Mehran Vahdani Moghaddam

Transforming carbon nanotube forest from darkest absorber to reflective mirror

Carbon nanotube (CNT) forests are known to be among the darkest materials on earth. They can absorb the entire visible range of electromagnetic wave more efficiently than any other known black material. We have attempted controlled mechanical processing of the CNTs and, surprisingly, observed mirror-like reflection from the processed area with 10%–15% reflectivity, a level higher than typical reflectivity of pure forests by over two orders of magnitude, for a wide range of the spectrum (570–1100 nm). Patterning of micro mirrors in the forest is demonstrated to show its potential application for producing monolithically integrated reflector-absorber arrays in the material.

Solar electron source and thermionic solar cell

Common solar technologies are either photovoltaic/thermophotovoltaic, or use indirect methods of electricity generation such as boiling water for a steam turbine. Thermionic energy conversion based on the emission of electrons from a hot cathode into vacuum and their collection by an anode is also a promising route. However, thermionic solar conversion is extremely challenging as the sunlight intensity is too low for heating a conventional cathode to thermionic emission temperatures in a practical manner. Therefore, compared to other technologies, little has been done in this area, and the devices have been mainly limited to large experimental apparatus investigated for space power applications. Based on a recently observed “Heat Trap” effect in carbon nanotube arrays, allowing their efficient heating with low-power light, we report the first compact thermionic solar cell. Even using a simple off-the-shelf focusing lens, the device delivered over 1 V across a load. The device also shows intrinsic storage ...

Polarization-dependent light-induced thermionic electron emission from carbon nanotube arrays using a wide range of wavelengths

Light-induced thermionic electron emission from arrays of carbon nanotubes is observed using low-power, continuous-wave lasers with a broad set of wavelengths ranging from violet to infrared. The thermionic emission current is highest when the electric field of the laser is parallel to the axis of the nanotubes and lowest when it is perpendicular. The polarization dependence is stronger for the longer-wavelength beam.

Nanostructured Thermionics for Conversion of Light to Electricity: Simultaneous Extraction of Device Parameters

Thermionic conversion involves the direct conversion of heat, including light-induced heat, from a heat source, e.g., solar energy, to electricity. Although the concept is almost a hundred years old, the progress of thermionic convertors has been limited by issues such as the space-charge effect and availability of materials with desirable mechanical and electrical properties, while maintaining a low work function. Nanotechnology could help address some of the main challenges that thermionic convertors face. However, existing models, which were developed for macroscopic convertors, are not capable of describing all aspects of nanostructured devices. We present a method to evaluate the output characteristics of thermionic convertors with a higher precision than the existing models and the ability to simulate a broader range of parameters, including temperatures, active surface areas, interelectrode distances, and work functions. These features are crucial for the characterization of emergent devices due to the unknowns involved in their internal parameters; the models high numerical precision and flexibility allows one to solve the reverse problem and to evaluate the internal parameters of the device from a set of simple experimental data. As an experimental case, a carbon nanotube forest was used as the emitter and locally heated to thermionic emission temperatures using a 50-mW-focused laser beam. The current-voltage characteristics were measured and used to solve the reverse problem to obtain the internal parameters of the device, which were shown to be consistent with the values obtained using other methods.

Photon-impenetrable, electron-permeable: the carbon nanotube forest as a medium for multiphoton thermal-photoemission.

Combining the photoelectric and thermionic mechanisms to generate free electrons has been of great interest since the early days of quantum physics as exemplified by the Fowler-DuBridge theory, and recently proposed for highly efficient solar conversion. We present experimental evidence of this combined effect over the entire range spanning room-temperature photoemission to thermionic emission. Remarkably, the optical stimulus alone is responsible for both heating and photoemission at the same time. Moreover, the current depends on optical intensity quadratically, indicating two-photon photoemission, for intensities of ca. 1-50 W/cm(2), which are orders of magnitude below the intensities required for two-photon photoemission from bulk metals. This surprising behavior appears to be enabled by the internal nanostructure of the carbon nanotube forest, which captures photons effectively, yet allows electrons to escape easily.

Visible-light induced electron emission from carbon nanotube forests

The authors report electron emission from forests of vertically aligned multiwalled carbon nanotubes under irradiation by continuous wave green and blue lasers with relatively low power and intensity (maximum intensity of ∼320 W cm−2). The electron emission shows nonlinear increase with laser power for both laser wavelengths of 488 and 532 nm. Thermionic emission and photofield-emission appear to play a role in different sections of the current-voltage characteristics.

Cone-shaped forest of aligned carbon nanotubes: An alternative probe for scanning microscopy

A scanning microscopy probe based on three-dimensionally shaped carbon nanotube (CNT) forests and its application to atomic-force microscopy (AFM) are reported. Micro-scale CNT forests directly grown on silicon cantilevers are patterned into cone shapes with the tips of a few individual nanotubes. The CNT-forest-based probes provide significantly higher mechanical stability/robustness than the common single-CNT probes. AFM imaging using the fabricated probes reveals their imaging ability comparable to that of commercial probes. The patterning process also improves the uniformity of the CNT forests grown on each cantilever. The results suggest a promising future for CNT scanning probes and their production approach.

The effects of three-dimensional shaping of vertically aligned carbon-nanotube contacts for micro-electro-mechanical switches

A micro-electro-mechanical switch integrated with vertically aligned carbon nanotubes (CNTs) as the contact material is presented. Arrays of the CNTs are three-dimensionally micropatterned using a pulsed micro-discharge process to have tapered contact surfaces with controlled angles, achieving maximized contact areas, while providing contact resistances in the 10 Ω range with an enhanced current capacity. A shape-memory-alloy actuator is integrated to demonstrate stable switching for ∼1.4 × 106 ON-OFF cycles with no sign of damage. The results prove that post-growth micropatterning of CNTs is a promising path to improved and reliable micro contact switches enabled by arrayed CNT contacts for high-power applications.

Stabilization of laser-induced thermionic electron emission from carbon nanotubes through rapid power switching

The electron emission stability of a carbon nanotube-based optically-activated cathode is investigated. The emission mechanism is thermionic, based on localized light-induced heating of the carbon nanotube forest - the Heat Trap effect. We demonstrate stabilized emission behavior by introducing a switching phase in the input optical power. This effectively eliminates or significantly reduces the decay in emission current with time.