Electronic Supporting Info</A></P>

PECVD를 이용한 MoS₂ 반도체 박막의 저온 합성 및 특성 파악

안치성(Chisung Ahn),이진환(Jinhwan Lee),김형우(Hyeong U Kim),박훈영(Hunyoung Bark),이창구(Changgu Lee),김태성(Taesung Kim) 대한기계학회 2015 대한기계학회 춘추학술대회 Vol.열공학 No.-