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Lai S.,SKKU Advanced Institute of Nanotechnology SAINT | Jeon J.,SKKU Advanced Institute of Nanotechnology SAINT | Song Y.-J.,SKKU Advanced Institute of Nanotechnology SAINT | Lee S.,SKKU Advanced Institute of Nanotechnology SAINT | And 2 more authors.
RSC Advances | Year: 2016

The transfer of two-dimensional (2D) material layers to arbitrary substrates from growth substrates is critical for many applications. Although several studies of transfer processes have been reported, a transfer method that does not degrade 2D layers and damage growth substrates is still required. In this paper, we report a method that with the assistance of water penetration enables the mechanical transfer of MoS2, one of the most widely studied 2D materials, from the growth substrate to a target substrate without any etching of the growth substrate or leaving any polymer residue on the MoS2 layer. The difference between the adhesion forces of the MoS2 and carrier films and the difference between the hydrophobicities of MoS2 and the growth substrate means that water can easily penetrate the interspace at the MoS2/growth substrate interface generated by the peeling off process. We also experimentally confirmed the usefulness of Cu carrier films as a contact material for MoS2 that enables its clean separation. Our transfer method protects the original quality and morphology of large area MoS2 without leaving any polymer residue, and enables the reuse of the growth substrate. This clean transfer approach is expected to facilitate the realization of industrial applications of MoS2 and other 2D materials. © 2016 The Royal Society of Chemistry.

Wang M.,SKKU Advanced Institute of Nanotechnology SAINT | Wang M.,Center for Human Interface Nanotechnology | Jang S.K.,SKKU Advanced Institute of Nanotechnology SAINT | Song Y.J.,SKKU Advanced Institute of Nanotechnology SAINT | And 4 more authors.
Materials Research Bulletin | Year: 2015

We have demonstrated a novel yet simple method for fabricating graphene-based vertical hybrid structures by performing the CVD growth of graphene at an h-BN/Cu interface. Our systematic Raman measurements combined with plasma etching process indicate that a graphene film is grown under exfoliated h-BN rather than on its top surface, and that an h-BN/graphene vertical hybrid structure has been fabricated. Electrical transport measurements of this h-BN/graphene, transferred on SiO2, show the carrier mobility up to approximately 2250 cm2 V-1 s-1. The developed method would enable the exploration of the possibility of novel hybrid structure integration with two-dimensional material systems. © 2014, Elsevier Ltd. All rights reserved.

Lai S.,SKKU Advanced Institute of Nanotechnology SAINT | Lai S.,Center for Human Interface Nanotechnology | Kyu Jang S.,SKKU Advanced Institute of Nanotechnology SAINT | Jae Song Y.,SKKU Advanced Institute of Nanotechnology SAINT | And 4 more authors.
Applied Physics Letters | Year: 2014

We report a simple and accurate method for detecting graphene defects that utilizes the mild, dry annealing of graphene/Cu films in air. In contrast to previously reported techniques, our simple approach with optical microscopy can determine the density and degree of dislocation of defects in a graphene film without inducing water-related damage or functionalization. Scanning electron microscopy, confocal Raman and atomic force microscopy, and X-ray photoelectron spectroscopy analysis were performed to demonstrate that our nondestructive approach to characterizing graphene defects with optimized thermal annealing provides rapid and comprehensive determinations of graphene quality. © 2014 AIP Publishing LLC.

Xu J.,SKKU Advanced Institute of Nanotechnology SAINT | Xu J.,Center for Human Interface Nanotechnology | Jang S.K.,SKKU Advanced Institute of Nanotechnology SAINT | Jang S.K.,Center for Human Interface Nanotechnology | And 6 more authors.
Journal of Physical Chemistry C | Year: 2014

We report a new method for the codoping of boron and nitrogen in a monolayer graphene film. After the CVD synthesis of monolayer graphene, BN-doped graphene is prepared by performing power-controlled plasma treatment and thermal annealing with borazine. BN-doped graphene films with various doping levels, which were controlled by altering the plasma treatment power, were found with Raman and electrical measurements to investigate exhibit p-doping behavior. Transmission electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy were used to demonstrate that the synthesized BN-doped graphene films have a sp2 hybridized hexagonal structure. This approach to tuning the distribution and doping levels of boron and nitrogen in monolayer sp2 hybridized BN-doped graphene is expected to be very useful for applications requiring large-area graphene with an opened band gap. © 2014 American Chemical Society.

Park H.-Y.,Sungkyunkwan University | Dugasani S.R.,Sungkyunkwan University | Kang D.-H.,Sungkyunkwan University | Jeon J.,Sungkyunkwan University | And 6 more authors.
ACS Nano | Year: 2014

(Figure Presented). Deoxyribonucleic acid (DNA) and two-dimensional (2D) transition metal dichalcogenide (TMD) nanotechnology holds great potential for the development of extremely small devices with increasingly complex functionality. However, most current research related to DNA is limited to crystal growth and synthesis. In addition, since controllable doping methods like ion implantation can cause fatal crystal damage to 2D TMD materials, it is very hard to achieve a low-level doping concentration (nondegenerate regime) on TMD in the present state of technology. Here, we report a nondegenerate doping phenomenon for TMD materials (MoS2 and WSe2, which represent n- and p-channel materials, respectively) using DNA and slightly modified DNA by metal ions (Zn2+, Ni2+, Co2+, and Cu2+), named as M-DNA. This study is an example of interdisciplinary convergence research between DNA nanotechnology and TMD-based 2D device technology. The phosphate backbone (PO4 -) in DNA attracts and holds hole carriers in the TMD region, n-doping the TMD films. Conversely, M-DNA nanostructures, which are functionalized by intercalating metal ions, have positive dipole moments and consequently reduce the electron carrier density of TMD materials, resulting in p-doping phenomenon. N-doping by DNA occurs at ∼6.4 × 1010 cm-2 on MoS2 and ∼7.3 × 109 cm-2 on WSe2, which is uniform across the TMD area. p-Doping which is uniformly achieved by M-DNA occurs between 2.3 × 1010 and 5.5 × 1010 cm-2 on MoS2 and between 2.4 × 1010 and 5.0 × 1010 cm-2 on WSe2. These doping levels are in the nondegenerate regime, allowing for the proper design of performance parameters of TMD-based electronic and optoelectronic devices (VTH, on-/off-currents, field-effect mobility, photoresponsivity, and detectivity). In addition, by controlling the metal ions used, the p-doping level of TMD materials, which also influences their performance parameters, can be controlled. This interdisciplinary convergence research will allow for the successful integration of future layered semiconductor devices requiring extremely small and very complicated structures. © 2014 American Chemical Society.

Lee D.-H.,Sungkyunkwan University | Lee D.-H.,Center for Human Interface Nanotechnology | Liu Y.-P.,Sungkyunkwan University | Jung E.,Sungkyunkwan University | And 3 more authors.
Japanese Journal of Applied Physics | Year: 2011

We fabricated polymer organic light-emitting devices (OLEDs) with an aluminum cathode transferred under ambient conditions from a separately prepared transfer film in order to achieve complete, vacuum-free fabrication of polymer OLEDs. Transfer of aluminum (Al) and lithium fluoride on aluminum (LiF/Al) onto polymer OLEDs as a cathode revealed problems in device performance due to native aluminum oxide and the stability of the LiF layer under ambient conditions, respectively. In contrast, the device fabricated with the transfer of cesium carbonate (Cs 2CO 3)-doped poly(vinyl alcohol) (PVA) on aluminum as a cathode showed lower turn-on voltage, and enhanced efficiency and stability. This method may provide an easy way to fabricate low-cost polymer OLEDs using complete, vacuum-free processes. © 2011 The Japan Society of Applied Physics.

Park H.-Y.,Sungkyunkwan University | Lim M.-H.,Sungkyunkwan University | Jeon J.,Sungkyunkwan University | Yoo G.,Sungkyunkwan University | And 12 more authors.
ACS Nano | Year: 2015

Despite growing interest in doping two-dimensional (2D) transition metal dichalcogenides (TMDs) for future layered semiconductor devices, controllability is currently limited to only heavy doping (degenerate regime). This causes 2D materials to act as metallic layers, and an ion implantation technique with precise doping controllability is not available for these materials (e.g., MoS2, MoSe2, WS2, WSe2, graphene). Since adjustment of the electrical and optical properties of 2D materials is possible within a light (nondegenerate) doping regime, a wide-range doping capability including nondegenerate and degenerate regimes is a critical aspect of the design and fabrication of 2D TMD-based electronic and optoelectronic devices. Here, we demonstrate a wide-range controllable n-doping method on a 2D TMD material (exfoliated trilayer and bulk MoS2) with the assistance of a phosphorus silicate glass (PSG) insulating layer, which has the broadest doping range among the results reported to date (between 3.6 × 1010 and 8.3 × 1012 cm-2) and is also applicable to other 2D semiconductors. This is achieved through (1) a three-step process consisting of, first, dopant out-diffusion between 700 and 900 °C, second, thermal activation at 500 °C, and, third, optical activation above 5 μW steps and (2) weight percentage adjustment of P atoms in PSG (2 and 5 wt %). We anticipate our widely controllable n-doping method to be a starting point for the successful integration of future layered semiconductor devices. © 2015 American Chemical Society.

Jang S.K.,Sungkyunkwan University | Jang J.-R.,Sungkyunkwan University | Choe W.-S.,Sungkyunkwan University | Lee S.,Sungkyunkwan University | Lee S.,Center for Human Interface Nanotechnology
ACS Applied Materials and Interfaces | Year: 2015

In this work, we demonstrated tunable p- and/or n-type doping of chemical vapor deposition-grown graphene with the use of protein bovine serum albumin (BSA) as a dopant. BSA undergoes protonation or deprotonation reaction subject to solution pH, thereby acting as either an electron donor or an electron acceptor on the graphene surface layered with denatured BSA through π-stacking interaction. This direct annealing of graphene with denatured BSA of amphoteric nature rendered facilitated fabrication of a p- and/or n-type graphene transistor by modulating pH-dependent net charges of the single dopant. Following AFM confirmation of the BSA/graphene interface assembly, the carrier transport properties of BSA-doped graphene transistors were assessed by I-V measurement and Raman spectra to show effective charge modulation of the graphene enabled by BSA doping at various pH conditions. The protein-mediated bipolar doping of graphene demonstrated in our work is simple, scalable, and straightforward; the proposed scheme is therefore expected to provide a useful alternative for fabricating graphene transistors of novel properties and promote their implementation in practice. (Chemical Equation Presented). © 2014 American Chemical Society.

Kang D.-H.,Sungkyunkwan University | Shim J.,Sungkyunkwan University | Jang S.K.,Sungkyunkwan University | Jeon J.,Sungkyunkwan University | And 7 more authors.
ACS Nano | Year: 2015

Despite heightened interest in 2D transition-metal dichalcogenide (TMD) doping methods for future layered semiconductor devices, most doping research is currently limited to molybdenum disulfide (MoS2), which is generally used for n-channel 2D transistors. In addition, previously reported TMD doping techniques result in only high-level doping concentrations (degenerate) in which TMD materials behave as near-metallic layers. Here, we demonstrate a controllable nondegenerate p-type doping (p-doping) technique on tungsten diselenide (WSe2) for p-channel 2D transistors by adjusting the concentration of octadecyltrichlorosilane (OTS). This p-doping phenomenon originates from the methyl (-CH3) functional groups in OTS, which exhibit a positive pole and consequently reduce the electron carrier density in WSe2. The controlled p-doping levels are between 2.1 × 1011 and 5.2 × 1011 cm-2 in the nondegenerate regime, where the performance parameters of WSe2-based electronic and optoelectronic devices can be properly designed or optimized (threshold voltage→, on-/off-currents→, field-effect mobility→, photoresponsivity, and detectivity as the doping level increases). The p-doping effect provided by OTS is sustained in ambient air for a long time showing small changes in the device performance (18-34% loss of ΔVTHinitially achieved by OTS doping for 60 h). Furthermore, performance degradation is almost completely recovered by additional thermal annealing at 120 °C. Through Raman spectroscopy and electrical/optical measurements, we have also confirmed that the OTS doping phenomenon is independent of the thickness of the WSe2films. We expect that our controllable p-doping method will make it possible to successfully integrate future layered semiconductor devices. © 2015 American Chemical Society.

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