K.L. Shepard

Columbia University, New York City, New York, United States

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Publications (160)524.73 Total impact

  • 1 edited by Eric Lagally, 05/2014; CRC Press., ISBN: 9781466594906
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    ABSTRACT: Despite advances in monitoring spatiotemporal expression patterns of genes and proteins with fluorescent probes, direct detection of metabolites and small molecules remains challenging. A technique for spatially resolved detection of small molecules would benefit the study of redox-active metabolites that are produced by microbial biofilms and can affect their development. Here we present an integrated circuit-based electrochemical sensing platform featuring an array of working electrodes and parallel potentiostat channels. 'Images' over a 3.25 × 0.9 mm(2) area can be captured with a diffusion-limited spatial resolution of 750 μm. We demonstrate that square wave voltammetry can be used to detect, identify and quantify (for concentrations as low as 2.6 μM) four distinct redox-active metabolites called phenazines. We characterize phenazine production in both wild-type and mutant Pseudomonas aeruginosa PA14 colony biofilms, and find correlations with fluorescent reporter imaging of phenazine biosynthetic gene expression.
    Nature Communications 02/2014; 5:3256. · 10.74 Impact Factor
  • R.M. Field, S. Realov, K.L. Shepard
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    ABSTRACT: A fully-integrated single-photon avalanche diode (SPAD) and time-to-digital converter (TDC) array for high-speed fluorescence lifetime imaging microscopy (FLIM) in standard 130 nm CMOS is presented. This imager is comprised of an array of 64-by-64 SPADs each with an independent TDC for performing time-correlated single-photon counting (TCSPC) at each pixel. The TDCs use a delay-locked-loop-based architecture and achieve a 62.5 ps resolution with up to a 64 ns range. A data-compression datapath is designed to transfer TDC data to off-chip buffers, which can support a data rate of up to 42 Gbps. These features, combined with a system implementation that leverages a x4 PCIe-cabled interface, allow for demonstrated FLIM imaging rates at up to 100 frames per second.
    IEEE Journal of Solid-State Circuits 01/2014; 49(4):867-880. · 3.06 Impact Factor
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    ABSTRACT: Oscillators, which produce continuous periodic signals from direct current power, are central to modern communications systems, with versatile applications including timing references and frequency modulators. However, conventional oscillators typically consist of macroscopic mechanical resonators such as quartz crystals, which require excessive off-chip space. Here, we report oscillators built on micrometre-size, atomically thin graphene nanomechanical resonators, whose frequencies can be electrostatically tuned by as much as 14%. Self-sustaining mechanical motion is generated and transduced at room temperature in these oscillators using simple electrical circuitry. The prototype graphene voltage-controlled oscillators exhibit frequency stability and a modulation bandwidth sufficient for the modulation of radiofrequency carrier signals. As a demonstration, we use a graphene oscillator as the active element for frequency-modulated signal generation and achieve efficient audio signal transmission.
    Nature Nanotechnology 11/2013; · 31.17 Impact Factor
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    ABSTRACT: Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal boron nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality electrical contact. Here, we report a contact geometry in which we metalize only the 1D edge of a 2D graphene layer. In addition to outperforming conventional surface contacts, the edge-contact geometry allows a complete separation of the layer assembly and contact metallization processes. In graphene heterostructures, this enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-temperature mobility comparable to the theoretical phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials.
    Science 11/2013; 342(6158):614-7. · 31.20 Impact Factor
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    ABSTRACT: Graphene-based photodetectors have attracted strong interest for their exceptional physical properties, which include an ultrafast response1, 2, 3 across a broad spectrum4, a strong electron–electron interaction5 and photocarrier multiplication6, 7, 8. However, the weak optical absorption of graphene2, 3 limits its photoresponsivity. To address this, graphene has been integrated into nanocavities9, microcavities10 and plasmon resonators11, 12, but these approaches restrict photodetection to narrow bands. Hybrid graphene–quantum dot architectures can greatly improve responsivity13, but at the cost of response speed. Here, we demonstrate a waveguide-integrated graphene photodetector that simultaneously exhibits high responsivity, high speed and broad spectral bandwidth. Using a metal-doped graphene junction coupled evanescently to the waveguide, the detector achieves a photoresponsivity exceeding 0.1 A W−1 together with a nearly uniform response between 1,450 and 1,590 nm. Under zero-bias operation, we demonstrate response rates exceeding 20 GHz and an instrumentation-limited 12 Gbit s−1 optical data link.
    Nature Photonics 09/2013; 7(11):883-887. · 27.25 Impact Factor
  • Jacob K Rosenstein, Kenneth L Shepard
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    ABSTRACT: Here we discuss the limits to temporal resolution in nanopore sensor recordings, which arise from considerations of both small-signal frequency response and accumulated noise power. Nanopore sensors have strong similarities to patch-clamp ion channel recordings, except that the magnitudes of many physical parameters are substantially different. We will present examples from our recent work developing high-speed nanopore sensing platforms, in which we physically integrated nanopores with custom low-noise complementary metal-oxide-semiconductor (CMOS) circuitry. Close physical proximity of the sensor and amplifier electronics can reduce parasitic capacitances, improving both the signal-to-noise ratio and the effective temporal resolution of the recordings.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 07/2013; 2013:4110-4113.
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    ABSTRACT: Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In two dimensions, electrons moving through a magnetic field also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. When subject to both a magnetic field and a periodic electrostatic potential, two-dimensional systems of electrons exhibit a self-similar recursive energy spectrum. Known as Hofstadter's butterfly, this complex spectrum results from an interplay between the characteristic lengths associated with the two quantizing fields, and is one of the first quantum fractals discovered in physics. In the decades since its prediction, experimental attempts to study this effect have been limited by difficulties in reconciling the two length scales. Typical atomic lattices (with periodicities of less than one nanometre) require unfeasibly large magnetic fields to reach the commensurability condition, and in artificially engineered structures (with periodicities greater than about 100 nanometres) the corresponding fields are too small to overcome disorder completely. Here we demonstrate that moiré superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodic modulation with ideal length scales of the order of ten nanometres, enabling unprecedented experimental access to the fractal spectrum. We confirm that quantum Hall features associated with the fractal gaps are described by two integer topological quantum numbers, and report evidence of their recursive structure. Observation of a Hofstadter spectrum in bilayer graphene means that it is possible to investigate emergent behaviour within a fractal energy landscape in a system with tunable internal degrees of freedom.
    Nature 05/2013; 497:598. · 38.60 Impact Factor
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    ABSTRACT: Graphene mechanical resonators are the ultimate two-dimensional nanoelectromechanical systems (NEMS) with applications in sensing and signal processing. While initial devices have shown promising results, an ideal graphene NEMS resonator should be strain engineered, clamped at the edge without trapping gas underneath, and electrically integratable. In this Letter, we demonstrate fabrication and direct electrical measurement of circular SU-8 polymer-clamped chemical vapor deposition graphene drum resonators. The clamping increases device yield and responsivity, while providing a cleaner resonance spectrum from eliminated edge modes. Furthermore, the clamping induces a large strain in the resonator, increasing its resonant frequency.
    Applied Physics Letters 04/2013; 102:153101. · 3.52 Impact Factor
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    ABSTRACT: We show the optimization of magnetic properties of ferromagnetic (FM)/SiO2/FM trilayer structures as potential candidates for the magnetic core in toroidal integrated inductors, with FM materials Co91.5Zr4.0Ta4.5 (CZT) and Ni80Fe20 (Py). In the single-layer parent films, we found a monotonic reduction of easy-axis coercivity (Hc down to 0.17 Oe in CZT, 0.4 Oe in Py) with increasing dc magnetron sputtering voltage. In the trilayer rectangular structures, with induced easy-axis in the short lateral dimension, we found proof of dipolar coupling between the two FM layers from BH loop measurements in the CZT system, showing linear response with minimal hysteresis loss when the external field is applied in the long axis. Py elements did not show this optimized property. Further investigation of domain configurations using scanning transmission x-ray microscopy suggests an insufficient induced anisotropy in Py compared with the shape anisotropy to realize the antiparallel-coupled state.
    Journal of Applied Physics 04/2013; 113(17). · 2.21 Impact Factor
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    ABSTRACT: The quantum spin Hall effect is characterized by spin-polarized counter-propagating edge states. It has been predicted that this edge state configuration could occur in graphene when spin-split electron- and hole-like Landau levels are forced to cross at the edge of the sample. In particular, a quantum-spin-Hall analogue has been predicted in bilayer graphene with a Landau level filling factor ν=0 if the ground state is a spin ferromagnet. Previous studies have demonstrated that the bilayer ν=0 state is an insulator in a perpendicular magnetic field, although the exact nature of this state has not been identified. Here we present measurements of the ν=0 state in a dual-gated bilayer graphene device in a tilted magnetic field. We map out a full phase diagram of the ν=0 state as a function of experimentally tunable in-plane magnetic field and perpendicular electric field. At large in-plane magnetic field we observe a quantum phase transition to a metallic state with conductance of the order of 4e2/h, consistent with predictions for the ferromagnet.
    Nature Physics 03/2013; 9(3):154-158. · 19.35 Impact Factor
  • S. Realov, K.L. Shepard
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    ABSTRACT: An on-chip system for combined capacitance-voltage (C-V) and current-voltage (I-V) characterization of a large integrated transistor array implemented in a 45-nm bulk CMOS process is presented. On-chip I-V characterization is implemented using a four-point Kelvin measurement technique with 12-bit sub-10 nA current measurement resolution, 10-bit sub-1 mV voltage measurement resolution, and sampling speeds on the order of 100 kHz. C-V characterization is performed using a novel leakage- and parasitics-insensitive charge-based capacitance measurement (CBCM) technique with atto-Farad resolution. The on-chip system is employed in studying both random and systematic sources of quasi-static device variability. For the first time, combined C-V/I-V characterization of circuit-representative devices is demonstrated and used to extract variations in the underlying physical characteristics of the device, including line-edge-roughness (LER) parameters and systematic device length variations across the die.
    IEEE Journal of Solid-State Circuits 01/2013; 48(3):814-826. · 3.06 Impact Factor
  • S. Realov, K.L. Shepard
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    ABSTRACT: An on-chip variability characterization system implemented in a 45-nm CMOS process is used for direct time-domain measurements of random telegraph noise (RTN) in small-area devices. A procedure for automated extraction of RTN parameters from large volumes of measured data is developed. Statistics for number of traps, $N_{T}$, and single-trap amplitudes, $Delta V_{rm{th}}$, are studied across device polarity, bias, and gate area. A Poisson distribution is used to model $N_{T}$ and a log-normal distribution is used to model $Delta V_{rm{th}}$. The scaling of the two statistics across gate dimensions is discussed; the expected value of $N_{T}$ is shown to scale with $(L-{Delta}L)^{-1}$, whereas the expected value of $Delta V_{rm{th}}$ is shown to scale with $W^{-1}(L-{Delta}L)^{-0.5}$. The two statistics are combined in a compact RTN probabilistic model representing the statistics of the overall $Delta V_{rm{th}}$ fluctuations because of RTN. This model is demonstrated to give accurate predictions of the tails of the measured RTN distributions at the 95th percentile level, which scale with $W^{-1}(L-{Delta}L)^{-1.5}$. A comparison between nMOS and pMOS devices shows that pMOS devices exhibit both a higher average number of traps and a larger average single-trap $Delta V_{rm{th}}$ amplitude, leading to a comparatively larger overall impact of RTN.
    IEEE Transactions on Electron Devices 01/2013; 60(5):1716-1722. · 2.06 Impact Factor
  • Jacob Rosenstein, Kenneth L. Shepard
    Biophysical Journal 01/2013; 104(2):518-. · 3.67 Impact Factor
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    R.M. Field, K.L. Shepard
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    ABSTRACT: A wide-field fluorescence lifetime imager capable of up to 100 frames per second (fps) is presented. The imager consists of a 64-by-64 array of low-noise single photon avalanche diodes (SPADs) in a standard 0.13-μm CMOS process, 4096 time-to-digital converters, and an application specific data path to enable continuous image acquisition at a total output data rate of 42 Gbps. These features combine to enable new lifetime-based diagnostic imaging.
    VLSI Circuits (VLSIC), 2013 Symposium on; 01/2013
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    ABSTRACT: In this work we present room-temperature measurements of graphene nanoelectromechanical resonators (GN-ERs) demonstrating quality factors (Qs) greater than 200 at resonance. A nominal resonant frequency (fo) of 200 MHz is attained by applying strain to the suspended graphene using an SU-8 polymer clamp. Additionally, the device fo can be tuned by more than 5% by application of a DC gate bias on the order of 5V. Chemical vapor deposited (CVD) graphene is used to demonstrate the scalability of the process.
    European Frequency and Time Forum & International Frequency Control Symposium (EFTF/IFC), 2013 Joint; 01/2013
  • H. Norian, I. Kymissis, K.L. Shepard
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    ABSTRACT: An integrated lab-on-chip capable of performing quantitative polymerase chain reaction (qPCR) is demonstrated in a high-voltage 0.35-μm CMOS process operating at a 3.3 V supply. PCR thermal cycling can be performed by physically moving droplets between three distinct temperature zones on the surface of chip or by thermal cycling a droplet in place. Droplet actuation is enabled by electrowetting-on-dielectric transport at 90 V. On-chip temperature regulation to 0.15°C is performed with on-chip resistive heaters and temperature sensors. PCR cycles are monitored by measuring the fluorescence signal of an intercalator dye using integrated single photon avalanche diodes (SPADs). Results are demonstrated for the recognition of DNA extracts from Staphylococcus aureus (S. aureus) at a detection limit of a few copies per nL target volume.
    VLSI Circuits (VLSIC), 2013 Symposium on; 01/2013
  • Inanc Meric, Nicholas Petrone, James Hone, Kenneth L. Shepard
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    ABSTRACT: The high-frequency characteristics of graphene field-effect transistors (GFETs) has received significant interest due the very high carrier velocities in graphene. In addition to excellent electronic performance, graphene possesses exceptional mechanical properties such as high flexibility and strength. Here, we demonstrate the potential of flexible-GFETs and show that the combination of electrical and mechanical advantages of graphene result in gigahertz-frequency operation at strain values up to 2%. These devices represent the only reported technology to achieve gigahertz-frequency power gain at strain levels above 0.5%.
    Microwave Symposium Digest (IMS), 2013 IEEE MTT-S International; 01/2013
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    ABSTRACT: Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In two dimensions, electrons moving through a magnetic field also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. In 1976 Douglas Hofstadter theoretically considered the intersection of these two problems and discovered that 2D electrons subjected to both a magnetic field and a periodic electrostatic potential exhibit a self-similar recursive energy spectrum. Known as Hofstadter's butterfly, this complex spectrum results from a delicate interplay between the characteristic lengths associated with the two quantizing fields, and represents one of the first quantum fractals discovered in physics. In the decades since, experimental attempts to study this effect have been limited by difficulties in reconciling the two length scales. Typical crystalline systems (<1 nm periodicity) require impossibly large magnetic fields to reach the commensurability condition, while in artificially engineered structures (>100 nm), the corresponding fields are too small to completely overcome disorder. Here we demonstrate that moire superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a nearly ideal-sized periodic modulation, enabling unprecedented experimental access to the fractal spectrum. We confirm that quantum Hall effect features associated with the fractal gaps are described by two integer topological quantum numbers, and report evidence of their recursive structure. Observation of Hofstadter's spectrum in graphene provides the further opportunity to investigate emergent behaviour within a fractal energy landscape in a system with tunable internal degrees of freedom.
    12/2012;
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    ABSTRACT: In graphene, as in most metals, electron-electron interactions renormalize the properties of electrons but leave them behaving like noninteracting quasiparticles. Many measurements probe the renormalized properties of electrons right at the Fermi energy. Uniquely for graphene, the accessibility of the electrons at the surface offers the opportunity to use scanned probe techniques to examine the effect of interactions at energies away from the Fermi energy, over a broad range of densities, and on a local scale. Using scanning tunneling spectroscopy, we show that electron interactions leave the graphene energy dispersion linear as a function of excitation energy for energies within �200 meV of the Fermi energy. However, the measured dispersion velocity depends on density and increases strongly as the density approaches zero near the charge neutrality point, revealing a squeezing of the Dirac cone due to interactions.
    Physical Review Letters 09/2012; 109:116802. · 7.73 Impact Factor

Publication Stats

3k Citations
524.73 Total Impact Points

Institutions

  • 1998–2014
    • Columbia University
      • Department of Electrical Engineering
      New York City, New York, United States
  • 2013
    • City College of New York
      • Department of Physics
      New York City, NY, United States
    • CUNY Graduate Center
      New York City, New York, United States
  • 2010
    • Seventh Sense Biosystems
      Cambridge, Massachusetts, United States
  • 2008
    • University of Virginia
      Charlottesville, Virginia, United States
  • 2001–2004
    • Cadence Design Systems, Inc.
      San Jose, California, United States
  • 1999
    • Microcosm, Inc.
      Hawthorne, California, United States