N. Chandrasekhar

Max Planck Institute for Polymer Research, Mayence, Rheinland-Pfalz, Germany

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Publications (38)169.14 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: Significant variation in the charge transport behaviour in graphene oxide (GO) ranging from Schottky to Poole-Frenkel and to space charge limited transport exists. These have been extensively reported in the literature. However, the validity of such conventional charge transport models meant for delocalized carriers, to study charge transport through localised states in GO, a disordered semiconductor is open to question. In this work, we use the concept of transport energy (TE) to model charge transport in lightly reduced GO (RGO) and demonstrate that the TE calculations match well with temperature dependent experimental I-V data on RGO. We report on a temperature dependent TE ranging from a few 10 meV to 0.1 eV in slightly reduced GO. Last, we point out that, despite the success of several delocalised charge transport models in estimating barrier heights that resemble the TE level, they remain largely accidental and lack the insight in which the TE concept provides in understanding charge transport in RGO.
    Journal of Applied Physics 02/2013; 113(6). · 2.21 Impact Factor
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    Electrochemical and Solid-State Letters 07/2012; · 2.01 Impact Factor
  • Xue-Feng Wang, Natarajan Chandrasekhar, Haibin Su
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    ABSTRACT: We investigate charge transport across metal-molecule-metal junctions, i.e. hexagonal and triangular nanographene molecular layers sandwiched between Pt and Pd thin films, as measured by ballistic-electron-emission spectroscopy (BEEM). The measured shape of current-voltage curves cannot be explained in the framework of existing BEEM theories of bulk inorganic semiconductors. We develop a tight-binding model for the BEEM process and propose that the energetic dispersion of molecular layers and the dephasing effect due to the interface states account for the anomalous BEEM current-voltage behavior and play an important role in determining the shape of the curve. The electron-phonon scattering can also affect the shape of current-voltage curves.
    Journal of Physics Condensed Matter 05/2012; 24(25):255303. · 2.36 Impact Factor
  • H S Wong, X Feng, K Müllen, N Chandrasekhar, C Durkan
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    ABSTRACT: We report selective tunnelling through a nanographene intermolecular tunnel junction achieved via scanning tunnelling microscope tip functionalization with hexa-peri-hexabenzocoronene (HBC) molecules. This leads to an offset in the alignment between the energy levels of the tip and the molecular assembly, resulting in the imaging of a variety of distinct charge density patterns in the HBC assembly, not attainable using a bare metallic tip. Different tunnelling channels can be selected by the application of an electric field in the tunnelling junction, which changes the condition of the HBC on the tip. Density functional theory-based calculations relate the imaged HBC patterns to the calculated molecular orbitals at certain energy levels. These patterns bear a close resemblance to the π-orbital states of the HBC molecule calculated at the relevant energy levels, mainly below the Fermi energy of HBC. This correlation demonstrates the ability of an HBC functionalized tip as regards accessing an energy range that is restricted to the usual operating bias range around the Fermi energy with a normal metallic tip at room temperature. Apart from relating to molecular orbitals, some patterns could also be described in association with the Clar aromatic sextet formula. Our observations may help pave the way towards the possibility of controlling charge transport between organic interfaces.
    Nanotechnology 03/2012; 23(9):095601. · 3.84 Impact Factor
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    H S Wong, X Feng, Z Y Yang, K Müllen, N Chandrasekhar, C Durkan
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    ABSTRACT: Molecular self-organization has the potential to serve as an efficient and versatile tool for the spontaneous creation of low-dimensional nanostructures on surfaces. We demonstrate how the subtle balance between intermolecular interactions and molecule-surface interactions can be altered by modifying the environment or through manipulation by means of the tip in a scanning tunnelling microscope (STM) at room temperature. We show how this leads to the distinctive ordering and disordering of a triangular nanographene molecule, the trizigzag-hexa-peri-hexabenzocoronenes-phenyl-6 (trizigzagHBC-Ph6), on two different surfaces: graphite and Au(111). The assembly of submonolayer films on graphite reveals a sixfold packing symmetry under UHV conditions, whereas at the graphite-phenyloctane interface, they reorganize into a fourfold packing symmetry, mediated by the solvent molecules. On Au(111) under UHV conditions in the multilayer films we investigated, although disorder prevails with the molecules being randomly distributed, their packing behaviour can be altered by the scanning motion of the tip. The asymmetric diode-like current-voltage characteristics of the molecules are retained when deposited on both substrates. This paper highlights the importance of the surrounding medium and any external stimulus in influencing the molecular organization process, and offers a unique approach for controlling the assembly of molecules at a desired location on a substrate.
    Nanotechnology 12/2011; 23(1):015606. · 3.84 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: We demonstrate the information on molecular vibrational modes via the second derivative (d(2)I(B)/dV(2)) of the ballistic electron emission spectroscopy (BEES) current. The proposed method does not create huge fields as in the case of conventional derivative spectroscopy and maintains a zero bias across the device. BEES studies carried out on three different types of large polycyclic aromatic hydrocarbon (PAH) molecular layers show that the d(2)I(B)/dV(2) spectra consist of uniformly spaced peaks corresponding to vibronic excitations. The peak spacing is found to be identical for molecules within the same PAH family though the BEES onset voltage varies for different molecules. In addition, injection into a particular orbital appears to correspond to a specific vibrational mode as the manifestation of the symmetry principle.
    Nanotechnology 09/2011; 22(43):435701. · 3.84 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: Quantum states of a trinaphthylene molecule were manipulated by putting its naphthyl branches in contact with single Au atoms. One Au atom carries 1-bit of classical information input that is converted into quantum information throughout the molecule. The Au-trinaphthylene electronic interactions give rise to measurable energy shifts of the molecular electronic states demonstrating a NOR logic gate functionality. The NOR truth table of the single molecule logic gate was characterized by means of scanning tunnelling spectroscopy.
    ACS Nano 02/2011; 5(2):1436-40. · 12.06 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: A logic gate has been implemented in a single trinaphthylene molecule. Each logical input controls the position of a surface Au atom that is brought closer or further away from the end of one of the naphthyl branch. Each Au atom carries 1 bit of information and is able to deform nonlocally and to shift in energy the molecular electronic states of the trinaphthylene. Probed at the end of the third naphthyl branch using scanning tunneling spectroscopy, the variations of the tunneling current intensity as a function of the Au atoms position measures the logical output of the gate. We demonstrate both theoretically and experimentally that these variations respect the truth table of a NOR logic gate.
    Physical review. B, Condensed matter 01/2011; 83(15).
  • Edward J McCumiskey, Natarajan Chandrasekhar, Curtis R Taylor
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    ABSTRACT: We report the mechanical characterization of a nanocomposite thin film consisting of CdSe quantum dots (QDs) and the electroluminescent polymer poly[2-methoxy-5-2(2'-ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV). The electrical and optical properties of this nanocomposite have been studied intensely for organic electronics research. However, the mechanical behaviour-which depends on several variables, such as the concentration of QDs, the interfacial surface area, deformation mechanisms, and the mechanical properties of the QDs and polymer-is not well understood. In this paper, thin films of CdSe QDs blended with MEH-PPV are prepared at different QD:polymer ratios. The QDs' surface ligands are removed to promote dispersion and to more realistically mimic QD-polymer devices. QD dispersion is verified using transmission electron microscopy, while the films' morphology and roughness are observed using atomic force microscopy. Finally, quasi-static nanoindentation is used to measure the elastic modulus, hardness, and creep of the films. The incorporation of QDs into the polymer matrix is seen to increase the elastic modulus and hardness by factors of 4 and 5, respectively, both of which scale linearly as a function of QD volume fraction. Furthermore, the QDs have the effect of suppressing the viscoelastic behaviour of the polymer, which is observed by studying the creep under a constant load. These results may have profound implications for future nanocomposite devices, such as increased stiffness, damage resistance, and long-term stability.
    Nanotechnology 06/2010; 21(22):225703. · 3.84 Impact Factor
  • Yi Zheng, Andrew Thye Shen Wee, Natarajan Chandrasekhar
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    ABSTRACT: In this letter, we show the feasibility to use scanning tunneling microscopy (STM) as a stand-alone technique in analyzing the structure of organic thin films grown on polycrystalline metal surfaces. At room temperature, by effectively suppressing the molecule-substrate interaction, pentacene resumes the typical quasi layer-by-layer growth with the "thin-film phase" structure due to intermolecule interaction, while substrate roughness does not play an important role. By elevating the substrate to 320 K, two different polycrystalline phases, that is, the "thin-film phase" and the "single-crystal phase" intermixed grow and form terraced and lamellar structures, respectively. Using STM distance-voltage spectroscopy, the energy level alignment of the underlying organic/metal interfaces can also be acquired.
    ACS Nano 03/2010; 4(4):2104-8. · 12.06 Impact Factor
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    ABSTRACT: The scientific and technical challenges involved in building the planar electrical connection of an atomic scale circuit to N electrodes (N > 2) are discussed. The practical, laboratory scale approach explored today to assemble a multi-access atomic scale precision interconnection machine is presented. Depending on the surface electronic properties of the targeted substrates, two types of machines are considered: on moderate surface band gap materials, scanning tunneling microscopy can be combined with scanning electron microscopy to provide an efficient navigation system, while on wide surface band gap materials, atomic force microscopy can be used in conjunction with optical microscopy. The size of the planar part of the circuit should be minimized on moderate band gap surfaces to avoid current leakage, while this requirement does not apply to wide band gap surfaces. These constraints impose different methods of connection, which are thoroughly discussed, in particular regarding the recent progress in single atom and molecule manipulations on a surface.
    Journal of Physics Condensed Matter 02/2010; 22(8):084025. · 2.36 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: In a planar configuration, multiple electrical connections to a single molecule require an atomic scale precision of the wiring and an atomically flat supporting surface. Current nanofabrication techniques cannot achieve this on the same surface of a wafer. A double sided interconnection process flow adopted from sensor technology is presented using silicon on insulator substrates. The top part of the wafer is exclusively reserved for atomic scale interconnect and constructions. The back side is reserved for all the other interconnection steps. To ensure the passage between the back and the top side of the wafer, nanoscale vias are fabricated through the full thickness of the wafer. The top end of each via is boron and phosphorus doped, and the interconnection leakage current-voltage characteristics are measured. At low voltage and for top inter-via distances in the 30–50 nm range, a high gigaohm range resistance is obtained.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2010;
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    ABSTRACT: Based on the assumption that the contact barrier height determines the current flow in organic semiconductor-based electronic devices, charge injection at metal-organic (MO) interfaces has been extensively investigated, while space-charge conduction in organic bulk is generally overlooked. Recent theoretical modeling and simulation have pointed out that such a simplification is questionable due to the hopping nature of charge injection and hopping-related space-charge conduction. Here we show experimentally that charge transport in MO diodes is a complex interplay between injection-limited current (ILC) and space-charge-limited current (SCLC). We report the experimental observation of ILC-to-SCLC transition in Ag/pentacene/Ag diodes as a function of temperature.
    Applied Physics Letters 11/2009; · 3.79 Impact Factor
  • Hong Seng Wong, Colm Durkan, Natarajan Chandrasekhar
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    ABSTRACT: With recent developments in carbon-based electronics, it is imperative to understand the interplay between the morphology and electronic structure in graphene and graphite. We demonstrate controlled and repeatable vertical displacement of the top graphene layer from the substrate mediated by the scanning tunneling microscopy (STM) tip-sample interaction, manifested at the atomic level as well as over superlattices spanning several tens of nanometers. Besides the full-displacement, we observed the first half-displacement of the surface graphene layer, confirming that a reduced coupling rather than a change in lateral layer stacking is responsible for the triangular/honeycomb atomic lattice transition phenomenon, clearing the controversy surrounding it. Furthermore, an atomic scale mechanical stress at a grain boundary in graphite, resulting in the localization of states near the Fermi energy, is revealed through voltage-dependent imaging. A method of producing graphene nanoribbons based on the manipulation capabilities of the STM is also implemented.
    ACS Nano 10/2009; 3(11):3455-62. · 12.06 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: Gears are microfabricated down to diameters of a few micrometres. Natural macromolecular motors, of tens of nanometres in diameter, also show gear effects. At a smaller scale, the random rotation of a single-molecule rotor encaged in a molecular stator has been observed, demonstrating that a single molecule can be rotated with the tip of a scanning tunnelling microscope (STM). A self-assembled rack-and-pinion molecular machine where the STM tip apex is the rotation axis of the pinion was also tested. Here, we present the mechanics of an intentionally constructed molecule-gear on a Au(111) surface, mounting and centring one hexa-t-butyl-pyrimidopentaphenylbenzene molecule on one atom axis. The combination of molecular design, molecular manipulation and surface atomic structure selection leads to the construction of a fundamental component of a planar single-molecule mechanical machine. The rotation of our molecule-gear is step-by-step and totally under control, demonstrating nine stable stations in both directions.
    Nature Material 08/2009; 8(7):576-9. · 35.75 Impact Factor
  • W-H Soe, C Manzano, A De Sarkar, N Chandrasekhar, C Joachim
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    ABSTRACT: Differential conductance (dI/dV) images taken with a low-temperature scanning tunneling microscope enabled the first observation of the electron probability distribution of the molecular orbitals of a pentacene molecule directly adsorbed on a metal surface. The three highest occupied molecular orbitals (HOMO, HOMO-1, and HOMO-2) and the lowest unoccupied molecular orbital are imaged. Thus dI/dV imaging without any intervening insulating layer permits the visualization of a large variety of molecular orbitals in the electronic cloud of a wide-gap molecule physisorbed on a metal surface.
    Physical Review Letters 06/2009; 102(17):176102. · 7.94 Impact Factor
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    Justin C W Song, Natarajan Chandrasekhar, Cedric Troadec, Kuan Eng J Goh
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    ABSTRACT: We study the electrostatic effects of thin organic films in modifying the interface physics of metal/ semiconductor Schottky contacts. We work out analytically the electrostatic parameter space pointing out where interface state effects exceed space-charge effects and vice versa. This is done by introducing another treatment of the electrostatic problem. We also find that the image force effect on the barrier height due to the insertion of a material with lower dielectric constant than the semiconductor in between the Schottky contact is small but positive. This is in contrast to what might be expected from effective-medium theory. We conclude with an examination of ballistic electron emission microscopy results of pentacene modified Au/ n-Si111 Schottky diodes as a case study. Using the tools fore mentioned, we infer the local charge neutrality level and density of interface gap states to an area of 500 500 nm 2 from barrier height statistics and pentacene monolayer heights.
    Physical Review B 04/2009; 79:165313. · 3.77 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: Chemically tagged hexaphenylbenzene molecules physisorbed on Au(1 1 1) surface display their tag differently under STM as compared to their physisorption on a Cu(1 1 1) surface. Our STM findings complemented by First Principles and Quantum Chemistry calculations have attributed this difference to two different conformations adopted by these molecules on Au(1 1 1) in comparison to its one conformation on Cu(1 1 1). The demonstration of the sensitivity of the tag to its electronic or chemical environment would have important implications in designing single molecule machinery where the motion of the molecule is to be discerned by tracking its tag induced intramolecular STM contrast.
    Surface Science 01/2009; · 1.84 Impact Factor
  • Xinliang Feng, Natarajan Chandrasekhar, Haibin Su, Klaus Müllen
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    ABSTRACT: We use scanning tunneling microscopy and ballistic electron emission spectroscopy and microscopy to study charge transport across Pt-nanographene-Pd interfaces. Four triangle-shaped nanographene molecules with different bulky substituents are studied. Modifications of highest occupied molecular orbital and lowest unoccupied molecular orbital levels resulting from hybridization with the metal substrate are observed for all molecules and compared with theoretical calculations. The substituents can influence the charge transport through the molecules by varying the distance between the metal substrate and the nanographene plane or providing additional electronic channels through iodo substituents. This effect can be quantified as a larger effective mass for carriers with increasing molecule-substrate distance, using tight binding. Our results address the critical coupling issue for metal contacts to devices using molecules as active layers.
    Nano Letters 01/2009; 8(12):4259-64. · 13.03 Impact Factor
  • Moh’d Rezeq, Christian Joachim, N. Chandrasekhar
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    ABSTRACT: The spatially controlled field assisted etching method for sharpening metallic tips, in a field ion microscope (FIM), is used to study the evolution of the field emission when the tip apex radius is decreased below 1nm. Unlike the conventional image formation in a field emission microscope (FEM), we demonstrate that at this scale the field emission is rather confined to atomic sites. A single atom apex fabricated at the end of such tips exhibits an outstanding brightness compared to other atomic tips. The measurements have been repeated for two double atom tips, with different atom-atom separations, and images of atomic field emission localization have also been obtained. We have found that the field emission intensity alternates between adjacent atoms when the applied voltage is gradually increased beyond a threshold value.
    Surface Science 01/2009; 603(4):697-702. · 1.84 Impact Factor

Publication Stats

65 Citations
169.14 Total Impact Points

Institutions

  • 2011
    • Max Planck Institute for Polymer Research
      Mayence, Rheinland-Pfalz, Germany
  • 2009–2010
    • National University of Singapore
      • Department of Physics
      Singapore, Singapore
    • University of Cambridge
      • Nanoscience Centre
      Cambridge, ENG, United Kingdom
  • 2005–2009
    • Agency for Science, Technology and Research
      • Institute of Materials Research and Engineering (IMRE)
      Singapore, Singapore