Kun Xu

Purdue University, West Lafayette, Indiana, United States

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Publications (11)28.39 Total impact

  • Kun Xu, Peide D. Ye
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    ABSTRACT: Graphene nanoribbons (GNRs), as an emerging class of material, hold great potential for the future high speed and low power electronic and spintronic devices. The fabrication of GNRs is of the utmost interest in terms of graphene based device research. Chemical narrowing of GNRs by oxidation is a promising technique in producing nanoribbons of desired widths. In this article, we hope to elucidate the etching mechanism of zigzag GNR (ZGNR) edge by oxidation through theoretical investigations. The oxidation mechanisms and dynamics of the ZGNR edge by O2 and O3 are fully revealed by density functional theory and statistical theory. The relationship between the reaction time and pressure as well as temperature is estimated dynamically. These theoretical results successfully interpret the recent experimental results and can be further used to predict the appropriate oxidation conditions for the precision etching of ZGNRs.
    The Journal of Physical Chemistry C. 05/2014; 118(19):10400–10407.
  • Kun Xu, Peide D. Ye
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    ABSTRACT: The electron spin states of zigzag graphene nanoribbon (ZGNR) edge play a pivotal role in the applications of graphene nanoribbons. However, the exact arrangements of the electron spins remain unclear to date. In this report, the electronic spin states of the ZGNR edge have been elucidated through a combination of quantum chemical investigation and previous electron spin resonance experiment observations. An alternating α and β spin configuration of the unpaired electrons along the ZGNR edge is established in ambient condition without any external magnetic field, and the origin of the spin magnetism of the ZGNR edge is revealed. It paves a pathway for the understanding and design of graphene based electronic and spintronic devices.
    Applied Physics Letters 01/2014; 104(16):163104-163104-4. · 3.79 Impact Factor
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    ABSTRACT: Advancing toward the rational design, fabrication, and implementation of graphene(GR)-based electronic and optical devices, the intrinsic barrier height of undoped GR (the Dirac point of GR to the conduction band(CB) edge of an insulator), as well as the intrinsic work function(WF) of GR must be accurately determined. We present an internal photoemission (IPE) investigation of a unique semi-transparent metal/high-k/GR/SiO2/Si structure, and focus our study on the photoemission phenomena at the GR/SiO2 interface. By taking advantage of the optical interference of the SiO2 cavity, the enhanced photoemission from GR was observed. As a result, a complete electronic band alignment at the GR/SiO2/Si interfaces is established. The intrinsic positions of the undoped GR Dirac point with respect to the CB of SiO2, 3.58 eV (Al2O3 TG) and 3.60 eV (HfO2 TG), are obtained. The intrinsic WF of graphene is found to be 4.50 eV. The determination of the WF of GR is of significant importance to the engineering of GR-base devices and the IPE spectroscopy, combined with specific interference cavity structures, would be a valuable measurement technique for other GR-like2-D material systems.
    03/2013;
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    ABSTRACT: We report band alignment for the widely studied organic semiconductor, Poly(3-hexylthiophene) (P3HT), by using internal photoemission (IPE). P3HT solution was spin coated onto 280 nm thick SiO2 on heavily doped P-type silicon. A 10 nm thick aluminum (Al) electrode with adjoining 70 nm thick Al contact pad were deposited onto the P3HT film through aligned shadow masks. Photocurrent in the IPE measurement was generated using a monochromator with photon energy ranging from 1.5eV to 6.0eV (0.05 eV steps) and with a DC voltage which ranged from 20V to -20V (-2V steps) applied between the silicon backside and the thick Al contact. Both positive photocurrent and negative photocurrent were observed. For the IPE measurement, the yield (Y) is defined as the ratio of the carriers contributing to the photocurrent to the incident photon flux, and the threshold at each applied voltage is obtained by extrapolating Y^1/3(hν) to zero. The barrier height is determined from Schottky plots extrapolated to zero field. By using this established method we extract a barrier height of 4.2 eV ± 0.1 eV for the Si:SiO2 interface and 4.0 eV ± 0.1 eV for the P3HT:SiO2 interface, respectively.
    03/2013;
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    ABSTRACT: We report the direct measurement of the Dirac point, the Fermi level, and the work function of graphene by performing internal photoemission measurements on a graphene/SiO2/Si structure with a unique optical-cavity enhanced test structure. A complete electronic band alignment at the graphene/SiO2/Si interfaces is accurately established. The observation of enhanced photoemission from a one-atom thick graphene layer was possible by taking advantage of the constructive optical interference in the SiO2 cavity. The photoemission yield was found to follow the well-known linear density-of-states dispersion in the vicinity of the Dirac point. At the flat band condition, the Fermi level was extracted and found to reside 3.3 eV ± 0.05 eV below the bottom of the SiO2 conduction band. When combined with the shift of the Fermi level from the Dirac point we are able to ascertain the position of the Dirac point at 3.6 eV ± 0.05 eV with respect to the bottom of the SiO2 conduction band edge, yielding a work function of 4.5 eV ± 0.05 eV which is in an excellent agreement with theory. The accurate determination of the work function of graphene is of significant importance to the engineering of graphene-based devices and the measurement technique we have advanced in this letter will have significant impact on numerous applications for emerging graphene-like 2-Dimensional material systems.
    Nano Letters 12/2012; · 13.03 Impact Factor
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    ABSTRACT: Iron metal is an opaque phase common in planetary materials both as an igneous mineral and as a space-weathering by-product. In either form, iron metal has a large influence on the interpretation of ultraviolet, visible, and near-infrared spectra of planetary surfaces obtained from Earth- or space-based observatories. Therefore, the optical properties of iron are a critical input necessary for accurate theoretical radiative-transfer mixing models for inversion of maturity and mineral proportions from reflectance spectra. Here we report new measurements of the optical constants of iron in the ultraviolet, visible, and near-infrared portions of the electromagnetic spectrum (˜160 to 1700 nm). These values are determined from an iron metal film vapor-deposited onto a fused silica prism. Optical constant determination is carried out using an ellipsometer that performs the measurement within the prism, sensing the side of the metal film unexposed to the ambient atmosphere. The data we report have important implications for modeling planetary spectra and for comparison of laboratory measurements of extraterrestrial samples with remotely sensed data sets.
    Geophysical Research Letters 05/2012; 39(10):10204-. · 3.98 Impact Factor
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    ABSTRACT: The electron energy band alignment of a metal-oxide-semiconductor tunnel field-effect transistor heterojunction, W/Al2O3/InGaAs/InAs/InP, is determined by internal photoemission spectroscopy. At the oxide flat-band condition, the barrier height from the top of the InGaAs/InAs valence band and the top of the InP valence band to the bottom of the Al2O3 conduction band is determined to be 3.5 and 2.8 eV, respectively. The simulated energy band diagram of the heterostructure is shown to be consistent with the measured band alignments if an equivalent positive charge of 6.0 × 1012 cm−2 is present at the Al2O3/InGaAs. This interface charge is in agreement with previously reported capacitance-voltage measurements.
    Applied Physics Letters 03/2012; 100(10). · 3.79 Impact Factor
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    ABSTRACT: We investigate the integration of Al2O3 high-k dielectric on two-dimensional (2D) crystals of boron nitride (BN) and molybdenum disulfide (MoS2) by atomic layer deposition (ALD). We demonstrate the feasibility of direct ALD growth with trimethylaluminum(TMA) and water as precursors on both 2D crystals. Through theoretical and experimental studies, we found that the initial ALD cycles play the critical role, during which physical adsorption dominates precursor adsorption at the semiconductor surface. We model the initial ALD growth stages at the 2D surface by analyzing Lennard-Jones Potentials, which could guide future optimization of the ALD process on 2D crystals.
    Applied Physics Letters 02/2012; 100(15). · 3.79 Impact Factor
  • Han Liu, Kun Xu, Peide Ye
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    ABSTRACT: We study the growth mechanism of atomic layer deposition (ALD) of Al2O3 on layered MoS2. We demonstrate the feasibility of direct growth of Al2O3 on this 2D material by trimethylaluminum (TMA) and water as precursors. Atomic force microscopy study shows that the quality of the Al2O3 film is degraded at elevated temperatures, originated from impeded surface absorption of precursors. We also apply density functional theory (DFT) study of the reaction which is in good agreement with our experimental observations. In addition, we fabricate dual gate MoS2 metal-oxide-semiconductor field effect transistors (MOSFET). From the transport study we find out the lowering the growth temperature will result in a huge negative threshold voltage shift, which can be improved by either forming gas anneal after Al2O3 deposition or insertion of an Al seeding layer which would facilitate higher growth temperature with better film quality. Further details will be provided in the presentation.
    02/2012;
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    ABSTRACT: We report the first direct measurement of the Dirac point, the Fermi level, and the work function of single layer gapless graphene by using photoemission threshold spectroscopy. Since the pioneering work of Novoselov et al in 2004, [1] graphene has attracted an immense amount of interest from all disciplines. [2] The knowledge of the physics of graphene-based devices has grown dramatically. Along with the recent success of large area chemical vapor deposition (CVD) growth of graphene, [3] it seems the industrial applications such as transparent electrodes, [4] field effect transistors, [5] and quantum well devices [6] are becoming more promising. However, the precise position of the Dirac point and Fermi level at the graphene/oxide interface has yet to be investigated; despite their importance in the design and modeling of graphene-based devices. In this paper, we present the study of a semi-transparent metal/high-k/graphene/SiO2/Si structure, and focus our study on the photoemission phenomena at the graphene/SiO2 interface. As a result, a complete electronic band alignment of the graphene/SiO2/Si system is accurately constructed for the first time.
    Device Research Conference (DRC), 2012 70th Annual; 01/2012
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    ABSTRACT: The tunneling field-effect transistor (TFET) has been attracting increasing attention for low-voltage logic application [1]. Recently, III-V semiconductor TFETs are being extensively explored, showing higher on-state current than Si TFETs [1]–[3]. However, low subthreshold swings (SS) have not yet been demonstrated on any III-V TFETs, possibly due to the high density of traps at the high-k oxide and semiconductor interface [2]–[3]. It has been shown that post deposition anneal (PDA) can improve SS by reducing the interface trap density in III V TFETs [2].
    01/2011;