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News Article | February 15, 2017
Site: www.eurekalert.org

Graphene's unusual electronic structure enables this extraordinary material to break many records of strength, electricity and heat conduction. Physicists at the Center for Theoretical Physics of Complex Systems (PCS), in collaboration with the Research Institute for Standards and Science (KRISS), used a model to explain the electronic structure of graphene measured by a new spectroscopic platform. These techniques, published in the journal Nano Letters, could promote future research on stable and accurate quantum measurements for new 2D electronics. Recently, interest in 2D materials has risen exponentially in both academia and industry. These materials are made by extremely thin sheets, which have different physical properties compared to conventional 3D materials. Moreover, when different 2D sheets are stacked on the top of each other, new electrical, optical, and thermal properties emerge. One of the most promising and much studied 2D materials is graphene: a single sheet of carbon atoms. In order to study the electronic properties of both single and double layer graphene, the team constructed a nanodevice with graphene sandwiched between two layers of an insulating material known as hexagonal boron nitride (hBN). On top of this device they placed graphite as electrode. Graphite is essentially made up of hundreds of thousands of layers of graphene. The bottom layer consisted of one layer of silicon and one of silica. By tuning the voltages applied via the graphite and the silicon, the scientists measured the changes in the conductance of graphene, which reflects its electronic properties. The electrons of graphene have a particular energy structure, represented by the so-called Dirac cone, which is actually made by two cones that look like a sandglass, with only an infinitesimally small point in between (Dirac Point). You can think of it as an unusual cocktail glass shaped liked a sandglass, where the drink plays the function of the graphene's electrons. At temperature close to zero Kelvin (-273 degrees Celsius), the electrons pack into the lowest available energy states and fill up the double-cone glass from the bottom up, until a certain energy level, called Fermi level, is reached. Applying a negative voltage via the silicon and graphite layers is equivalent to drinking from the glass, while a positive voltage has the same effect as adding liquid to the glass. Through modulating the applied voltages, the scientists could deduce the electronic structure of graphene by following the Fermi level. In particular, they noticed that when the voltage applied to graphite is around 350 milliVolts, there is a dip in the conductance measurement, by which the Fermi level matches with the Dirac point. This is a well-known property of single layer graphene. Finally, the electrical properties change again when a magnetic field is applied to the single layer graphene. In this case, instead of a sandglass cocktail glass, the energy of the electrons is more similar to a ladder where electrons of increasing energies can be found on the higher rungs. Gaps between the ladder rungs are devoid of electrons, while the steps fill with electrons from the bottom upwards. Interestingly, the data obtained by the scientists of KRISS was successfully reproduced by the theoretical physicists at IBS showed more than 40 rungs, technically known as Landau levels. Each level clearly distinguished because of the low background noise. Indeed, the scientists could also match the theoretical and experimental data relative to the electronic properties of bilayer graphene. Double layer graphene, has a different conductance behavior with a broader dip, better known as an energy gap. In the presence of an electric field perpendicular to it, this energy gap makes double layer graphene more similar to the current tunable semiconductors. "We used an intuitive model to reproduce the experimental measurement and we gave a theoretical explanation to why these energy configurations form with single and double layer graphene," explains MYOUNG Nojoon, first co-author of this study. "This model provides a gauge between voltages and energy in spectroscopic measurements, and we believe that this is a fundamental step to study graphene's electronic properties further."


News Article | February 14, 2017
Site: www.rdmag.com

Graphene's unusual electronic structure enables this extraordinary material to break many records of strength, electricity and heat conduction. Physicists at the Center for Theoretical Physics of Complex Systems (PCS), in collaboration with the Research Institute for Standards and Science (KRISS), used a model to explain the electronic structure of graphene measured by a new spectroscopic platform. These techniques, published in the journal Nano Letters, could promote future research on stable and accurate quantum measurements for new 2D electronics. Recently, interest in 2D materials has risen exponentially in both academia and industry. These materials are made by extremely thin sheets, which have different physical properties compared to conventional 3D materials. Moreover, when different 2D sheets are stacked on the top of each other, new electrical, optical, and thermal properties emerge. One of the most promising and much studied 2D materials is graphene: a single sheet of carbon atoms. In order to study the electronic properties of both single and double layer graphene, the team constructed a nanodevice with graphene sandwiched between two layers of an insulating material known as hexagonal boron nitride (hBN). On top of this device they placed graphite as electrode. Graphite is essentially made up of hundreds of thousands of layers of graphene. The bottom layer consisted of one layer of silicon and one of silica. By tuning the voltages applied via the graphite and the silicon, the scientists measured the changes in the conductance of graphene, which reflects its electronic properties. The electrons of graphene have a particular energy structure, represented by the so-called Dirac cone, which is actually made by two cones that look like a sandglass, with only an infinitesimally small point in between (Dirac Point). You can think of it as an unusual cocktail glass shaped liked a sandglass, where the drink plays the function of the graphene's electrons. At temperature close to zero Kelvin (-273 degrees Celsius), the electrons pack into the lowest available energy states and fill up the double-cone glass from the bottom up, until a certain energy level, called Fermi level, is reached. Applying a negative voltage via the silicon and graphite layers is equivalent to drinking from the glass, while a positive voltage has the same effect as adding liquid to the glass. Through modulating the applied voltages, the scientists could deduce the electronic structure of graphene by following the Fermi level. In particular, they noticed that when the voltage applied to graphite is around 350 milliVolts, there is a dip in the conductance measurement, by which the Fermi level matches with the Dirac point. This is a well-known property of single layer graphene. Finally, the electrical properties change again when a magnetic field is applied to the single layer graphene. In this case, instead of a sandglass cocktail glass, the energy of the electrons is more similar to a ladder where electrons of increasing energies can be found on the higher rungs. Gaps between the ladder rungs are devoid of electrons, while the steps fill with electrons from the bottom upwards. Interestingly, the data obtained by the scientists of KRISS was successfully reproduced by the theoretical physicists at IBS showed more than 40 rungs, technically known as Landau levels. Each level clearly distinguished because of the low background noise. Indeed, the scientists could also match the theoretical and experimental data relative to the electronic properties of bilayer graphene. Double layer graphene, has a different conductance behavior with a broader dip, better known as an energy gap. In the presence of an electric field perpendicular to it, this energy gap makes double layer graphene more similar to the current tunable semiconductors. "We used an intuitive model to reproduce the experimental measurement and we gave a theoretical explanation to why these energy configurations form with single and double layer graphene," explains MYOUNG Nojoon, first co-author of this study. "This model provides a gauge between voltages and energy in spectroscopic measurements, and we believe that this is a fundamental step to study graphene's electronic properties further."


Yong Kim I.,KAIST | Joong Kim K.,KRISS | Shin J.H.,KAIST
Applied Physics Letters | Year: 2012

Luminescence from Er 3+ ions sensitized by nanocluster Si is investigated using finite-element, Monte-Carlo simulations. We find that we can reproduce and explain many conflicting results that have been reported using only a simple Förster-type interaction. In particular, we show that Er-Er energy migration plays a major role in Er 3+ excitation such quantities such as excitation distance and sensitized fraction depend on optically active Er fraction and pumping power. Based on simulation results, we identify optically active fraction as the critical factor and suggest a multi-layered structure as being ideal for achieving population inversion. © 2012 American Institute of Physics.


Suherlan,KRISS | Suherlan,Indonesian Institute of Sciences | Kim Y.-G.,KRISS | Yang I.,KRISS
Metrologia | Year: 2013

Five Si-SiC eutectic fixed-point cells were constructed for use in thermocouple thermometry. Two cells were made from a silicon and carbon mixture within a graphite crucible; the other three were made from pure silicon. The first broke after 12 melt-freeze cycles due to weaknesses in the crucible; the other four, made with a modified crucible that was thicker and shorter than the first type, were more resistant to failure. The second, subjected to various furnace settings, was able to withstand up to 36 cycles. The third, subjected to only the furnace setting of 5 °C above and below the transition temperature, was able to withstand 56 cycles. The fourth was rapidly cooled in an unpowered furnace from 600 °C, then removed from the furnace at 300 °C and allowed to cool to room temperature. With this repeated treatment, the cell broke after only 25 cycles. The fifth was treated gently with a slow rate of 1 °C min-1 through both melting and freezing, and 2 °C min-1 to 3 °C min-1 when cooling. This cell was successfully tested through 80 melt-freeze cycles without any mechanical failure. The melting point of the five Si-SiC cells based on its maximum drift agreed within 1.2 °C. © 2013 BIPM & IOP Publishing Ltd.


Park S.H.,Yonsei University | Kim H.J.,Yonsei University | Cho M.-H.,Yonsei University | Yi Y.,KRISS | And 3 more authors.
Applied Physics Letters | Year: 2011

The interfacial electronic structures of zinc oxide (ZnO)/copper- phthalocyanine (CuPc) were investigated by in situ x-ray and ultraviolet photoelectron spectroscopy (UPS) to determine the effects of air contamination on the ZnO substrate. UPS spectra showed that the 0.2 eV of the interface dipole is generated at the interface of the air exposed ZnO/CuPc while the interface of the annealed ZnO/CuPc generated -0.2 eV. In both cases, no band bending was observed. On the other hand, band bending at 0.3 eV and an interface dipole of 0.2 eV were observed at the interface of the sputter cleaned ZnO/CuPc. The energy offset between the conduction band maximum of ZnO and the highest occupied molecular orbital of CuPc was determined to be 0.6-0.7 eV for the contaminated ZnO interface while the offset was 1.0 eV for the cleaned ZnO interface. Contaminating moisture has little effect on the offset while the charge transfer was blocked and the offset was decreased in the presence of hydrocarbons. © 2011 American Institute of Physics.


Lee S.,KRISS
22nd IMEKO TC3 International Conference on Measurement of Force, Mass and Torque 2014, Held Together with TC5 and TC22 | Year: 2014

The necessity of making a new transfer standard was proposed to disseminate of mass scale during or after establishing new definition of kilogram unit. It is worthy to find new material or perfect surface characterization of mass transfer standard. It should be confirmed whether a material is suitable to use as a standard after much chemical or mechanical analysis of its surface. In this presentation, we would like to present the first plan for development project of new transfer standard materials.


Ryu H.Y.,KRISS | Lee S.H.,KRISS | Suh H.S.,KRISS
IEEE Photonics Technology Letters | Year: 2010

We report an ultrastable external cavity laser diode (ECLD) that can be selectively injection locked to an optical frequency comb (OFC) generated by acetylene stabilized laser (ASL) seeding. The carrier envelope offset (position) and repetition rate (mode spacing) of the OFC, which functions as the master oscillator, was stabilized to the ASL (1.1×10-12 at 1 s) and H-maser (2× 10-13 at 1 s), respectively. The ECLD can be tuned over 70 nm in the 1.5-μm region and can be discretely locked to the comb modes with 25-GHz spacing over a 10-THz span. The frequency stability of the ECLD injection locked to the OFC reached as high as 1.1×10-12 for 1-s averaging time. © 2010 IEEE.


Lee S.,KRISS | Chung J.W.,KRISS
XXI IMEKO World Congress "Measurement in Research and Industry" | Year: 2015

To provide insights for analysis of Key Comparisons (KCs), we have estimated previous KCs with Monte Carlo Simulation. Two similar previous KCs in mass metrology of 50 kg scale were taken in this work as the typical example. The KC Reference Value (KCRV) was evaluated with several popular estimators as well as median in this analysis. Their uncertainties were considered with Guide to the Expression of Uncertainty in Measurement (GUM) and numerical simulation by Monte Carlo Method (MCM).


Park S.H.,Yonsei University | Jeong J.G.,Yonsei University | Kim H.-J.,Yonsei University | Park S.-H.,Yonsei University | And 5 more authors.
Applied Physics Letters | Year: 2010

The interfacial electronic structures of fullerene (C60) /zinc -phthalocyanine (ZnPc) and C60 /ZnPc: C60 (50 wt %) containing a blended layer were investigated by in situ ultraviolet photoelectron spectroscopy (UPS), in an attempt to understand the role of the blended layer in improving the performance of organic photovoltaic devices that contain such layers. From the UPS spectra, the band bending found to be 0.30 eV in the ZnPc layer and 0.43 eV in the C60 layer at the C60 /ZnPc interface. On the other hand, the band bending was 0.25 eV in both of the organic layers at the ZnPc: C60 /ZnPc interface and no significant band bending in the C60 layer at the C60 /ZnPc: C 60 interface was found. The observed interface dipole was 0.06 eV at the C60 /ZnPc interface and 0.26 eV at the ZnPc: C60 /ZnPc interface. The offset between the highest unoccupied molecular orbital of ZnPc and the lowest occupied molecular orbital of C60 was 0.75 eV at C60 /ZnPc and was 1.04 eV at the ZnPc: C60 /ZnPc interface. The increased offset can be attributed to an increase in the interface dipole, caused by the blending donor and acceptor material. The blending facilitates charge transfer between the donor and acceptor, resulting in an increase in the interface dipole, resulting in a larger offset. © 2010 American Institute of Physics.


News Article | February 15, 2017
Site: phys.org

Recently, interest in 2-D materials has risen exponentially in both academia and industry. These materials are made by extremely thin sheets, which have different physical properties compared to conventional 3-D materials. Moreover, when different 2-D sheets are stacked on the top of each other, new electrical, optical, and thermal properties emerge. One of the most promising and much studied 2-D materials is graphene: a single sheet of carbon atoms. In order to study the electronic properties of both single and double layer graphene, the team constructed a nanodevice with graphene sandwiched between two layers of an insulating material known as hexagonal boron nitride (hBN). On top of this device they placed graphite as electrode. Graphite is essentially made up of hundreds of thousands of layers of graphene. The bottom layer consisted of one layer of silicon and one of silica. By tuning the voltages applied via the graphite and the silicon, the scientists measured the changes in the conductance of graphene, which reflects its electronic properties. The electrons of graphene have a particular energy structure, represented by the so-called Dirac cone, which is actually made by two cones that look like a sandglass, with only an infinitesimally small point in between (Dirac Point). You can think of it as an unusual cocktail glass shaped liked a sandglass, where the drink plays the function of the graphene's electrons. At temperature close to zero Kelvin (-273 degrees Celsius), the electrons pack into the lowest available energy states and fill up the double-cone glass from the bottom up, until a certain energy level, called Fermi level, is reached. Applying a negative voltage via the silicon and graphite layers is equivalent to drinking from the glass, while a positive voltage has the same effect as adding liquid to the glass. Through modulating the applied voltages, the scientists could deduce the electronic structure of graphene by following the Fermi level. In particular, they noticed that when the voltage applied to graphite is around 350 milliVolts, there is a dip in the conductance measurement, by which the Fermi level matches with the Dirac point. This is a well-known property of single layer graphene. Finally, the electrical properties change again when a magnetic field is applied to the single layer graphene. In this case, instead of a sandglass cocktail glass, the energy of the electrons is more similar to a ladder where electrons of increasing energies can be found on the higher rungs. Gaps between the ladder rungs are devoid of electrons, while the steps fill with electrons from the bottom upwards. Interestingly, the data obtained by the scientists of KRISS was successfully reproduced by the theoretical physicists at IBS showed more than 40 rungs, technically known as Landau levels. Each level clearly distinguished because of the low background noise. Indeed, the scientists could also match the theoretical and experimental data relative to the electronic properties of bilayer graphene. Double layer graphene, has a different conductance behavior with a broader dip, better known as an energy gap. In the presence of an electric field perpendicular to it, this energy gap makes double layer graphene more similar to the current tunable semiconductors. "We used an intuitive model to reproduce the experimental measurement and we gave a theoretical explanation to why these energy configurations form with single and double layer graphene," explains MYOUNG Nojoon, first co-author of this study. "This model provides a gauge between voltages and energy in spectroscopic measurements, and we believe that this is a fundamental step to study graphene's electronic properties further." More information: Suyong Jung et al. Direct Probing of the Electronic Structures of Single-Layer and Bilayer Graphene with a Hexagonal Boron Nitride Tunneling Barrier, Nano Letters (2017). DOI: 10.1021/acs.nanolett.6b03821

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