-
Contact Engineering High Performance n-Type MoTe2 Transistors
Authors:
Michal J. Mleczko,
Andrew C. Yu,
Christopher M. Smyth,
Victoria Chen,
Yong Cheol Shin,
Sukti Chatterjee,
Yi-Chia Tsai,
Yoshio Nishi,
Robert M. Wallace,
Eric Pop
Abstract:
Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains under-explored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400…
▽ More
Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains under-explored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400 $μA/μm$ at 80 K and >200 $μA/μm$ at 300 K) and relatively low contact resistance (1.2 to 2 $kΩ\cdotμm$ from 80 to 300 K), achieved with Ag contacts and AlOx encapsulation. We also investigate other contact metals, extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer h-BN between MoTe2 and Sc. These metal-insulator-semiconductor (MIS) contacts partly de-pin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.
△ Less
Submitted 4 July, 2019;
originally announced July 2019.
-
Application of single-electron effects to fingerprints of chips using image recognition algorithms
Authors:
T. Tanamoto,
Y. Nishi,
K. Ono
Abstract:
Single-electron effects have been widely investigated as a typical physical phenomenon in nanoelectronics. The single-electron effect caused by trap sites has been observed in many devices. In general, traps are randomly distributed and not controllable; therefore, different current--voltage characteristics are observed through traps even in silicon transistors having the same device parameters (e…
▽ More
Single-electron effects have been widely investigated as a typical physical phenomenon in nanoelectronics. The single-electron effect caused by trap sites has been observed in many devices. In general, traps are randomly distributed and not controllable; therefore, different current--voltage characteristics are observed through traps even in silicon transistors having the same device parameters (e.g., gate length). This allows us to use single-electron effects as fingerprints of chips. In this study, we analyze the single-electron effect of traps in conventional silicon transistors and show the possibility of their use as fingerprints of chips through image recognition algorithms. Resonant tunneling parts in the Coulomb diagram can also be used to characterize each device. These results show that single-electron effects can provide a quantum version of a physically unclonable function (quantum-PUF).
△ Less
Submitted 24 July, 2019; v1 submitted 15 April, 2019;
originally announced April 2019.
-
Oxygen migration during resistance switching and failure of hafnium oxide memristors
Authors:
Suhas Kumar,
Ziwen Wang,
Xiaopeng Huang,
Niru Kumari,
Noraica Davila,
John Paul Strachan,
David Vine,
A. L. David Kilcoyne,
Yoshio Nishi,
R. Stanley Williams
Abstract:
While the recent establishment of the role of thermophoresis/diffusion-driven oxygen migration during resistance switching in metal oxide memristors provided critical insights required for memristor modeling, extended investigations of the role of oxygen migration during ageing and failure remain to be detailed. Such detailing will enable failure-tolerant design, which can lead to enhanced perform…
▽ More
While the recent establishment of the role of thermophoresis/diffusion-driven oxygen migration during resistance switching in metal oxide memristors provided critical insights required for memristor modeling, extended investigations of the role of oxygen migration during ageing and failure remain to be detailed. Such detailing will enable failure-tolerant design, which can lead to enhanced performance of memristor-based next-generation storage-class memory. Here we directly observed lateral oxygen migration using in-situ synchrotron x-ray absorption spectromicroscopy of HfOx memristors during initial resistance switching, wear over millions of switching cycles, and eventual failure, through which we determined potential physical causes of failure. Using this information, we reengineered devices to mitigate three failure mechanisms, and demonstrated an improvement in endurance of about three orders of magnitude.
△ Less
Submitted 8 March, 2017;
originally announced March 2017.
-
Spatially uniform resistance switching of low current, high endurance titanium-niobium-oxide memristors
Authors:
Suhas Kumar,
Noraica Davila,
Ziwen Wang,
Xiaopeng Huang,
John Paul Strachan,
David Vine,
A. L. David Kilcoyne,
Yoshio Nishi,
R. Stanley Williams
Abstract:
We analyzed micrometer-scale titanium-niobium-oxide prototype memristors, which exhibited low write-power (<3 μW) and energy (<200 fJ/bit/μm2), low read-power (~nW), and high endurance (>millions of cycles). To understand their physico-chemical operating mechanisms, we performed in-operando synchrotron x-ray transmission nanoscale spectromicroscopy using an ultra-sensitive time-multiplexed techniq…
▽ More
We analyzed micrometer-scale titanium-niobium-oxide prototype memristors, which exhibited low write-power (<3 μW) and energy (<200 fJ/bit/μm2), low read-power (~nW), and high endurance (>millions of cycles). To understand their physico-chemical operating mechanisms, we performed in-operando synchrotron x-ray transmission nanoscale spectromicroscopy using an ultra-sensitive time-multiplexed technique. We observed only spatially uniform material changes during cell operation, in sharp contrast to the frequently detected formation of a localized conduction channel in transition-metal-oxide memristors. We also associated the response of assigned spectral features distinctly to non-volatile storage (resistance change) and writing of information (application of voltage and Joule heating). These results provide critical insights into high-performance memristors that will aid in device design, scaling and predictive circuit-modeling, all of which are essential for the widespread deployment of successful memristor applications.
△ Less
Submitted 6 January, 2017;
originally announced January 2017.
-
Conduction Channel Formation and Dissolution Due to Oxygen Thermophoresis/Diffusion in Hafnium Oxide Memristors
Authors:
Suhas Kumar,
Ziwen Wang,
Xiaopeng Huang,
Niru Kumari,
Noraica Davila,
John Paul Strachan,
David Vine,
A. L. David Kilcoyne,
Yoshio Nishi,
R. Stanley Williams
Abstract:
Transition metal oxide memristors, or resistive random-access memory (RRAM) switches, are under intense development for storage-class memory because of their favorable operating power, endurance, speed, and density. Their commercial deployment critically depends on predictive compact models based on understanding nanoscale physico-chemical forces, which remains elusive and controversial owing to t…
▽ More
Transition metal oxide memristors, or resistive random-access memory (RRAM) switches, are under intense development for storage-class memory because of their favorable operating power, endurance, speed, and density. Their commercial deployment critically depends on predictive compact models based on understanding nanoscale physico-chemical forces, which remains elusive and controversial owing to the difficulties in directly observing atomic motions during resistive switching, Here, using scanning transmission synchrotron x-ray spectromicroscopy to study in-situ switching of hafnium oxide memristors, we directly observed the formation of a localized oxygen-deficiency-derived conductive channel surrounded by a low-conductivity ring of excess oxygen. Subsequent thermal annealing homogenized the segregated oxygen, resetting the cells towards their as-grown resistance state. We show that the formation and dissolution of the conduction channel are successfully modeled by radial thermophoresis and Fick diffusion of oxygen atoms driven by Joule heating. This confirmation and quantification of two opposing nanoscale radial forces that affect bipolar memristor switching are important components for any future physics-based compact model for the electronic switching of these devices.
△ Less
Submitted 3 January, 2017;
originally announced January 2017.
-
High Current Density and Low Thermal Conductivity of Atomically Thin Semimetallic WTe2
Authors:
Michal J. Mleczko,
Runjie,
Xu,
Kye Okabe,
Hsueh-Hui Kuo,
Ian R. Fisher,
H. -S. Philip Wong,
Yoshio Nishi,
Eric Pop
Abstract:
Two-dimensional (2D) semimetals beyond graphene have been relatively unexplored in the atomically-thin limit. Here we introduce a facile growth mechanism for semimetallic WTe2 crystals, then fabricate few-layer test structures while carefully avoiding degradation from exposure to air. Low-field electrical measurements of 80 nm to 2 um long devices allow us to separate intrinsic and contact resista…
▽ More
Two-dimensional (2D) semimetals beyond graphene have been relatively unexplored in the atomically-thin limit. Here we introduce a facile growth mechanism for semimetallic WTe2 crystals, then fabricate few-layer test structures while carefully avoiding degradation from exposure to air. Low-field electrical measurements of 80 nm to 2 um long devices allow us to separate intrinsic and contact resistance, revealing metallic response in the thinnest encapsulated and stable WTe2 devices studied to date (3 to 20 layers thick). High-field electrical measurements and electro-thermal modeling demonstrate that ultra-thin WTe2 can carry remarkably high current density (approaching 50 MA/cm2, higher than most common interconnect metals) despite a very low thermal conductivity (of the order ~3 W/m/K). These results suggest several pathways for air-stable technological viability of this layered semimetal.
△ Less
Submitted 2 August, 2016;
originally announced August 2016.
-
Direct Observation of Localized Radial Oxygen Migration in Functioning Tantalum Oxide Memristors
Authors:
Suhas Kumar,
Catherine E. Graves,
John Paul Strachan,
Emmanuelle Merced Grafals,
Arthur L. David Kilcoyne,
Tolek Tyliszczak,
Johanna Nelson Weker,
Yoshio Nishi,
R. Stanley Williams
Abstract:
Oxygen migration in tantalum oxide, a promising next-generation storage material, is studied using in-operando x-ray absorption spectromicroscopy and is used to microphysically describe accelerated evolution of conduction channel and device failure. The resulting ring-like patterns of oxygen concentration are modeled using thermophoretic forces and Fick diffusion, establishing the critical role of…
▽ More
Oxygen migration in tantalum oxide, a promising next-generation storage material, is studied using in-operando x-ray absorption spectromicroscopy and is used to microphysically describe accelerated evolution of conduction channel and device failure. The resulting ring-like patterns of oxygen concentration are modeled using thermophoretic forces and Fick diffusion, establishing the critical role of temperature-activated oxygen migration that has been under question lately.
△ Less
Submitted 4 February, 2016; v1 submitted 3 February, 2016;
originally announced February 2016.
-
Characterization of electronic structure of periodically strained graphene
Authors:
Marjan Aslani,
C. Michael Garner,
Suhas Kumar,
Dennis Nordlund,
Piero Pianetta,
Yoshio Nishi
Abstract:
We induced periodic biaxial tensile strain in polycrystalline graphene by wrapping it over a substrate with repeating pillar-like structures with a periodicity of 600 nm. Using Raman spectroscopy, we determined to have introduced biaxial strains in graphene in the range of 0.4% to 0.7%. Its band structure was characterized using photoemission from valance bands, shifts in the secondary electron em…
▽ More
We induced periodic biaxial tensile strain in polycrystalline graphene by wrapping it over a substrate with repeating pillar-like structures with a periodicity of 600 nm. Using Raman spectroscopy, we determined to have introduced biaxial strains in graphene in the range of 0.4% to 0.7%. Its band structure was characterized using photoemission from valance bands, shifts in the secondary electron emission, and x-ray absorption from the carbon 1s levels to the unoccupied graphene conduction bands. It was observed that relative to unstrained graphene, strained graphene had a higher work function and higher density of states in the valence and conduction bands. We measured the conductivity of the strained and unstrained graphene in response to a gate voltage and correlated the changes in their behavior to the changes in the electronic structure. From these sets of data, we propose a simple band diagram representing graphene with periodic biaxial strain.
△ Less
Submitted 5 November, 2015;
originally announced November 2015.
-
Sequential Electronic and Structural Transitions in VO2 Observed Using X-ray Absorption Spectromicroscopy
Authors:
Suhas Kumar,
John Paul Strachan,
Matthew D. Pickett,
Alexander Bratkovsky,
Yoshio Nishi,
R. Stanley Williams
Abstract:
The popular dual electronic (Mott) and structural (Peierls) transitions in VO2 are explored using x-ray absorption spectromicroscopy with high spatial and spectral resolutions. It is found that during both heating and cooling, the electronic transition always precedes the structural Peierls transition. Between the two transitions, there are intermediate states that are spectrally isolated here.
The popular dual electronic (Mott) and structural (Peierls) transitions in VO2 are explored using x-ray absorption spectromicroscopy with high spatial and spectral resolutions. It is found that during both heating and cooling, the electronic transition always precedes the structural Peierls transition. Between the two transitions, there are intermediate states that are spectrally isolated here.
△ Less
Submitted 19 November, 2015; v1 submitted 5 November, 2015;
originally announced November 2015.
-
Local Temperature Redistribution and Structural Transition During Joule-Heating-Driven Conductance Switching in VO2
Authors:
Suhas Kumar,
Matthew D. Pickett,
John Paul Strachan,
Gary Gibson,
Yoshio Nishi,
R. Stanley Williams
Abstract:
Joule-heating induced conductance-switching is studied in VO2, a Mott insulator. Complementary in-situ techniques including optical characterization, blackbody microscopy, scanning transmission x-ray microscopy (STXM) and numerical simulations are used. Abrupt redistribution in local temperature is shown to occur upon conductance-switching along with a structural phase transition, at the same curr…
▽ More
Joule-heating induced conductance-switching is studied in VO2, a Mott insulator. Complementary in-situ techniques including optical characterization, blackbody microscopy, scanning transmission x-ray microscopy (STXM) and numerical simulations are used. Abrupt redistribution in local temperature is shown to occur upon conductance-switching along with a structural phase transition, at the same current.
△ Less
Submitted 22 October, 2015;
originally announced October 2015.
-
In-operando synchronous time-multiplexed O K-edge x-ray absorption spectromicroscopy of functioning tantalum oxide memristors
Authors:
Suhas Kumar,
Catherine E. Graves,
John Paul Strachan,
A. L. David Kilcoyne,
Tolek Tyliszczak,
Yoshio Nishi,
R. Stanley Williams
Abstract:
Memristors are receiving keen interest because of their potential varied applications and promising large-scale information storage capabilities. Tantalum oxide is a memristive material that has shown promise for high-performance nonvolatile computer memory. The microphysics has been elusive because of the small scale and subtle physical changes that accompany conductance switching. In this study,…
▽ More
Memristors are receiving keen interest because of their potential varied applications and promising large-scale information storage capabilities. Tantalum oxide is a memristive material that has shown promise for high-performance nonvolatile computer memory. The microphysics has been elusive because of the small scale and subtle physical changes that accompany conductance switching. In this study, we probed the atomic composition, local chemistry and electronic structure of functioning tantalum oxide memristors through spatially mapped O K-edge x-ray absorption. We developed a time-multiplexed spectromicroscopy technique to enhance the weak and possibly localized oxide modifications with spatial and spectral resolutions of <30 nm and 70 meV, respectively. During the initial stages of conductance switching of a micrometer sized crosspoint device, the spectral changes were uniform within the spatial resolution of our technique. When the device was further driven with millions of high voltage-pulse cycles, we observed lateral motion and separation of ~100 nm-scale agglomerates of both oxygen interstitials and vacancies. We also demonstrate a unique capability of this technique by identifying the relaxation behavior in the material during electrical stimuli by identifying electric field driven changes with varying pulse widths. In addition, we show that changes to the material can be localized to a spatial region by modifying its topography or uniformity, as against spatially uniform changes observed here during memristive switching. The goal of this report is to introduce the capability of time-multiplexed x-ray spectromicroscopy in studying weak-signal transitions in inhomogeneous media through the example of the operation and temporal evolution of a memristor.
△ Less
Submitted 16 October, 2015;
originally announced October 2015.
-
Prediction of semi-metallic tetragonal Hf2O3 and Zr2O3 from first-principles
Authors:
Kan-Hao Xue,
Philippe Blaise,
Leonardo R. C. Fonseca,
Yoshio Nishi
Abstract:
A tetragonal phase is predicted for Hf2O3 and Zr2O3 using density functional theory. Starting from atomic and unit cell relaxations of substoichiometric monoclinic HfO2 and ZrO2, such tetragonal structures are only reached at zero temperature by introducing the oxygen vacancy pair with the lowest formation energy. The tetragonal Hf2O3 and Zr2O3 structures belong to space group P-4m2 and are more s…
▽ More
A tetragonal phase is predicted for Hf2O3 and Zr2O3 using density functional theory. Starting from atomic and unit cell relaxations of substoichiometric monoclinic HfO2 and ZrO2, such tetragonal structures are only reached at zero temperature by introducing the oxygen vacancy pair with the lowest formation energy. The tetragonal Hf2O3 and Zr2O3 structures belong to space group P-4m2 and are more stable than their corundum structure counterparts. These phases are semi-metallic, as confirmed through further G0W0 calculations. The carrier concentrations are estimated to be 1.77E21 cm^{-3} for both electrons and holes in tetragonal Hf2O3, and 1.75E21 cm^{-3} for both electrons and holes in tetragonal Zr2O3. The tetragonal Hf2O3 phase is probably related to the low resistivity state of hafnia-based resistive random access memory (RRAM).
△ Less
Submitted 11 October, 2012;
originally announced October 2012.
-
Electroluminescence from Strained Ge membranes and Implications for an Efficient Si-Compatible Laser
Authors:
Donguk Nam,
David Sukhdeo,
Szu-Lin Cheng,
Arunanshu Roy,
Kevin Chih-Yao Huang,
Mark Brongersma,
Yoshio Nishi,
Krishna Saraswat
Abstract:
We demonstrate room-temperature electroluminescence (EL) from light-emitting diodes (LED) on highly strained germanium (Ge) membranes. An external stressor technique was employed to introduce a 0.76% bi-axial tensile strain in the active region of a vertical PN junction. Electrical measurements show an on-off ratio increase of one order of magnitude in membrane LEDs compared to bulk. The EL spectr…
▽ More
We demonstrate room-temperature electroluminescence (EL) from light-emitting diodes (LED) on highly strained germanium (Ge) membranes. An external stressor technique was employed to introduce a 0.76% bi-axial tensile strain in the active region of a vertical PN junction. Electrical measurements show an on-off ratio increase of one order of magnitude in membrane LEDs compared to bulk. The EL spectrum from the 0.76% strained Ge LED shows a 100nm redshift of the center wavelength because of the strain-induced direct band gap reduction. Finally, using tight-binding and FDTD simulations, we discuss the implications for highly efficient Ge lasers.
△ Less
Submitted 16 February, 2012;
originally announced February 2012.
-
Accurate Model of a Vertical Pillar Quantum Dot
Authors:
P. A. Maksym,
Y. Nishi,
D. G. Austing,
T. Hatano,
L. P. Kouwenhoven,
H. Aoki,
S. Tarucha
Abstract:
An accurate model of a vertical pillar quantum dot is described. The full three dimensional structure of the device containing the dot is taken into account and this leads to an effective two dimensional model in which electrons move in the two lateral dimensions, the confinement is parabolic and the interaction potential is very different from the bare Coulomb potential. The potentials are foun…
▽ More
An accurate model of a vertical pillar quantum dot is described. The full three dimensional structure of the device containing the dot is taken into account and this leads to an effective two dimensional model in which electrons move in the two lateral dimensions, the confinement is parabolic and the interaction potential is very different from the bare Coulomb potential. The potentials are found from the device structure and a few adjustable parameters. Numerically stable calculation procedures for the interaction potential are detailed and procedures for deriving parameter values from experimental addition energy and chemical potential data are described. The model is able to explain magnetic field dependent addition energy and chemical potential data for an individual dot to an accuracy of about 5%, the accuracy level needed to determine ground state quantum numbers from experimental transport data. Applications to excited state transport data are also described.
△ Less
Submitted 20 April, 2008;
originally announced April 2008.
-
Fano-Kondo effect in a two-level system with triple quantum dots: shot noise characteristics
Authors:
Tetsufumi Tanamoto,
Yoshifumi Nishi,
Shinobu Fujita
Abstract:
We theoretically compare transport properties of Fano-Kondo effect with those of Fano effect. We focus on shot noise characteristics of a triple quantum dot (QD) system in the Fano-Kondo region at zero temperature, and discuss the effect of strong electric correlation in QDs. We found that the modulation of the Fano dip is strongly affected by the on-site Coulomb interaction in QDs.
We theoretically compare transport properties of Fano-Kondo effect with those of Fano effect. We focus on shot noise characteristics of a triple quantum dot (QD) system in the Fano-Kondo region at zero temperature, and discuss the effect of strong electric correlation in QDs. We found that the modulation of the Fano dip is strongly affected by the on-site Coulomb interaction in QDs.
△ Less
Submitted 3 October, 2007;
originally announced October 2007.
-
Quantum Capacitance Spectroscopy of Single Nanotube Molecules
Authors:
Yuerui Lu,
Ryan Tu,
Xinran Wang,
Yoshio Nishi,
Hongjie Dai
Abstract:
some errors in equations
some errors in equations
△ Less
Submitted 19 July, 2007; v1 submitted 2 July, 2007;
originally announced July 2007.
-
Measuring the Capacitance of Individual Semiconductor Nanowires for Carrier Mobility Assessment
Authors:
Ryan Tu,
Li Zhang,
Yoshio Nishi,
Hongjie Dai
Abstract:
Capacitance-voltage characteristics of individual germanium nanowire field effect transistors were directly measured and used to assess carrier mobility in nanowires for the first time; thereby removing uncertainties in calculated mobility due to device geometries, surface and interface states and gate dielectric constants and thicknesses. Direct experimental evidence showed that surround-gated…
▽ More
Capacitance-voltage characteristics of individual germanium nanowire field effect transistors were directly measured and used to assess carrier mobility in nanowires for the first time; thereby removing uncertainties in calculated mobility due to device geometries, surface and interface states and gate dielectric constants and thicknesses. Direct experimental evidence showed that surround-gated nanowire transistors exhibit higher capacitance and better electrostatic gate control than top-gated devices, and are the most promising structure for future high performance nanoelectronics.
△ Less
Submitted 17 May, 2007;
originally announced May 2007.
-
Fano-Kondo effect through two-level system based on quantum dots
Authors:
Tetsufumi Tanamoto,
Yoshifumi Nishi
Abstract:
We theoretically study the Fano-Kondo effect in a triple quantum dot (QD) system where two QDs constitute a two-level system and the other QD works in a detector with electrodes. We found that the Fano dip is clearly modulated by strongly coupled QDs in a two-level system and a slow detector with no interacting QD. This setup suggests a new method of reading out qubit states.
We theoretically study the Fano-Kondo effect in a triple quantum dot (QD) system where two QDs constitute a two-level system and the other QD works in a detector with electrodes. We found that the Fano dip is clearly modulated by strongly coupled QDs in a two-level system and a slow detector with no interacting QD. This setup suggests a new method of reading out qubit states.
△ Less
Submitted 16 March, 2008; v1 submitted 29 April, 2007;
originally announced April 2007.
-
DNA Functionalization of Carbon Nanotubes for Ultra-Thin Atomic Layer Deposition of High k Dielectrics for Nanotube Transistors with 60mV/decade Switching
Authors:
Yuerui Lu,
Sarunya Bangsaruntip,
Xinran Wang,
Li Zhang,
Yoshio Nishi,
Hongjie Dai
Abstract:
For single-walled carbon nanotube (SWNT) field effect transistors, vertical scaling of high k dielectrics by atomic layer deposition (ALD) currently stands at ~8nm with subthreshold swing S~70-90 mV/decade at room temperature. ALD on as-grown pristine SWNTs is incapable of producing a uniform and conformal dielectric layer due to the lack of functional groups on nanotubes and that nucleation of…
▽ More
For single-walled carbon nanotube (SWNT) field effect transistors, vertical scaling of high k dielectrics by atomic layer deposition (ALD) currently stands at ~8nm with subthreshold swing S~70-90 mV/decade at room temperature. ALD on as-grown pristine SWNTs is incapable of producing a uniform and conformal dielectric layer due to the lack of functional groups on nanotubes and that nucleation of an oxide dielectric layer in the ALD process hinges upon covalent chemisorption on reactive groups on surfaces. Here, we show that by non-covalent functionalization of SWNTs with ploy-T DNA molecules (dT40-DNA), one can impart functional groups of sufficient density and stability for uniform and conformal ALD of high k dielectrics on SWNTs with thickness down to 2-3nm. This enables approaching the ultimate vertical scaling limit of nanotube FETs and reliably achieving S ~ 60mV/decade at room temperature, and S~50mV/decade in band to band tunneling regime of ambipolar transport. We have also carried out microscopy investigations to understand ALD processes on SWNTs with and without DNA functionalization.
△ Less
Submitted 20 February, 2006;
originally announced February 2006.
-
Intermediate Low Spin States in a Few-electron Quantum Dot in the $ν\le 1$ Regime
Authors:
Y. Nishi,
P. A. Maksym,
D. G. Austing,
T. Hatano,
L. P. Kouwenhoven,
H. Aoki,
S. Tarucha
Abstract:
We study the effects of electron-electron interactions in a circular few-electron vertical quantum dot in such a strong magnetic field that the filling factor $ν\le 1$. We measure excitation spectra and find ground state transitions beyond the maximum density droplet ($ν=1$) region. We compare the observed spectra with those calculated by exact diagonalization to identify the ground state quantu…
▽ More
We study the effects of electron-electron interactions in a circular few-electron vertical quantum dot in such a strong magnetic field that the filling factor $ν\le 1$. We measure excitation spectra and find ground state transitions beyond the maximum density droplet ($ν=1$) region. We compare the observed spectra with those calculated by exact diagonalization to identify the ground state quantum numbers, and find that intermediate low-spin states occur between adjacent spin-polarized magic number states.
△ Less
Submitted 12 December, 2005;
originally announced December 2005.
-
Ultra-High Yield Growth of Vertical Single-Walled Carbon Nanotubes: Hidden Roles of Hydrogen and Oxygen
Authors:
Guangyu Zhang,
David Mann,
Li Zhang,
Ali Javey,
Yiming Li,
Erhan Yenilmez,
Qian Wang,
James McVittie,
Yoshio Nishi,
James Gibbons,
Hongjie Dai
Abstract:
An oxygen assisted hydrocarbon chemical vapor deposition (CVD) method is developed to afford large-scale highly reproducible ultra high-yield growth of vertical single-walled carbon nanotubes (SWNT). It is revealed that reactive hydrogen (H)-species, inevitable in hydrocarbon-based growth, are damaging to the formation of sp2-like SWNTs. The addition of oxygen scavenges H-species and provides a…
▽ More
An oxygen assisted hydrocarbon chemical vapor deposition (CVD) method is developed to afford large-scale highly reproducible ultra high-yield growth of vertical single-walled carbon nanotubes (SWNT). It is revealed that reactive hydrogen (H)-species, inevitable in hydrocarbon-based growth, are damaging to the formation of sp2-like SWNTs. The addition of oxygen scavenges H-species and provides a powerful control over the C/H ratio to favor SWNT growth. The revelation of the roles played by hydrogen and oxygen leads to a unified and universal optimum growth condition for SWNTs. Further, a versatile method is developed to form vertical SWNT films on any substrate, lifting a major substrate-type limitation for aligned SWNTs.
△ Less
Submitted 1 November, 2005;
originally announced November 2005.