Heemin Park

Terahertz pulse propagation in a plastic photonic crystal fiber

Guided-wave single-mode propagation of sub-ps terahertz (THz) pulses in a plastic photonic crystal fiber has been experimentally demonstrated for the first time to the best of our knowledge. The plastic photonic crystal fiber is fabricated from high density polyethylene tubes and filaments. The fabricated fiber exhibits low loss and relatively low dispersive propagation of THz pulses within the experimental bandwidth of 0.1 /spl sim/ 3 THz. The measured loss and group velocity dispersion are less than 0.5 cm/sup -1/ and -0.3 ps/THz/spl middot/cm above 0.6 THz, respectively.

Spin-induced band modifications of graphene through intercalation of magnetic iron atoms

Intercalation of magnetic iron atoms through graphene formed on the SiC(0001) surface is found to induce significant changes in the electronic properties of graphene due mainly to the Fe-induced asymmetries in charge as well as spin distribution. From our synchrotron-based photoelectron spectroscopy data together with ab initio calculations, we observe that the Fe-induced charge asymmetry results in the formation of a quasi-free-standing bilayer graphene while the spin asymmetry drives multiple spin-split bands. We find that Fe adatoms are best intercalated upon annealing at 600 °C, exhibiting split linear π-bands, characteristic of a bilayer graphene, but much diffused. Subsequent changes in the C 1s, Si 2p, and Fe 3p core levels are consistently described in terms of Fe-intercalation. Our calculations together with a spin-dependent tight binding model ascribe the diffuse nature of the π-bands to the multiple spin-split bands originated from the spin-injected carbon atoms residing only in the lower graphene layer.

Proximity Effect Induced Electronic Properties of Graphene on Bi2Te2Se

We report that the π-electrons of graphene can be spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene-interfaced topological insulator hybrid material. We have grown epitaxial Bi2Te2Se (BTS) films on a chemical vapor deposition (CVD) graphene. We observe two linear surface bands from both the CVD graphene notably flattened and BTS coexisting with their DPs separated by 0.53 eV in the photoemission data measured with synchrotron photons. We further demonstrate that the separation between the two DPs, Δ(D-D), can be artificially fine-tuned by adjusting the amount of Cs atoms adsorbed on the graphene to a value as small as Δ(D-D) = 0.12 eV to find any proximity effect induced by the DPs. Our density functional theory calculation shows the opening of a spin-orbit gap of ∼20 meV in the π-band, enhanced by 3 orders of magnitude from that of a pristine graphene, and a concomitant phase transition from a semimetallic to a quantum spin Hall phase when Δ(D-D) ≤ 0.20 eV. We thus present a practical means of spin-polarizing the π-band of graphene, which can be pivotal to advance graphene-based spintronics.

Band gap engineering for single-layer graphene by using slow Li(+) ions.

In order to utilize the superb electronic properties of graphene in future electronic nano-devices, a dependable means of controlling the transport properties of its Dirac electrons has to be devised by forming a tunable band gap. We report on the ion-induced modification of the electronic properties of single-layer graphene (SLG) grown on a SiC(0001) substrate by doping low-energy (5 eV) Li(+) ions. We find the opening of a sizable and tunable band gap up to 0.85 eV, which depends on the Li(+) ion dose as well as the following thermal treatment, and is the largest band gap in the π-band of SLG by any means reported so far. Our Li 1s core-level data together with the valence band suggest that Li(+) ions do not intercalate below the topmost graphene layer, but cause a significant charge asymmetry between the carbon sublattices of SLG to drive the opening of the band gap. We thus provide a route to producing a tunable graphene band gap by doping Li(+) ions, which may play a pivotal role in the utilization of graphene in future graphene-based electronic nano-devices.

Cerium-induced changes in the π-band of graphene

Modifying or controlling the intrinsic properties of graphene, such as by controlling its band gap and carrying out spin injection of its π-electrons, have been a recent focus of research in graphene technology in order to promote the industrial applications of its superb properties. Here, we carried out photoemission spectroscopy experiments using synchrotron photons and showed several unique changes in the electronic and structural properties of graphene resulting from its adsorption of magnetic cerium (Ce) atoms. A band gap as large as Eg = 0.50 eV opened when the Ce-adsorbed graphene was cooled to 41 K after a brief annealing at a temperature Ta of 1200 °C. As the temperature of this sample was then increased to room temperature (RT), the size of the band gap decreased gradually to an Eg of 0.36 eV, indicative of a temperature-dependent structural and/or spin-ordering phase transition. We also observed the presence of two different stages of Ce-intercalation upon annealing the graphene with Ce adsorbed at RT: the Ce atoms first intercalated below graphene at a Ta of 530 °C and then below the buffer layer at a Ta of 1050 °C. We discuss the physical implications of these temperature-dependent features of the Ce-adsorbed graphene.

Topological modification of the electronic structure by Bi-bilayers lying deep inside bulk Bi2Se3.

We observe the modified surface states of an epitaxial thin film of a homologous series of (Bi2)m(Bi2Se3)n, as a topological insulator (TI), by angle-resolved photoemission spectroscopy measurements. A thin film with m : n  =  1 : 3 (Bi8Se9) has been grown with Bi2 bilayers embedded every other three quintuple layers (QLs) of Bi2Se3. Despite the reduced dimension of continuous QLs due to the Bi2 heterolayers, we find that the topological surface states stem from the inverted Bi and Se states and the topologically nontrivial structures are mainly based on the prototype of 3D TI Bi2Se3 without affecting the overall topological order.

Band gap engineering for graphene by using Na+ ions

Despite the noble electronic properties of graphene, its industrial application has been hindered mainly by the absence of a stable means of producing a band gap at the Dirac point (DP). We report a new route to open a band gap (Eg) at DP in a controlled way by depositing positively charged Na+ ions on single layer graphene formed on 6H-SiC(0001) surface. The doping of low energy Na+ ions is found to deplete the π* band of graphene above the DP, and simultaneously shift the DP downward away from Fermi energy indicating the opening of Eg. The band gap increases with increasing Na+ coverage with a maximum Eg≥0.70 eV. Our core-level data, C 1s, Na 2p, and Si 2p, consistently suggest that Na+ ions do not intercalate through graphene, but produce a significant charge asymmetry among the carbon atoms of graphene to cause the opening of a band gap. We thus provide a reliable way of producing and tuning the band gap of graphene by using Na+ ions, which may play a vital role in utilizing graphene in future nano-el...

Band modification of graphene by using slow Cs + ions

We report new wide band gap engineering for graphene using slow Cs+ ions, which allows both fine-tuning and on–off switching capability of the band gap in a range suitable for most applications without modifying or deteriorating the relativistic nature of the Dirac fermions. The doping of Cs+ ions opens the band gap up to Eg = 0.68 eV, which can be closed completely by adding neutral Cs atoms, as observed in angle-resolved photoemission spectroscopy. The operating mechanism of this band gap engineering is understood by a simple capacitor model, which is fully supported by the density-functional theory calculations.

Observation of variable hybridized-band gaps in Eu-intercalated graphene

We report europium (Eu)-induced changes in the π-band of graphene (G) formed on the 6H-SiC(0001) surface by a combined study of photoemission measurements and density functional theory (DFT) calculations. Our photoemission data reveal that Eu intercalates upon annealing at 120 °C into the region between the graphene and the buffer layer (BL) to form a G/Eu/BL system, where a band gap of 0.29 eV opens at room temperature. This band gap is found to increase further to 0.48 eV upon cooling down to 60 K. Our DFT calculations suggest that the increased band gap originates from the enhanced hybridization of the graphene π-band with the Eu 4f band due to the increased magnetic ordering upon cooling. These Eu atoms continue to intercalate further down below the BL to produce bilayer graphene (G/BL/Eu) upon annealing at 300 °C. The π-band stemming from the BL then exhibits another band gap of 0.37 eV, which appears to be due to the strong hybridization between the π-band of the BL and the Eu 4f band. The Eu-intercalated graphene thus illustrates an example of versatile band gaps formed under different thermal treatments, which may play a critical role for future applications in graphene-based electronics.

Modification of electronic properties of graphene by using low-energy K+ ions

Despite its superb electronic properties, the semi-metallic nature of graphene with no band gap (Eg) at the Dirac point has been a stumbling block for its industrial application. We report an improved means of producing a tunable band gap over other schemes by doping low energy (10 eV) potassium ions (K+) on single layer graphene formed on 6H-SiC(0001) surface, where the noble Dirac nature of the π-band remains almost unaltered. The changes in the π-band induced by K+ ions reveal that the band gap increases gradually with increasing dose (θ) of the ions up to Eg = 0.65 eV at θ = 1.10 monolayers, demonstrating the tunable character of the band gap. Our core level data for C 1s, Si 2p, and K 2p suggest that the K+-induced asymmetry in charge distribution among carbon atoms drives the opening of band gap, which is in sharp contrast with no band gap when neutral K atoms are adsorbed on graphene. This tunable K+-induced band gap in graphene illustrates its potential application in graphene-based nano-electronics.