Applied Physics Letters

/Applied Physics Letters

Superconducting-contact-induced resistance-anomalies in the 3D topological insulator Bi2Te3

2015-10-30T14:13:25+00:00October 30th, 2015|Categories: Publications|

This study examines the magnetotransport response observed in flakes of the 3D topological insulator (TI) Bi2Te3, including indium superconducting electrodes, and demonstrates two critical transitions in the magnetoresistive response with decreasing temperatures below . The first transition is attributed to superconductivity in the indium electrodes, and the second transition, with a critical field exceeding the transition field of indium, is attributed to a proximity effect at the 2D planar interface of this hybrid TI/superconductor structure.

Published in: "Applied Physics Letters".

Evidence for bandgap opening in buckled epitaxial graphene from ultrafast time-resolved terahertz spectroscopy

2015-10-28T18:04:48+00:00October 28th, 2015|Categories: Publications|Tags: |

We utilize ultrafast time-resolved terahertz (THz) spectroscopy as a direct, sensitive, and non-contact all-optical probe to investigate the hot-carrier relaxation and cooling dynamics of buckled epitaxial graphene. This special form of graphene is grown epitaxially on nitrogen-seeded single-crystal silicon carbide (SiC( )) substrates by thermal decomposition of Si atoms. The pre-deposited interfacial nitrogen atoms pin the first graphene layer to the SiC substrate, and cause it and subsequent graphene layers to buckle into nanoscale folds, which opens an energy gap of up to ∼0.7 eV. We observe a remarkable increase of up to two orders of magnitude in the relaxation rate of the THz carrier dynamics of this semiconducting form of epitaxial graphene relative to pristine epitaxial graphene, which we attribute to a large enhancement of the optical-phonon-mediated carrier cooling and recombination over a wide range of electron temperatures due to the finite bandgap. Our results suggest that the introduced bandgap is spatially non-homogenous, with local values close to the optical phonon energy of ∼200 meV, which allows the conduction and the valence band to be bridged by optical phonon emission. We also demonstrate that carrier relaxation times can be modified by orders of magnitude by careful bandgap engineering, which could find application in novel graphene-based devices that incorporate both metallic and semiconducting forms of graphene.

Published in: "Applied Physics Letters".

Magneto-transport and Kondo effect in cobalt doped Bi2Se3 topological insulators

2015-10-28T15:12:58+00:00October 28th, 2015|Categories: Publications|Tags: |

Weak magnetic perturbation on the surface of topological insulators breaks the time reversal symmetry and opens the energy gap. We report the effect of cobalt doping in Bi2Se3 single crystals grown by the modified Bridgeman technique. The magnetic susceptibility of the cobalt doped Bi2Se3 changes from diamagnetic to paramagnetic at room temperature and then to ferromagnetic at 2 K. The Kondo effect was observed in bulk crystals for Bi1.8Co0.2Se3 (i.e., 10% Co doped in Bi2Se3), whereas the lower doping of cobalt results in a simple metallic behavior. In order to study the surface properties, the devices were fabricated on mechanically exfoliated thin (∼70 nm) flakes of Bi1.8Co0.2Se3 obtained from the bulk crystal. Semiconducting behavior can be clearly seen in Bi1.8Co0.2Se3 devices at T > 40 K, and the Kondo effect was observed when the temperature was below 40 K. In the Bi1.8Co0.2Se3 device, the weak antilocalization to weak localization transition observed in magnetoresistance behavior at T ∼ 40 K indicates the band opening at the Dirac point.

Published in: "Applied Physics Letters".

Nonlinear mode coupling and internal resonances in MoS2 nanoelectromechanical system

2015-10-28T15:12:57+00:00October 28th, 2015|Categories: Publications|Tags: , |

Atomically thin two dimensional (2D) layered materials have emerged as a new class of material for nanoelectromechanical systems (NEMS) due to their extraordinary mechanical properties and ultralow mass density. Among them, graphene has been the material of choice for nanomechanical resonator. However, recent interest in 2D chalcogenide compounds has also spurred research in using materials such as MoS2 for the NEMS applications. As the dimensions of devices fabricated using these materials shrink down to atomically thin membrane, strain and nonlinear effects have become important. A clear understanding of the nonlinear effects and the ability to manipulate them is essential for next generation sensors. Here, we report on all electrical actuation and detection of few-layer MoS2 resonator. The ability to electrically detect multiple modes and actuate the modes deep into the nonlinear regime enables us to probe the nonlinear coupling between various vibrational modes. The modal coupling in our device is strong enough to detect three distinct internal resonances.

Published in: "Applied Physics Letters".

Electronic structure of a superconducting topological insulator Sr-doped Bi2Se3

2015-10-27T15:13:30+00:00October 27th, 2015|Categories: Publications|Tags: |

Using high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy, the atomic and low energy electronic structure of the Sr-doped superconducting topological insulators (Sr x Bi2Se3) was studied. Scanning tunneling microscopy shows that most of the Sr atoms are not in the van der Waals gap. After Sr doping, the Fermi level was found to move further upwards when compared with the parent compound Bi2Se3, which is consistent with the low carrier density in this system. The topological surface state was clearly observed, and the position of the Dirac point was determined in all doped samples. The surface state is well separated from the bulk conduction bands in the momentum space. The persistence of separated topological surface state combined with small Fermi energy makes this superconducting material a very promising candidate for the time reversal invariant topological superconductor.

Published in: "Applied Physics Letters".

Effect of ferroelectric substrate on carrier mobility in graphene field-effect transistors

2015-10-27T15:13:28+00:00October 27th, 2015|Categories: Publications|Tags: |

Effect of LiNbO3 ferroelectric substrate on the carrier mobility in top gated graphene field-effect transistors (G-FETs) is demonstrated. It is shown that, at the same residual concentration of the charge carriers, the mobility in the G-FETs on the LiNbO3 substrate is higher than that on the SiO2/Si substrate. The effect is associated with reduction of Coulomb scattering via screening the charged impurity field by the field induced in the ferroelectric substrate, but significant only for mobilities below 1000 cm2/V s. Raman spectra analysis and correlations established between mobility and microwave loss tangent of the Al2O3 gate dielectric indicate that the charged impurities are located predominantly at the gate dielectric and/or at the gate dielectric/graphene interface and are likely associated with oxygen vacancies.

Published in: "Applied Physics Letters".

Oxidation of ultrathin GaSe

2015-10-26T15:14:26+00:00October 26th, 2015|Categories: Publications|Tags: |

Oxidation of exfoliated gallium selenide (GaSe) is investigated through Raman, photoluminescence, Auger, and X-ray photoelectron spectroscopies. Photoluminescence and Raman intensity reductions associated with spectral features of GaSe are shown to coincide with the emergence of signatures emanating from the by-products of the oxidation reaction, namely, Ga2Se3 and amorphous Se. Photoinduced oxidation is initiated over a portion of a flake highlighting the potential for laser based patterning of two-dimensional heterostructures via selective oxidation.

Published in: "Applied Physics Letters".

Surface confined quantum well state in MoS2(0001) thin film

2015-10-22T16:13:15+00:00October 22nd, 2015|Categories: Publications|Tags: |

Surface confined quantum well state (scQWS) is a QWS confined around the surface of a thin film whose electronic energy is smaller than the work function of the film. The scQWS is rather rare in most thin films. Here, we show the existence of scQWS in thin films of transition metal dichalcogenides, MoS2. Signatures of scQWS are identified as the overall downward band dispersion in the bulk gap of 2 H-MoS2 thin film at larger binding energy range. These scQWSs are also characterized with a Shockley-type surface state having an inverse parabolic decay into the film and a symmetric (asymmetric) distribution of projected charge density at the two surfaces of odd-layer (even-layer) films. Our findings of scQWS in MoS2 shed some light on understanding the electronic properties of 2D materials with implications in future 2D electronic devices.

Published in: "Applied Physics Letters".

A general method for large-area and broadband enhancing photoresponsivity in graphene photodetectors

2015-10-22T16:13:14+00:00October 22nd, 2015|Categories: Publications|Tags: |

We report on a general method for broadband responsivity enhancement in graphene photodetectors based on the sandwiched graphene structure under total internal reflection. The optical absorption is ∼25% for transverse electric waves for pure monolayer graphene, and the responsivity of pure monolayer graphene photodetectors is 0.012 A/W, which is one or two orders of magnitude larger than the normal incidence excitation. The enhanced responsivity covers a wide wavelength range from 300 to 1550 nm. Further, this method is not limited by the device, and it is a general method used to improve the light-graphene coupling, thus increasing the responsivity of graphene photodetectors largely. And this method allows large area preparation to meet the needs of the free space light detections.

Published in: "Applied Physics Letters".

Effect of impurity doping in gapped bilayer graphene

2015-10-21T16:14:05+00:00October 21st, 2015|Categories: Publications|Tags: |

Impurity doping plays a pivotal role in semiconductor electronics. We study the doping effect in a two-dimensional semiconductor, gapped bilayer graphene. By employing in situ deposition of calcium on the bilayer graphene, dopants are controllably introduced. Low temperature transport results show a variable range hopping conduction near the charge neutrality point persisting up to 50 K, providing evidence for the impurity levels inside the gap. Our experiment confirms a predicted peculiar effect in the gapped bilayer graphene, i.e., formation of in-gap states even if the bare impurity level lies in the conduction band. The result provides perspective on the effect of doping and impurity levels in semiconducting bilayer graphene.

Published in: "Applied Physics Letters".

High temperature and current density induced degradation of multi-layer graphene

2015-10-20T16:13:55+00:00October 20th, 2015|Categories: Publications|Tags: |

We present evidence of moderate current density, when accompanied with high temperature, promoting migration of foreign atoms on the surface of multi-layer graphene. Our in situ transmission electron microscope experiments show migration of silicon atoms at temperatures above 800 °C and current density around 4.2 × 107 A/cm2. Originating from the micro-machined silicon structures that clamp the freestanding specimen, the atoms are observed to react with the carbon atoms in the multi-layer graphene to produce silicon carbide at temperatures of 900–1000 °C. In the absence of electrical current, there is no migration of silicon and only pyrolysis of polymeric residue is observed.

Published in: "Applied Physics Letters".

Transfer characteristics and low-frequency noise in single- and multi-layer MoS2 field-effect transistors

2015-10-19T16:13:11+00:00October 19th, 2015|Categories: Publications|Tags: |

Leveraging nanoscale field-effect transistors (FETs) in integrated circuits depends heavily on its transfer characteristics and low-frequency noise (LFN) properties. Here, we report the transfer characteristics and LFN in FETs fabricated with molybdenum disulfide (MoS2) with different layer (L) counts. 4L to 6L devices showed highest ION-IOFF ratio (≈108) whereas LFN was maximum for 1L device with normalized power spectral density (PSD) ≈1.5 × 10−5 Hz−1. For devices with L ≈ 6, PSD was minimum (≈2 × 10−8 Hz−1). Further, LFN for single and few layer devices satisfied carrier number fluctuation (CNF) model in both weak and strong accumulation regimes while thicker devices followed Hooge’s mobility fluctuation model in the weak accumulation regime and CNF model in strong accumulation regime, respectively. Transfer-characteristics and LFN experimental data are explained with the help of model incorporating Thomas-Fermi charge screening and inter-layer resistance coupling.

Published in: "Applied Physics Letters".

Transition metal contacts to graphene

2015-10-15T16:14:18+00:00October 15th, 2015|Categories: Publications|Tags: |

Achieving low resistance contacts to graphene is a common concern for graphene device performance and hybrid graphene/metal interconnects. In this work, we have used the circular Transfer Length Method (cTLM) to electrically characterize Ag, Au, Ni, Ti, and Pd as contact metals to graphene. The consistency of the obtained results was verified with the characterization of up to 72 cTLM structures per metal. Within our study, the noble metals Au, Ag and Pd, which form a weaker bond with graphene, are shown to result in lower contact resistance (Rc) values compared to the more reactive Ni and Ti. X-ray Photo Electron Spectroscopy and Transmission Electron Microscopy characterization for the latter have shown the formation of Ti and Ni carbides. Graphene/Pd contacts show a distinct intermediate behavior. The weak carbide formation signature and the low Rc values measured agree with theoretical predictions of an intermediate state of weak chemisorption of Pd on graphene.

Published in: "Applied Physics Letters".

Gate tunable monolayer MoS2/InP heterostructure solar cells

2015-10-14T16:15:55+00:00October 14th, 2015|Categories: Publications|Tags: , |

We demonstrate monolayer molybdenum disulfide (MoS2)/indium phosphide (InP) van der Waals heterostructure with remarkable photovoltaic response. Furthermore, benefiting from the atomically thin and semiconductor nature of MoS2, we have designed the gate tunable MoS2/InP heterostructure. Applied with a top gate voltage, the Fermi level of MoS2 is effectively tuned, and the barrier height at the MoS2/InP heterojunction correspondingly changes. The power conversion efficiency of MoS2/InP solar cells has reached a value of 7.1% under AM 1.5G illumination with a gate voltage of +6 V. The tunable MoS2/InP heterostructure may be promising for highly efficient solar cells.

Published in: "Applied Physics Letters".

Ferroelectric modulation of terahertz waves with graphene/ultrathin-Si:HfO2/Si structures

2015-10-13T16:17:53+00:00October 13th, 2015|Categories: Publications|Tags: |

Ferroelectric-field-effect-tunable modulation of terahertz waves in graphene/Si:HfO2/Si stack structure was observed. The modulation shows distinct behaviors when the samples under different gate polarities. At a negative voltage, a transmission modulation depth up to ∼74% was present without depending on the photo illumination power, whereas, at a positive voltage, the modulation of Thz wave shows dependence on the illumination power, which is ascribed to the creation/elimination of an extra barrier in Si layer in response to the polarization in the ferroelectric Si:HfO2 layer. Considering the good compatibility of HfO2 on Si-based semiconductor process, the ferroelectricity layer of Si:HfO2 may open up an avenue for the tunable modulation of Thz wave.

Published in: "Applied Physics Letters".

Pt-functionalized reduced graphene oxide for excellent hydrogen sensing at room temperature

2015-10-13T16:17:52+00:00October 13th, 2015|Categories: Publications|Tags: , |

Cost effective and faster detection of H2 has always remained a challenge. We report synthesis of reduced graphene oxide (RGO)–Pt composite and its application as highly sensitive and selective H2 sensors at room temperature. Four samples by varying the ratio of RGO and Pt were prepared to test their sensing performance. The tests were carried out in inert (N2) ambience as well as air ambience. It was observed that the RGO:Pt (1:3) 1 h reduced sample demonstrated the best H2 sensing performance in terms of sensitivity, response time, and recovery time at room temperature. Its response varied from ∼19% (200 ppm) to 57% (5000 ppm) against H2 in air ambience. Also, the response time and recovery time of the RGO:Pt (1:3) sample were found to be as fast as 65 s and 230 s against 5000 ppm, respectively, in air ambience. In N2 ambience, the RGO:Pt (1:3) sample demonstrated the best response of −97% (500 ppm), but its recovery was found to be poor. The RGO–Pt composite formation was verified by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The detailed physics behind the sensing mechanisms have been explained and experimentally verified in this work.

Published in: "Applied Physics Letters".

Broadband ultrafast spatial self-phase modulation for topological insulator Bi2Te3 dispersions

2015-10-12T16:18:40+00:00October 12th, 2015|Categories: Publications|

Ultrathin topological insulator bismuth telluride (Bi2Te3) nanosheets with uniform hexagonal nanostructure have been synthesized by cost-effective solvothermal method. Broadband spatial self-phase modulation phenomena of these topological insulator nanosheets have been observed with 400 nm, 800 nm, and 1070 nm ultrafast lasers. The experimental results suggest that this coherent light scattering is due to the broadband, ultrafast, and large third-order optical nonlinearity of Bi2Te3. With the pulsed laser excitation, the nonlinear refractive index (n 2) of Bi2Te3 dispersion solution was measured to be ∼10−12 m2/W, and the third-order nonlinear susceptibility ∼10−7 esu. Our work may provide an inroad for developing the nonlinear optical applications based on topological insulators.

Published in: "Applied Physics Letters".

Giant enhancement of nanoscale thermal radiation based on hyperbolic graphene plasmons

2015-10-09T14:18:02+00:00October 9th, 2015|Categories: Publications|Tags: |

Excitation of surface plasmons enables super-Planckian thermal radiation far beyond the blackbody limit. By patterning a single layer of graphene sheet into ribbons, the closed circular dispersion of graphene plasmons is opened to become hyperbolic, leading to broadband singularities of density of states. Extremely high-k evanescent waves can now couple with hyperbolic graphene plasmons. Consequently, a giant enhancement of the near-field radiative heat flux, by more than one order of magnitude, is demonstrated in this study using rigorous numerical simulations. The findings may open promising pathways for highly efficient thermal management, energy harvesting, and sub-wavelength thermal imaging.

Published in: "Applied Physics Letters".

Graphite to ultrafine nanocrystalline diamond phase transition model and growth restriction mechanism induced by nanosecond laser processing

2015-10-08T16:16:00+00:00October 8th, 2015|Categories: Publications|Tags: |

To have a clear insight into nanocrystal growth from graphite to diamond upon high energy pulsed laser irradiation of graphite suspension, synthesis of ultrafine nanocrystalline diamonds with laser energy set up from 0.3 J to 12 J, repetition rate of 10 Hz has been studied. The method allows synthesizing ultrafine nanocrystalline particles continuously at the ambient temperature and normal pressure. The particle size is shown independent of laser energy, which is ultrafine and ranges in 2–6 nm. The theoretical grown size of nano-diamonds is found in well agreement with the experiment results. Four kinds of production were found: nano-diamond, spherical carbon nano-particles, flocculent amorphous carbon, and graphene nano-ribbon rolls. A solid-vapor-plasma-liquid coexistence model describing phase transition from graphite to diamond induced by nanosecond laser processing was proposed. Graphene nano-ribbon rolls might be the intermediate phase in the conversion from graphite to diamond.

Published in: "Applied Physics Letters".

Correlating spin transport and electrode magnetization in a graphene spin valve: Simultaneous magnetic microscopy and non-local measurements

2015-10-08T16:15:59+00:00October 8th, 2015|Categories: Publications|Tags: |

Using simultaneous magnetic force microscopy and transport measurements of a graphene spin valve, we correlate the non-local spin signal with the magnetization of the device electrodes. The imaged magnetization states corroborate the influence of each electrode within a one-dimensional spin transport model and provide evidence linking domain wall pinning to additional features in the transport signal.

Published in: "Applied Physics Letters".

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