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Hongjie Dai

Publications and source records attributed to Hongjie Dai.

At least 91 records · Page 5Linked to original sources
arXiv2005

Electron Transport in Very Clean, As-Grown Suspended Carbon Nanotubes

Single walled carbon nanotubes (SWNT) have displayed a wealth of quantum transport phenomena thus far. Defect free, unperturbed SWNTs with wellbehaved or tunable metal contacts are important to probing the intrinsic electrical properties of nanotubes. Meeting these conditions experimentally is nontrivial due to numerous disorder and randomizing factors. Here we show that ~ 1 um long fully suspended SWNTs grown-in-place between metal contacts afford SWNT devices exhibiting well-defined characteristics over much wider energy ranges than nanotubes pinned on substrates. Various low temperature transport regimes in true-metallic, small and large bandgap semiconducting nanotubes are observed including quantum states shell-filling, -splitting and -crossing in magnetic fields for medium conductance devices. The clean transport data reveals a correlation between the contact junction resistance and the various transport regimes in SWNT devices. Further, we show that electrical transport data can be used to probe the band structures of nanotubes including nonlinear band dispersion.

cond-mat.mes-hall↗
arXiv2005

Electrical Contacts to Carbon Nanotubes Down to 1nm in Diameter

Rhodium (Rh) is found similar to Palladium (Pd) in making near-ohmic electrical contacts to single-walled carbon nanotubes (SWNTs) with diameters d > ~ 1.6 nm. Non-negligible positive Schottky barriers (SBs) exist between Rh or Pd and semiconducting SWNTs (S-SWNTs) with d < ~ 1.6 nm. With Rh and Pd contacts, the characteristics of SWNT field-effect transistors (FETs) and SB heights at the contacts are largely predictable based on the SWNT diameters, without random variations among devices. Surprisingly, electrical contacts to metallic SWNTs (M-SWNTs) also appear to be diameter dependent especially for small SWNTs. Ohmic contacts are difficult for M-SWNTs with diameters < ~ 1.0 nm possibly due to tunnel barriers.

cond-mat.mes-hall↗
arXiv2005

Negative Differential Conductance and Hot Phonons in Suspended Nanotube Molecular Wires

Freely suspended metallic single-wall carbon nanotubes (SWNTs) exhibit reduced current carrying ability compared to those lying on substrates, and striking negative differential conductance (NDC) at low electric fields. Theoretical analysis reveals significant self-heating effects including electron scattering by hot non-equilibrium optical phonons. Electron transport characteristics under strong self-heating are exploited for the first time to probe the thermal conductivity of individual SWNTs (~ 3600 Wm-1K-1 at T=300 K) up to ~700 K, and reveal a 1/T dependence expected for Umklapp phonon scattering at high temperatures.

cond-mat.mes-hall↗
arXiv2005

Carbon nanotubes as multifunctional biological transporters and near-infrared agents for selective cancer cell destruction

Biological systems are known to be highly transparent to 700-1100nm near-infrared (NIR) light. It is shown here that the strong optical absorbance of single-walled carbon nanotubes (SWNTs) in this special spectral window, an intrinsic property of SWNTs, can be utilized for optical stimulation of nanotubes inside living cells to afford multifunctional nanotube biological transporters. For oligonucleotides transported inside living cells by nanotubes, the oligos can translocate into cell nucleus upon endosomal rupture triggered by NIR laser pulses. Continuous NIR radiation can cause cell death due to excessive local heating of SWNT in vitro. Selective cancer cell destruction can be achieved via functionalization of SWNT with a folate moiety, selective internalization of SWNTs inside cells labeled with folate receptor tumor-markers and NIR triggered cell death, without harming receptor-free normal cells. Thus, the transporting capabilities of carbon nanotubes combined with suitable functionalization chemistry and their intrinsic optical properties can lead to new classes of novel nanomaterials for drug delivery and cancer therapy.

cond-mat.mtrl-sci↗
arXiv2005

Deterministic Synthesis of Carbon Nanotubes

A novel method is developed to enable the formation, positioning and patterning of individual metal nano-clusters with controllable and monodispersed sizes down to 1-2 nm-scale. Among wide range of potential applications, the nanoparticle arrays are used for deterministic synthesis of monodispersed single walled carbon nanotubes. The carbon nanotubes are grown at individually controlled locations with near one-to-one yield, reaching one of the ultimate goals of nanotube synthesis.

cond-mat.mtrl-sci↗
arXiv2005

Temperature and Gas-Environment Dependent Electron and Phonon Transport in Suspended Carbon Nanotubes Up to Electrical Breakdown

High bias electrical transport characteristics of freely suspended metallic single-walled carbon nanotubes (SWNTs) are investigated at 250-400K in vacuum and various gases. Self-heating is exploited to examine the temperature dependence of phonon transport and optical phonon decay in SWNTs. The acoustic phonon thermal conductivity of a SWNT follows ~1/T at high temperatures. Non-equilibrium optical phonon effects in suspended nanotubes decrease as the ambient temperature increases. Gas molecules assist the relaxation of hot optical phonons along the tube length and enable enhanced current flow. In vacuum, high bias breakdown of suspended SWNTs can occur via melting caused by electrically emitted hot optical phonons at a low acoustic phonon temperature.

cond-mat.mes-hall↗
arXiv2005

Oxidation Resistant Germanium Nanowires: Bulk Synthesis, Long Chain Alkanethiol Functionalization and Langmuir-Blodgett Assembly

A simple method is developed to synthesize gram quantities of uniform Ge nanowires (GeNWs) by chemical vapor deposition on preformed, monodispersed seed-particles loaded onto high surface area silica support. Various chemical functionalization schemes are investigated to passivate the GeNW surfaces using alkanethiols and alkyl Grignard reactions. The stability of functionalization against oxidation of germanium for various alkyl chain lengths is elucidated by X-ray photoelectron spectroscopy. Among all schemes tested, long chain alkanethiols (>=C12) are found to impart the most stable GeNW passivation against oxidation upon extended exposure to ambient air. Further, the chemically functionalized oxidation-resistant nanowires are soluble in organic solvents and can be readily assembled into close-packed Langmuir-Blodgett films potentially useful for future high performance electronic devices.

cond-mat.mtrl-sci↗
arXiv2005

Deterministic One-to-One Synthesis of Germanium Nanowires and Individual Gold Nano-Seed Patterning for Aligned Nanowire Arrays

It is demonstrated by the current work controlled one-to-one synthesis of Ge nanowires (GeNWs) with ~100% yield from Au seed particles. The optimum GeNW growth conditions are found to be size dependent. For < 50 nm Au seeds, under-growth, one-to-one growth and over-growth can occur depending on the growth conditions. Growth of large GeNWs appears to be diffusion limited. These results should have generic implications to the synthesis of other types of nanowires via vapor-liquid-solid (VLS) processes. Patterning and positioning of individual Au nanoparticles are achieved by lithographic patterning and used for successful one-to-one nanowire growth. Finally, post-growth flow-alignment is used to obtain quasi-parallel nanowires originating from well-controlled single-particle sites. These results are important to the fundamental science of nanomaterials synthesis and may find important applications in various fields including high performance nanoelectronics.

cond-mat.mtrl-sci↗
arXiv2005

Carbon Nanotubes as Intracellular Protein Transporters: Generality and Biological Functionality

Various proteins adsorb spontaneously on the sidewalls of acid-oxidized single-walled carbon nanotubes. This simple non-specific binding scheme can be used to afford non-covalent protein-nanotube conjugates. The proteins are found to be readily transported inside various mammalian cells with nanotubes acting as the transporter via the endocytosis pathway. Once released from the endosomes, the internalized protein-nanotube conjugates can enter into the cytoplasm of cells and perform biological functions, evidenced by apoptosis induction by transported cytochrome c. Carbon nanotubes represent a new class of molecular transporters potentially useful for future in-vitro and in-vivo protein delivery applications.

cond-mat.other↗
arXiv2005

High Performance N-Type Carbon Nanotube Field Effect Transistors with Chemically Doped Contacts

Short channel (~80 nm) n-type single-walled carbon nanotube (SWNT) field-effect transistors (FETs) with potassium (K) doped source and drain regions and high-k gate dielectrics (ALD HfO2) are obtained. For nanotubes with diameter ~ 1.6 nm and bandgap ~ 0.55 eV, we obtain n-MOSFET-like devices exhibiting high on-currents due to chemically suppressed Schottky barriers at the contacts, subthreshold swing of 70mV/decade, negligible ambipolar conduction and high on/off ratios up to 10^6 at a bias voltage of 0.5V. The results compare favorably with the state-of-the-art silicon n-MOSFETs and demonstrate the potential of SWNTs for future complementary electronics. The effects of doping level on the electrical characteristics of the nanotube devices are discussed.

cond-mat.mtrl-sci↗
arXiv2004

On the Origin of Preferential Growth of Semiconducting Single-Walled Carbon Nanotubes

A correlation is observed between the diameter (d) distribution of single walled carbon nanotubes and the percentages of metallic and semiconducting tubes in materials synthesized at low temperature (600 C) by plasma-assisted chemical vapor deposition. Small diameter nanotubes (average d~1.1 nm) show semiconducting-tube percentage much higher than expected for random chirality distribution. Density functional theory calculations reveal discernable differences in the cohesive energies and heat of formation energies for similar-diameter metallic, quasi-metallic and semiconducting nanotubes. Semiconducting nanotubes exhibit the lowest energies and the stabilization effect scales with ~1/d2. This is a likely thermodynamic factor in preferential growth of small diameter semiconducting nanotubes.

cond-mat.mes-hall↗
arXiv2004

Suspended Carbon Nanotube Quantum Wires with Two Gates

Suspended single-walled carbon nanotube devices comprised of high quality electrical contacts and two electrostatic gates per device are obtained. Compared to nanotubes pinned on substrates, the suspended devices exhibit little hysteresis related to environmental factors and manifest as cleaner Fabry-Perot interferometers or single electron transistors. The high-field saturation currents in suspended nanotubes related to optical phonon or zone boundary phonon scattering are significantly lower due to the lack of efficient heat sinking. The multiple gate design may also facilitate future investigation of electromechanical properties of nanotube quantum systems.

cond-mat.mes-hall↗
arXiv2004

The Aharonov-Bohm Interference and Beating in Single-Walled Carbon Nanotube Interferometers

Relatively low magnetic fields applied parallel to the axis of a chiral single-walled carbon nanotube are found causing large modulations to the p-channel or valence band conductance of the nanotube in the Fabry-Perot interference regime. Beating in the Aharonov-Bohm type of interference between two field-induced non-degenerate sub-bands of spiraling electrons is responsible for the observed modulation with a pseudo period much smaller than that needed to reach the flux quantum F0 = h/e through the nanotube cross-section. We show that single-walled nanotubes represent the smallest cylinders exhibiting the Aharonov-Bohm effect with rich interference and beating phenomena arising from well-defined molecular orbitals reflective of the nanotube chirality.

cond-mat.mes-hall↗
arXiv2004

10 to 50 nm Long Quasi Ballistic Carbon Nanotube Devices Obtained Without Complex Lithography

A simple method combining photolithography and shadow (or angle) evaporation is developed to fabricate single-walled carbon nanotube (SWCNT) devices with tube lengths L~10-50 nm between metal contacts. Large numbers of such short devices are obtained without the need of complex tools such as electron beam lithography. Metallic SWCNTs with lengths ~ 10 nm, near the mean free path (mfp) of lop~15 nm for optical phonon scattering, exhibit near-ballistic transport at high biases and can carry unprecedented 100 mA currents per tube. Semiconducting SWCNT field-effect transistors (FETs) with ~ 50 nm channel lengths are routinely produced to achieve quasi-ballistic operations for molecular transistors. The results demonstrate highly length-scaled and high-performance interconnects and transistors realized with SWCNTs.

cond-mat.mtrl-sci↗
arXiv2004

Miniature Organic Transistors With Carbon Nanotubes as Quasi-One Dimensional Electrodes

As the dimensions of electronic devices approach those of molecules, the size, geometry and chemical composition of the contact electrodes play increasingly dominant roles in device functions. It is shown here that single-walled carbon nanotubes (SWNT) can be used as quasi one-dimensional (1D) electrodes to construct organic field effect transistors (FET) with molecular scale width (~2 nm) and channel length (1-3 nm). An important feature owing to the quasi 1D electrode geometry is the favorable gate electrostatics that allows for efficient switching of ultra-short organic channels. This affords room temperature conductance modulation by orders of magnitude for organic transistors that are only several-molecules in length, with switching characteristics superior to similar devices with lithographically patterned metal electrodes. With nanotubes, covalent carbon-carbon bonds could be utilized to form contacts to molecular materials. The unique geometrical, physical and chemical properties of carbon nanotube electrodes may lead to various interesting molecular devices.

cond-mat.mtrl-sci↗
arXiv2004

Surface Chemistry and Electrical Properties of Germanium Nanowires

Germanium nanowires with p- and n-dopants were synthesized by chemical vapor deposition and used to construct complementary field effect transistors . Electrical transport and x-ray photoelectron spectroscopy data are correlated to glean the effects of Ge surface chemistry to the electrical characteristics of GeNWs. Large hysteresis due to water molecules strongly bound to GeO2 on GeNWs is revealed. Different oxidation behavior and hysteresis characteristics and opposite band bending due to Fermi level pinning by interface states between Ge and surface oxides are observed for p- and n-type GeNWs. Vacuum annealing above 400C is used to remove surface oxides and eliminate hysteresis in GeNW FETs. High-k dielectric HfO2 films grown on clean GeNW surfaces by atomic layer deposition (ALD) using an alkylamide precursor is effective serving as the first layer of surface passivation. Lastly, the depletion length along the radial direction of nanowires is evaluated. The result suggests that surface effects could be dominant over the bulk properties of small diameter wires.

cond-mat.mtrl-sci↗
arXiv2004

Self-Aligned Ballistic Molecular Transistors and Electrically Parallel Nanotube Arrays

Carbon nanotube field-effect transistors with structures and properties near the scaling limit with short (down to 50 nm) channels, self aligned geometries, palladium electrodes with low contact resistance and high-k dielectric gate insulators are realized. Electrical transport in these miniature transistors is near ballistic up to high biases at both room and low temperatures. Atomic layer deposited (ALD) high-k films interact with nanotube sidewalls via van der Waals interactions without causing weak localization at 4 K. New fundamental understanding of ballistic transport, optical phonon scattering and potential interfacial scattering mechanisms in nanotubes are obtained.

cond-mat.other↗