Santiago Esconjauregui

Growth of ultrahigh density vertically aligned carbon nanotube forests for interconnects.

We present a general catalyst design to synthesize ultrahigh density, aligned forests of carbon nanotubes by cyclic deposition and annealing of catalyst thin films. This leads to nanotube forests with an area density of at least 10(13) cm(-2), over 1 order of magnitude higher than existing values, and close to the limit of a fully dense forest. The technique consists of cycles of ultrathin metal film deposition, annealing, and immobilization. These ultradense forests are needed to use carbon nanotubes as vias and interconnects in integrated circuits and thermal interface materials. Further density increase to 10(14) cm(-2) by reducing nanotube diameter is possible, and it is also applicable to nanowires.

Growth of high-density vertically aligned arrays of carbon nanotubes by plasma-assisted catalyst pretreatment

A plasma-assisted thermal pretreatment of catalyst films (Ni, Co, or Fe) greatly facilitates the direct growth of high-density vertically aligned arrays of small diameter carbon nanotubes (CNTs) on conductive TiN by purely thermal chemical vapor deposition. Purely thermal catalyst pretreatment gives limited or no growth. The plasma-assisted pretreatment promotes a stronger catalyst-support interaction, which reduces catalyst mobility and hence stabilizes smaller catalyst particles with a higher number density.

Measurement of area density of vertically aligned carbon nanotube forests by the weight-gain method

The area density of vertically aligned carbon nanotubes forests is measured and analysed by the weight gain method. The mass density of a close packed array of single- and multi-walled nanotubes is analysed as a function of the average nanotube diameter and number of walls, and this is used to derive the area density, from which the filling factor can be extracted. Densities of order 1013 cm−2 tubes are grown from cyclic catalyst methods.

Capacitive nanoelectromechanical switch based on suspended carbon nanotube array

We present the fabrication and high frequency characterization of a capacitive nanoelectromechanical system (NEMS) switch using a dense array of horizontally aligned single-wall carbon nanotubes (CNTs). The nanotubes are directly grown onto metal layers with prepatterned catalysts with horizontal alignment in the gas flow direction. Subsequent wetting-induced compaction by isopropanol increases the nanotube density by one order of magnitude. The actuation voltage of 6 V is low for a NEMS device, and corresponds to CNT arrays with an equivalent Young’s modulus of 4.5–8.5 GPa, and resistivity of under 0.0077 Ω⋅cm. The high frequency characterization shows an isolation of −10 dB at 5 GHz.

Low temperature growth of ultra-high mass density carbon nanotube forests on conductive supports

We grow ultra-high mass density carbon nanotube forests at 450 °C on Ti-coated Cu supports using Co-Mo co-catalyst. X-ray photoelectron spectroscopy shows Mo strongly interacts with Ti and Co, suppressing both aggregation and lifting off of Co particles and, thus, promoting the root growth mechanism. The forests average a height of 0.38 μm and a mass density of 1.6 g cm−3. This mass density is the highest reported so far, even at higher temperatures or on insulators. The forests and Cu supports show ohmic conductivity (lowest resistance ∼22 kΩ), suggesting Co-Mo is useful for applications requiring forest growth on conductors.

Use of plasma treatment to grow carbon nanotube forests on TiN substrate

Hydrogen plasma pretreatment is used to enforce the growth of vertically-aligned carbon nanotube forests on TiN substrates. The evolution of the substrate, catalyst, and nanotubes are studied by in situ and ex-situ photoemission and X-ray diffraction in order to understand the growth mechanism. We find that TiN retains its crystallographic structure and its conductivity during plasma pretreatment and nanotube growth, which is confirmed by electrical measurements. Plasma pretreatment is found to favor the growth of nanotube forests by root growth, as it binds the catalyst nanoparticles more strongly to the substrate than thermal pretreatment. We find that plasma pretreatment time should be limited, otherwise poor or no growth is found.

Manipulation of the catalyst-support interactions for inducing nanotube forest growth

We show how an oxidative pretreatment of Fe, Co, or Ni growth catalyst on SiO2 support can be used to switch the growth mode of carbon nanotubes from tip growth to root growth, thus favoring the growth of dense, vertically aligned nanotube forests. The oxidative treatment creates a strong catalyst–support interaction at the catalyst–silica interface, which limits the surface diffusion and sintering of the catalyst nanoparticles and binds the catalyst to the SiO2 surface. This shows that the alignment and growth mode of nanotubes can be controlled, increasing the range of support materials giving dense nanotube forests.

Stability of graphene doping with MoO3 and I2

We dope graphene by evaporation of MoO3 or by solution-deposition of I2 and assess the doping stability for its use as transparent electrodes. Electrical measurements show that both dopants increase the graphene sheet conductivity and find that MoO3-doped graphene is significantly more stable during thermal cycling. Raman spectroscopy finds that neither dopant creates defects in the graphene lattice. In-situ photoemission determines the minimum necessary thickness of MoO3 for full graphene doping.

Growth of Continuous Monolayer Graphene with Millimeter-sized Domains Using Industrially Safe Conditions

We demonstrate the growth of continuous monolayer graphene films with millimeter-sized domains on Cu foils under intrinsically safe, atmospheric pressure growth conditions, suitable for application in roll-to-roll reactors. Previous attempts to grow large domains in graphene have been limited to isolated graphene single crystals rather than as part of an industrially useable continuous film. With both appropriate pre-treatment of the Cu and optimization of the CH4 supply, we show that it is possible to grow continuous films of monolayer graphene with millimeter scale domains within 80 min by chemical vapour deposition. The films are grown under industrially safe conditions, i.e., the flammable gases (H2 and CH4) are diluted to well below their lower explosive limit. The high quality, spatial uniformity, and low density of domain boundaries are demonstrated by charge carrier mobility measurements, scanning electron microscope, electron diffraction study, and Raman mapping. The hole mobility reaches as high as ~5,700 cm2 V−1 s−1 in ambient conditions. The growth process of such high-quality graphene with a low H2 concentration and short growth times widens the possibility of industrial mass production.

Efficient Transfer Doping of Carbon Nanotube Forests by MoO3

We dope nanotube forests using evaporated MoO3 and observe the forest resistivity to decrease by 2 orders of magnitude, reaching values as low as ∼5 × 10(-5) Ωcm, thus approaching that of copper. Using in situ photoemission spectroscopy, we determine the minimum necessary MoO3 thickness to dope a forest and study the underlying doping mechanism. Homogenous coating and tube compaction emerge as key factors for decreasing the forest resistivity. When all nanotubes are fully coated with MoO3 and packed, conduction channels are created both inside the nanotubes and on the outside oxide layer. This is supported by density functional theory calculations, which show a shift of the Fermi energy of the nanotubes and the conversion of the oxide into a layer of metallic character. MoO3 doping removes the need for chirality control during nanotube growth and represents a step forward toward the use of forests in next-generation electronics and in power cables or conductive polymers.