Publications by authors named "Taekyeong Kim"

18 Publications

  • Page 1 of 1

Observing the Layer-Number-Dependent Local Dielectric Response of WSe by Electrostatic Force Microscopy.

J Phys Chem Lett 2020 Aug 6;11(16):6684-6690. Epub 2020 Aug 6.

Department of Physics, Hankuk University of Foreign Studies, 81 Oedae-ro, Mohyeon-myeon Cheoin-gu, Yongin-si 17035, Republic of Korea.

We investigate the layer-number-dependent dielectric response of WSe by measuring the phase shift (Φ) through an electrostatic force microscopy (EFM). The measured Φ results stem mainly from the capacitive coupling between the tip and WSe based on the plane capacitor model, leading to changes in the second derivative of the capacitance ('') values, which increase in a few layers and saturate to the bulk value under an applied EFM tip bias. The '' value is related to the dielectric polarization, reflecting the charge carrier concentration and mobility of WSe flakes with different numbers of layers. This implies that the dielectric constant of WSe shows layer-number-dependent behavior which increases with the number of layers, approaching the bulk value. Furthermore, we also construct a spatially resolved '' map to observe the local dielectric response of WSe flakes. Our work could be significant in that it can improve the performance of novel electronic devices based on the controllable dielectric properties of 2D vdW semiconductor materials.
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http://dx.doi.org/10.1021/acs.jpclett.0c01521DOI Listing
August 2020

High Efficiency Doping of Conjugated Polymer for Investigation of Intercorrelation of Thermoelectric Effects with Electrical and Morphological Properties.

ACS Appl Mater Interfaces 2020 Jan 19;12(1):1151-1158. Epub 2019 Dec 19.

Division of Chemical Engineering , Konkuk University , Seoul 05029 , Republic of Korea.

Intercorrelation of thermoelectric properties of a doped conjugated semiconducting polymer (PIDF-BT) with charge carrier density, conductive morphology, and crystallinity are systematically investigated. Upon being doped with F4-TCNQ by the sequential doping method, PIDF-BT exhibited a high electrical conductivity over 210 S cm. The significant enhancement of electrical conductivity resulted from a high charge carrier density, which is attributed to the effective charge-transfer-based integer doping between PIDF-BT and dopant molecules. Based on the systemic characterization on the optical, electrical, and structural properties of doped PIDF-BT annealed at different temperatures, we investigated the characteristic correlations between thermoelectric properties of PIDF-BT films and their four-probe electrical conductivity, charge carrier density, and charge carrier mobility obtained from AC Hall effect measurements. This study revealed that exercising fine control over the crystallinity and conductive migration of the conjugated polymer films can be a strategic approach to suppressing the degradation of the Seebeck coefficient at high charge carrier density and ultimately to maximizing the power factors of organic thermoelectric devices.
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http://dx.doi.org/10.1021/acsami.9b17825DOI Listing
January 2020

Direct Mapping of the Gate Response of a Multilayer WSe/MoS Heterostructure with Locally Different Degrees of Charge Depletion.

J Phys Chem Lett 2019 Jul 5;10(14):4010-4016. Epub 2019 Jul 5.

Department of Physics , Hankuk University of Foreign Studies , Yongin 17035 , Korea.

Understanding the interlayer charge coupling mechanism in a two-dimensional van der Waals (vdW) heterojunction is crucial for optimizing the performance of heterostructure-based (opto)electronic devices. Here, we report mapping the gate response of a multilayer WSe/MoS heterostructure with locally different degrees of charge depletion through mobile carrier measurements based on electrostatic force microscopy. We observed ambipolar or unipolar behavior depending on the degree of charge depletion in the heterojunction under tip gating. Interestingly, the WSe on MoS shows gating behavior that is more efficient than that on the SiO/Si substrate, which can be explained by the high dielectric environment and screening of impurities on the SiO surface by the MoS. Furthermore, we found that the gate-induced majority carriers in the heterojunction reduce the carrier lifetime, leading to the enhanced interlayer recombination of the photogenerated carriers under illumination. Our work provides a comprehensive understanding of the interfacial phenomena at the vdW heterointerface with charge depletion.
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http://dx.doi.org/10.1021/acs.jpclett.9b01192DOI Listing
July 2019

Molecularly Controlled Stark Effect Induces Significant Rectification in Polycyclic-Aromatic-Hydrocarbon-Terminated n-Alkanethiolates.

Nano Lett 2019 01 28;19(1):545-553. Epub 2018 Dec 28.

Department of Chemistry , Korea University , Seoul 136-701 , Korea.

The variation of the electronic structure of individual molecules as a function of the applied bias matters for the performance of molecular and organic electronic devices. Understanding the structure-electric-field relationship, however, remains a challenge because of the lack of in-operando spectroscopic technique and complexity arising from the ill-defined on-surface structure of molecules and organic-electrode interfaces within devices. We report that a reliable and reproducible molecular diode can be achieved by control of the conjugation length in polycyclic-aromatic-hydrocarbon (PAH)-terminated n-alkanethiolate (denoted as SCPAH), incorporated into liquid-metal-based large-area tunnel junctions in the form of a self-assembled monolayer. By taking advantage of the structural simplicity and tunability of SCPAH and the high-yielding feature of the junction technique, we demonstrate that the increase in the conjugation length of the PAH terminal group leads to a significant rectification ratio up to ∼1.7 × 10 at ±740 mV. Further study suggests that the Stark shift of the molecular energy resonance of the PAH breaks the symmetry of the energy topography across the junction and induces rectification in a temperature-independent charge-transport regime.
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http://dx.doi.org/10.1021/acs.nanolett.8b04488DOI Listing
January 2019

Intrinsic Correlation between Electronic Structure and Degradation: From Few-Layer to Bulk Black Phosphorus.

Angew Chem Int Ed Engl 2019 Mar 18;58(12):3754-3758. Epub 2019 Jan 18.

Institute of Physics and Applied Physics, Yonsei University, 50 Yonsei-ro, Seodaemun-Gu, Seoul, Republic of Korea.

Black phosphorus (BP) has received much attention owing to its fascinating properties, such as a high carrier mobility and tunable band gap. However, these advantages have been overshadowed by the fast degradation of BP under ambient conditions. To overcome this obstacle, the exact degradation mechanisms need to be unveiled. Herein, we analyzed two sequential degradation processes and the layer-dependent degradation rates of BP in the dark by scanning Kelvin probe microscopy (SKPM) measurements and theoretical modeling. The layer-dependent degradation was successfully interpreted by considering the oxidation model based on the Marcus-Gerischer theory (MGT). In the dark, the electron transfer rate from BP to oxygen molecules depends on the number of layers as these systems have different carrier concentrations. This work not only provides a deeper understanding of the degradation mechanism itself but also suggest new strategies for the design of stable BP-based electronics.
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http://dx.doi.org/10.1002/anie.201811743DOI Listing
March 2019

Vertical and In-Plane Current Devices Using NbS/n-MoS van der Waals Schottky Junction and Graphene Contact.

Nano Lett 2018 03 8;18(3):1937-1945. Epub 2018 Feb 8.

Department of Physics , Hankuk University of Foreign Studies , 81 Oedae-ro , Chein-gu, Yongin-si 17035 , South Korea.

A van der Waals (vdW) Schottky junction between two-dimensional (2D) transition metal dichalcogenides (TMDs) is introduced here for both vertical and in-plane current devices: Schottky diodes and metal semiconductor field-effect transistors (MESFETs). The Schottky barrier between conducting NbS and semiconducting n-MoS appeared to be as large as ∼0.5 eV due to their work-function difference. While the Schottky diode shows an ideality factor of 1.8-4.0 with an on-to-off current ratio of 10-10, Schottky-effect MESFET displays little gate hysteresis and an ideal subthreshold swing of 60-80 mV/dec due to low-density traps at the vdW interface. All MESFETs operate with a low threshold gate voltage of -0.5 ∼ -1 V, exhibiting easy saturation. It was also found that the device mobility is significantly dependent on the condition of source/drain (S/D) contact for n-channel MoS. The highest room temperature mobility in MESFET reaches to approximately more than 800 cm/V s with graphene S/D contact. The NbS/n-MoS MESFET with graphene was successfully integrated into an organic piezoelectric touch sensor circuit with green OLED indicator, exploiting its predictable small threshold voltage, while NbS/n-MoS Schottky diodes with graphene were applied to extract doping concentrations in MoS channel.
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http://dx.doi.org/10.1021/acs.nanolett.7b05338DOI Listing
March 2018

Contact Effect of ReS/Metal Interface.

ACS Appl Mater Interfaces 2017 Aug 27;9(31):26325-26332. Epub 2017 Jul 27.

Department of Mechanical Engineering, Yonsei University , Seoul 120-749, Republic of Korea.

Rhenium disulfide (ReS) has attracted immense interest as a promising two-dimensional material for optoelectronic devices owing to its outstanding photonic response based on its energy band gap's insensitivity to the layer thickness. Here, we theoretically calculated the electrical band structure of mono-, bi-, and trilayer ReS and experimentally found the work function to be 4.8 eV, which was shown to be independent of the layer thickness. We also evaluated the contact resistance of a ReS field-effect transistor using a Y-function method with various metal electrodes, including graphene. The ReS channel is a strong n-type semiconductor, thus a lower work function than that of metals tends to lead to a lower contact resistance. Moreover, the graphene electrodes, which were not chemically or physically bonded to ReS, showed the lowest contact resistance, regardless of the work function, suggesting a significant Fermi-level pinning effect at the ReS/metal interface. In addition, an asymmetric Schottky diode device was demonstrated using Ti or graphene for ohmic contacts and Pt or Pd for Schottky contacts. The ReS-based transistor used in this study on the work function of ReS achieved the possibility of designing the next-generation nanologic devices.
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http://dx.doi.org/10.1021/acsami.7b06432DOI Listing
August 2017

Charge transport and rectification in molecular junctions formed with carbon-based electrodes.

Proc Natl Acad Sci U S A 2014 Jul 14;111(30):10928-32. Epub 2014 Jul 14.

Department of Applied Physics and Mathematics, Columbia University, New York, NY 10027;

Molecular junctions formed using the scanning-tunneling-microscope-based break-junction technique (STM-BJ) have provided unique insight into charge transport at the nanoscale. In most prior work, the same metal, typically Au, Pt, or Ag, is used for both tip and substrate. For such noble metal electrodes, the density of electronic states is approximately constant within a narrow energy window relevant to charge transport. Here, we form molecular junctions using the STM-BJ technique, with an Au metal tip and a microfabricated graphite substrate, and measure the conductance of a series of graphite/amine-terminated oligophenyl/Au molecular junctions. The remarkable mechanical strength of graphite and the single-crystal properties of our substrates allow measurements over few thousand junctions without any change in the surface properties. We show that conductance decays exponentially with molecular backbone length with a decay constant that is essentially the same as that for measurements with two Au electrodes. More importantly, despite the inherent symmetry of the oligophenylamines, we observe rectification in these junctions. State-of-art ab initio conductance calculations are in good agreement with experiment, and explain the rectification. We show that the highly energy-dependent graphite density of states contributes variations in transmission that, when coupled with an asymmetric voltage drop across the junction, leads to the observed rectification. Together, our measurements and calculations show how functionality may emerge from hybrid molecular-scale devices purposefully designed with different electrodes beyond the so-called "wide band limit," opening up the possibility of assembling molecular junctions with dissimilar electrodes using layered 2D materials.
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http://dx.doi.org/10.1073/pnas.1406926111DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4121811PMC
July 2014

Determination of energy level alignment and coupling strength in 4,4'-bipyridine single-molecule junctions.

Nano Lett 2014 Feb 24;14(2):794-8. Epub 2014 Jan 24.

Department of Applied Physics and Applied Mathematics, Columbia University , 500 W. 120th Street, New York, New York 10027, United States.

We measure conductance and thermopower of single Au-4,4'-bipyridine-Au junctions in distinct low and high conductance binding geometries accessed by modulating the electrode separation. We use these data to determine the electronic energy level alignment and coupling strength for these junctions, which are known to conduct through the lowest unoccupied molecular orbital (LUMO). Contrary to intuition, we find that, in the high-conductance junction, the LUMO resonance energy is further away from the Au Fermi energy than in the low-conductance junction. However, the LUMO of the high-conducting junction is better coupled to the electrode. These results are in good quantitative agreement with self-energy corrected zero-bias density functional theory calculations. Our calculations show further that measurements of conductance and thermopower in amine-terminated oligophenyl-Au junctions, where conduction occurs through the highest occupied molecular orbitals, cannot be used to extract electronic parameters as their transmission functions do not follow a simple Lorentzian form.
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http://dx.doi.org/10.1021/nl404143vDOI Listing
February 2014

Graphene nanonet for biological sensing applications.

Nanotechnology 2013 Sep 21;24(37):375302. Epub 2013 Aug 21.

Department of Physics and Astronomy, Seoul National University, Seoul, Korea.

We report a simple but efficient method to fabricate versatile graphene nanonet (GNN)-devices. In this method, networks of V2O5 nanowires (NWs) were prepared in specific regions of single-layer graphene, and the graphene layer was selectively etched via a reactive ion etching method using the V2O5 NWs as a shadow mask. The process allowed us to prepare large scale patterns of GNN structures which were comprised of continuous networks of graphene nanoribbons (GNRs) with chemical functional groups on their edges. The GNN can be easily functionalized with biomolecules for fluorescent biochip applications. Furthermore, electrical channels based on GNN exhibited a rather high mobility and low noise compared with other network structures based on nanostructures such as carbon nanotubes, which was attributed to the continuous connection of nanoribbons in GNN structures. As a proof of concept, we built DNA sensors based on GNN channels and demonstrated the selective detection of DNA. Since our method allows us to prepare high-performance networks of GNRs over a large surface area, it should open up various practical biosensing applications.
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http://dx.doi.org/10.1088/0957-4484/24/37/375302DOI Listing
September 2013

Multilayered nano-prism vertex tips for tip-enhanced Raman spectroscopy and imaging.

Analyst 2013 Oct;138(19):5588-93

Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea.

We presented a scalable fabrication method for the preparation of multilayered nano-prism vertex (NV)-tips whose dimensions can be controlled for tip-enhanced Raman spectroscopy (TERS). The NV-tip had sharp vertices (diameter ~20 nm) originated from the chemical lift-off process after the angle-grinding process, enabling high resolution imaging. TERS measurements were performed on brilliant cresyl blue (BCB) molecules using a Ag/Au NV-tip, revealing the enhanced field localization at the vertices of the NV-tip. Furthermore, we could observe the polarization effect of the NV-tip. Our NV-tips should be a powerful tool for basic research on TERS experiments and SPM applications.
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http://dx.doi.org/10.1039/c3an00808hDOI Listing
October 2013

Conductance of molecular junctions formed with silver electrodes.

Nano Lett 2013 Jul 5;13(7):3358-64. Epub 2013 Jun 5.

Department of Applied Physics and Applied Mathematics, Columbia University , New York, New York 10027, United States.

We compare the conductance of a series of amine-terminated oligophenyl and alkane molecular junctions formed with Ag and Au electrodes using the scanning tunneling microscope based break-junction technique. For these molecules that conduct through the highest occupied molecular orbital, junctions formed with Au electrodes are more conductive than those formed with Ag electrodes, consistent with the lower work function for Ag. The measured conductance decays exponentially with molecular backbone length with a decay constant that is essentially the same for Ag and Au electrodes. However, the formation and evolution of molecular junctions upon elongation are very different for these two metals. Specifically, junctions formed with Ag electrodes sustain significantly longer elongation when compared with Au due to a difference in the initial gap opened up when the metal point-contact is broken. Using this observation and density functional theory calculations of junction structure and conductance we explain the trends observed in the single molecule junction conductance. Our work thus opens a new path to the conductance measurements of a single molecule junction in Ag electrodes.
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http://dx.doi.org/10.1021/nl401654sDOI Listing
July 2013

Nanotube-bridged wires with sub-10 nm gaps.

Nano Lett 2012 Apr 28;12(4):1879-84. Epub 2012 Mar 28.

Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea.

We report a simple but efficient method to synthesize carbon nanotube-bridged wires (NBWs) with gaps as small as 5 nm. In this method, we have combined a strategy for assembling carbon nanotubes (CNTs) inside anodized aluminum oxide pores and the on-wire lithography technique to fabricate CNT-bridged wires with gap sizes deliberately tailored over the 5-600 nm range. As a proof-of-concept demonstration of the utility of this architecture, we have prepared NBW-based chemical and biosensors which exhibit higher analyte sensitivity (lower limits of detection) than those based on planar CNT networks. This observation is attributed to a greater surface-to-volume ratio of CNTs in the NBWs than those in the planar CNT devices. Because of the ease of synthesis and high yield of NBWs, this technique may enable the further incorporation of CNT-based architectures into various nanoelectronic and sensor platforms.
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http://dx.doi.org/10.1021/nl204259tDOI Listing
April 2012

Floating electrode transistor based on purified semiconducting carbon nanotubes for high source-drain voltage operation.

Nanotechnology 2012 Mar 1;23(8):085204. Epub 2012 Feb 1.

Department of Physics and Astronomy, Seoul National University, Seoul, Korea.

We report floating-electrode-based thin-film transistors (F-TFTs) based on a purified semiconducting single-walled carbon nanotube (swCNT) network for a high source-drain voltage operation. At a high source-drain voltage, a conventional swCNT-TFT exhibited poor transistor performance with a small on-off ratio, which was attributed to the reduced Schottky barrier modulation at a large bias. In the F-TFT device, an swCNT network channel was separated into a number of channels connected by floating electrodes. The F-TFTs exhibited a much higher on-off ratio than a conventional swCNT-TFT with a single channel. This work should provide an important guideline in designing swCNT-TFTs for high voltage applications such as displays.
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http://dx.doi.org/10.1088/0957-4484/23/8/085204DOI Listing
March 2012

Scanning noise microscopy on graphene devices.

ACS Nano 2011 Nov 6;5(11):8620-8. Epub 2011 Oct 6.

Department of Physics and Astronomy,Seoul National University, Seoul 151-747, Korea.

We developed a scanning noise microscopy (SNM) method and demonstrated the nanoscale noise analysis of a graphene strip-based device. Here, a Pt tip made a direct contact on the surface of a nanodevice to measure the current noise spectrum through it. Then, the measured noise spectrum was analyzed by an empirical model to extract the noise characteristics only from the device channel. As a proof of concept, we demonstrated the scaling behavior analysis of the noise in graphene strips. Furthermore, we performed the nanoscale noise mapping on a graphene channel, allowing us to study the effect of structural defects on the noise of the graphene channel. The SNM method is a powerful tool for nanoscale noise analysis and should play a significant role in basic research on nanoscale devices.
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http://dx.doi.org/10.1021/nn202135gDOI Listing
November 2011

Fibronectin-carbon-nanotube hybrid nanostructures for controlled cell growth.

Small 2011 Jan;7(1):56-61

Department of Physics and Astronomy, Seoul National University, Seoul, 151-747, Korea.

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http://dx.doi.org/10.1002/smll.201001513DOI Listing
January 2011

Robust single-nanoparticle probe for contact-mode analysis and dip-pen nanolithography.

Small 2008 Aug;4(8):1072-5

School of Physics, Seoul National University NS50 Shilim-Dong, Kwanak-Gu, Seoul 151-742, Korea.

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http://dx.doi.org/10.1002/smll.200701102DOI Listing
August 2008

"Lens" effect in directed assembly of nanowires on gradient molecular patterns.

J Phys Chem B 2006 Jun;110(21):10217-9

We report a new phenomenon, named here as the "lens" effect, in the directed-assembly process of nanowires (NWs) on self-assembled monolayer (SAM) patterns. In this process, the adsorption of NWs is focused in the nanoscale regions at the center of microscale SAM patterns with gradient surface molecular density just like an optical lens focuses light. As a proof of concepts, we successfully demonstrated the massive assembly of V2O5 NWs and single-walled carbon nanotubes (swCNTs) with a nanoscale resolution using only microscale molecular patterning methods. This work provides us with important insights about the directed-assembly process, and from a practical point of view, it allows us to generate nanoscale patterns of NWs over a large area for mass fabrication of NW-based devices.
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http://dx.doi.org/10.1021/jp062108sDOI Listing
June 2006